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I N S T I T U T E OF T E C H N O L O G Y
ELECTRON BEAM WELDING:
REVIEW OF LITERATURE
by
June 1972
CRANFIELD INSTITUTE OF TECHNOLOGY
ELECTRON BEAM WELDING: REVIEW OF LITERATURE
W.F. Chambers, M.Phil., D.A.E., M.Weld.I., and
R.L. Apps, B . S c , Ph.D., F.I.M., M.Weld.I.
SUMMARY;
The literature on electron beam welding has been reviewed with regard to (1) weldability and material properties; and
(2) production applications.
Each section has been sub-divided to allow easy reference to publications on specific materials or particular industries. This report should be read in conjunction with CIT Memo 69, "Bibliography on Electron Beam Welding".
CONTENTS
Page
1. INTRODUCTION 1 2. . WELDABILITY AND MATERIALS PROPERTIES 2
2.1 Aluminium and aluminium alloys 2
2.1.1 Pure aluminium 2 2.1.2 Aluminium - copper alloys 2
2.1.3 Aluminium - zinc - magnesium alloys 3
2.2 Beryllium 3 2.3 Chromium ^ 2.4 Copper * 2.4.1 Metallic copper ^ 2.4.2 Copper alloys 5 2.5 Ferrous alloys 5 2.5.1 Mild steels 5 2.5.2 Plain carbon steels °
2.5.3 Low carbon alloy steels ° 2.5.4. Medium carbon low alloy steels 6
2.5.5 Maraging steels 7 2.5.6 High alloy ferritic steels 8
2.5.7 Austenitic steels 8 2.5.8 Precipitation hardening stainless steels 9
2.5.9 High chromium martensitic steels 9
2.6 Graphite 10 2.7 Magnesium 10 2.8 Molybdenum 10 2.9 Nickel alloys 10 2.10 Niobium 11 2.11 Tantalum 12 2.12 Titanium and titanium alloys 12
2.13 Tungsten 13 2.14 Uranium 14 2.15 Zirconium lA 2.16 Dissimilar metal combinations lA
3. PRODUCTION APPLICATIONS 16 3.1 Miniature components 16 3.2 Nuclear engineering 16 3.3 Aeronautical engineering 16
3.4 Spacecraft 17 3.5 General light and medium engineering 17
ELECTRON BEAM WELDING : REVIEW OF LITERATURE
INTRODUCTION
Most welding processes have a fairly long history, and articles relating to their use are very many, the earliest probably being now
inapplicable with the recent advances in equipment design and material specifica-tion. Any literature survey attempted would necessarily have to be selective, and would even then be encyclopaedic.
Electron beam welding is a comparatively new process so that it should be possible to collect published works directly related to it and make some claim to be comprehensive. In the accompanying bibliography a start has been made to collect such references, but at present no claim is made in respect
to completeness. Nonetheless a representative number of the more important articles dealing with research into the various aspects of electron beam
welding are included. From this bibliography the following literature review has been prepared on the material weldability and properties, and the production application aspects only. It has been considered that electron beam users and prospective users will be most interested in these two aspects rather than machine design and characteristics of the welding process itself, although in some instances articles dealing with the determination of electron beam
characteristics necessarily produce results showing weldability and material properties.
The section dealing with weldability and material properties has been divided into metallic elements and their alloys with a section dealing with the welding of dissimilar metals. The section on ferrous alloys has been further sub-divided into:
i. Mild steels (0.06-0.25% C with only Si or Mn). ii. Plain carbon steels (>0.25% C with only Si or Mn). iii. Low carbon, alloy steels (0.1-0.25% C, up to 5%
alloy elements).
iv. Medium carbon, low alloy steels (0.25-0.5% C, up to 5% alloy).
v. Maraging steels (0.01-0.1% C ) .
vi. High alloy, ferritic steels (>0.1% C, up to 20% alloy). vii. Austenitic steels.
viii. Precipitation hardening stainless steels. ix. High chromium martensitic steels.
Under each of the above sub-headings where work has been reported, an attempt is made to define the weldability problems, such as porosity and cracking in the various thicknesses of material welded, and to show the joint properties
obtained. Of approximately forty references to the welding of ferrous materials, it will be seen that by far the greatest amount of research has been in the area of medium carbon alloy steels since these are most widely used in the aerospace industry, especially for rocket motor components. It follows therefore that most reports of this work originate in the U.S.A. and U.S.S.R.
The section on production engineering applications has been compiled to show the relative distribution of effort in the practical utilization of research results in the field of electron beam welding. The review has been divided into five sub-sections.
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-1. Miniature components, e.g. instruments and electronics.
2. Nuclear engineering, e.g. fuel cans, reactor components and isotope encapsulation.
3. Aerospace engineering, (a) Aircraft, (b) Spacecraft. 4. General light engineering, e.g. automobile components. 5. Heavy engineering and pipework.
It can be seen from the attendant bibliography that by far the largest reported users are in the aerospace industry, reflecting the interest in joining more exotic materials and the resources available to investigate and exploit new joining methods.
2. WELDABILITY AND MATERIALS PROPERTIES 2.1 Aluminium and aluminium alloys
2.1.1 Pure aluminium
In welding 99.9999% pure aluminium, Cheever et al (ref. 50) sought to obtain the most economical production process and ultimately decided to use T.I.G. welding since less expensive tooling was needed. However, 0.06 inch to 0.24 inch thick sheet was welded by the electron beam process in high vacuum. The deterioration of electrical resistivity was the main object of the investigation, and it was shown that very little rise in resistivity resulted from the welding.
2.1.2 Aluminium-copper alloys
Work on the high strength binary aluminium-copper alloy 2219-T87 (6,3 Cu, 0.3 Mn, O.IOV, 0.152V) has shown that up to 80% parent material tensile strength can be obtained (ref. 41). Higher powers than expected needed to be used, possibly due to the absence of magnesium, giving a lower vaporization potential. The greatest strength was obtained with the narrowest welds and with the highest speeds, but the most uniform root reinforcement was obtained by using low travel speed, low voltage, high current, out-of-focus and overpowered type weld settings (ref. 297). The as-welded fracture toughness and tensile properties were better than those obtained by T.I.G. welding. In i inch tkick plate pressure vessels,
gradual 'slope-out' at the weld start/stop point produced the best defect free welds, and in heavier plate a wide weld was needed to avoid defects in the
'slope-out' in circumferential welds. Induced porosity near a surface or intersecting a surface in { inch thick material governed fatigue life, and was more significant than much larger internal flaws. Weld spatter could be a problem, but anti-spatter coatings, if welded through, reduced the weld quality.
Sanderson et al (ref. 239) in welding 4.5% copper, aluminium alloy (H-15, BS1477) found that welding speeds below 2.5 m/min needed to be used on 8 mm thick material to avoid cracking, and that cleanliness in and around the joint face was essential to avoid contamination. Post-weld aging treatment was necessary to restore full U.T.S. but the 0.1% proof stress and elongation were still below parent material levels, and artificial aging gave a brittle H.A.Z. structure. The conclusion that low travel speeds and therefore wide welds are needed for maximum tensile strength, arrived at by Sanderson
(ref. 239) is at variance with the findings of both Brennecke (ref. 41) and Trzil (ref. 297), who showed that the fastest speeds gave the highest strength welds. Whilst all three workers used 30 KV machines, the only approximately comparable thicknesses welded were 8 mm and i inch. The low
travel speeds advocated by Sanderson gave the other two investigators
drop through and top bead undercut, and even the highest travel speeds necessi-tated smoothing passes on both sides to produce good surface profiles. The cracking found by Sanderson in the narrow welds was not seen by Brennecke or Trzil. The narrow welds, having very narrow H.A.Zs., would be expected to possess a more limited ability to deform considering that there would be little resolution or annealing of the parent metal when welded in either the heat treated or heat treated and cold worked conditions. The softest material would be seen in the weld metal where the yield point would be lowest. The highest overall yield strengths would therefore be expected in the thickest material with narrow welds where triaxial stress conditions would most
likely be found.
Out of vacuum welding using both low (6KW) and high (12KW) powers with helium shielding gas (ref. 350) showed that a narrow weld cross section which reduced the heat input per unit of weld run gave the best butt weld
tensile breaking strength and kept porosity to a minimum. Flow control of the helium from the exit nozzle was necessary to produce narrow welds over a variety of power levels and welding speeds.
2.1.3 Aluminium-zinc-magnesium alloys
In welding { inch thick 4% Zn - 2.5% Mg, aluminium alloy in the solution treated and aged condition, it was found that almost full parent plate strength was obtained after natural aging for 30 days. (ref. 5 1 ) .
Fine porosity in the weld metal, and slight cracking in the weld metal/H.A.Z. junction were seen. Zinc loss in the weld metal was expected and proved to be about 16%. Discoloration of the fatigue fracture face in specimens used for fracture toughness tests was thought to be due to a concentration of zinc at the grain boundaries formed during the welding cycle. The granular
type of fatigue crack was attributed to grain boundary accumulation of long, straight dislocations, the movement of which through the matrix and sub-grain boundaries was facilitated by the less well developed
precipitates in the HAZ. When comparing the weldability of Aluminium-zinc-magnesium alloy with aluminium-silicon-magnesium alloy, it was found (ref. 239) that higher mechanical properties were obtained in the post-weld naturally aged condition of the 4% Zn - 2.5% Mg, than in the unwelded sheet of a 1% Si - 1% Mg alloy, and that there appeared to be no H.A.Z. embrittle-ment.
2.2 Beryllium
Electron beam welds have been made in beryllium in thicknesses up to approximately | inch. (refs. 116, 117, 123, 321). In material i inch thick and above, surface roughness and undercut were the only real obstacles to producing satisfactory welds. Low travel speed with low power beam density and multiple passes were found to be beneficial in reducing porosity, which was probably due to beryllium oxide. Surface preparation to reduce oxygen contamination was also important since tensile strength and ductility were reduced as the oxygen content increased. Tensile strengths in single pass welds could be between 60% and 95% efficient in the temperature range of 22 C to 426 C, but elongations were low. Longitudinal welded specimens were seen to be significantly more ductile than transverse welded specimens. Lap, tee and corner joints tended to produce cracks in the weld metal which
could be reduced with aluminium filler, giving room temperature strengths of greater than 70% efficiency.
1
4
-Electron beam braze welding of hot pressed beryllium (refs. 123, 125, 310) has been effected by the addition of aluminium shims and filler, and in some cases joint strengths greater than the hot pressed
material were obtained. The most successful methods of joint alloying were found to be surface pre-coating by the T.I.G. process and vapour deposition, provided that at least a thickness of 0.01 inch was deposited and tapered to match the beam taper. The best joint strengths were obtained when full alloying was obtained between the aluminium and beryllium. As with the vacuum cast sheet material, beryllium oxide led to porosity which reduced
the tensile weld strength. This porosity could be reduced with an increasing number of weld passes, but welding speed appeared to have no significant
effect on porosity or strength. To promote the wetting of the beryllium surface in braze welding with various metals and alloys including pure aluminium, commercially pure aluminium, silver-copper eutectic and gold-nickel alloys, a vapour deposition of titanium has been used (ref. 308). Metallographic examination in conjunction with micro-probe analysis showed
the wetting mechanism to be by initial penetration and displacement of the titanium vapour deposited film, and subsequent 'tunnelling' of the liquid along the interface of the oxidized metallic surface and the vapour
deposit. The titanium vapour deposit then alloys and reacts with the liquid surrounding it from the top and bottom surface while the wetting and spreading phenomena continue.
2.3 Chromium
The only known reference to welding technically pure chromium (containing not more than 0.1% impurities by weight), refers to 1 to 2mm thick sheet, (ref. 97) and compares the electron beam process to both T.I.G. and manual metal arc. Undercut of the top weld bead was obtained with electron beam welding but was reduced by using a pulsed beam. There was less weld contamination from oxygen, nitrogen and hydrogen than that found by the other processes, and joint efficiency could be 85% to 95% of parent material values, with only slightly reduced ductility. The metal in the electron beam welds appeared to become brittle at a lower temperature than that observed for T.I.G. or manual metal arc welding. Any improvement in weld quality could only be obtained by reducing the amount of impurities in the parent material and hence improving the overall ductility and toughness. This would be expected since electron beam welding in high vacuum would not
introduce any atmospheric impurities. 2.4 Copper
2.4.1 Metallic Copper
In the electron beam welding of essentially pure copper, both oxygen-free and boron-deoxidized material up to 0.03 inch thick, it was found that both materials were weldable where freedom from porosity was the weldability criterion (ref. 39). The number of passes was the most
influential variable in reducing porosity, with the boron-deoxidized copper giving the least porosity initially and requiring four passes to produce satisfactory welds.
In welding i inch thick pure copper which was either oxygen free or phosphorous-deoxidized, the effects of varying beam power, focus,
travel speed and slight amounts of impurities were seen (ref. 143).
Measurement of the relative porosity, penetration and area of the fusion zone showed that the penetration was greatest for the greatest depth of focus, but that porosity also increased with increase in depth of focus. The highest travel speed and beam power possible gave the greatest joint efficiency and least porosity. Travel speed appeared to be the single variable having the greatest effect on porosity. Penetration varied inversely with the impurity level, the highest impurity content giving the least penetration, but the impurity level appeared to have no significant effect on porosity.
The effects of varying the amount of cold working in the base metal, and the resultant effects on the H.A.Z. were also studied in oxygen-free and phosphorous-deoxidized copper, either fully annealed or cold rolled to 11% or 75% reduction in thickness giving i inch thick plates. It was seen that the width of the H.A.Z. (recrystallized zone) for a high impurity copper was about two thirds that of a low impurity level copper. The highest rates of heat input completely suppressed equilibrium recrystall-ization and growth.
2.4.2 Copper alloys
Alloys containing between 0.25% and 3% beryllium have been welded by the electron beam process in material thicknesses up to 0.10 inch (refs. 168, 307). Because the thermal conductivity and melting point decrease with increased amounts of beryllium, it is found that the alloys containing
the highest amounts of beryllium are more easily welded, with a lower susceptibility to hot cracking. Weld metal properties can reach 95% efficiency after post-weld solution annealing and age hardening heat
treatments. Alloys up to 0.09 inch thick containing 0.25% Ni or Co, have high tensile weld strength but show short transverse defects parallel to the weld centre-line, which are believed to be caused by beryllide or cobalt segregation.
Cast A.B.2. aluminium bronze 13mm thick was successfully welded (ref. 239) with the use of filler, backing plates and beam spinning
techniques to reduce the otherwise gross porosity produced and the rough weld beads with internal and surface porosity. Both high and low voltage machines were used to produce satisfactory welds, but it was found that
tighter control of machine parameters, in particular accelerating voltage and beam current, were needed on the low voltage equipment to give
consistent penetration and weld profile. Welded joints showed 0.1% and 0.5% proof stress values roughly equivalent to parent material values, but elongation was much reduced in the 'as welded' condition due to hard and brittle weld metal and H.A.Z. Ultimate tensile strength however could be in excess of 80% of the parent plate.
It was also found (ref. 239) that both wrought and cast high strength cupro-nickel could be readily welded. In 13mm thick material no cracking or porosity problems were found, although both high and low voltage equipment produced weld bead sinking when using a vertically downward electron beam, with low voltage welds the worse. With the
wrought material and using a high voltage machine, beam oscillation was used to reduce top bead undercut, but filler was needed on the cast material to overcome the problem. An ultimate tensile strength of 80% parent plate values with a slight loss in ductility was found for the wrought material, and 0.1% proof strength was retained. The welded cast material showed all joint material properties to be comparable to the parent plate except that
there was some loss in ductility. 2.5 Ferrous alloys
2.5.1 Mild steels (0.06% - 0.25%C with only Si and Mn)
Very few references to investigations into the weldability and material properties were found ,for unalloyed steels with less than 0.25%C., commonly called mild steels or structural steels (refs. 67, 149, 172, 265). These steels are usually of a specification which allows wide limits in constituent proportions including impurities, consequently considerable non-uniformity has been found in electron beam welding. Porosity due to trapped gases was common and most reports dealt with measures for reducing this defect. The most successful method has been the introduction of
6
-aluminium shims into the joints in { inch and 1.0 inch thick plates of rimmed and semi-killed steels, to absorb the oxygen (ref. 149). Joint design is considered to be important, so that with rimmed steels the maximum amount of clean skin is included in the weld, and with surface
treated (carburized) steels the minimum amount of skin is included.
Beam spinning to give more time for gases to escape was found in some cases to be effective, and one researcher (ref. 265) successfully used pure iron interlayers to avoid porosity. Weld metal and H.A.Z. mechanical properties were equal or superior to the parent material, and with care in selecting
the welding parameters, adequate top and bottom bead profiles were obtained. The only known work on the fatigue performance of mild steel has been done at Cranfield and published as an accompanying report. This work has shown that with careful selection of welding parameters, porosity can be kept down to a level below which it only marginally affects the fatigue life of transverse butt welds in { inch thick material under pulsating tensile loading. Below approximately 5% porosity, surface defects are more significant in initiating fatigue cracks even when the porosity is concentrated to form a few large defects, provided the pores are roughly in the centre of the weld depth. Above approximately 5% porosity the internal pores are more significant than all but the most severe surface defects in reducing fatigue life. The reduction in life due to internal pores is roughly equivalent to the reduction in life associated with the higher actual stress level when the loss of cross sectional area is considered, whereas the reduction in life due to surface defects is much more severe. With weld top surface profile and underbead intact and up to 5% porosity on the mid-depth, the endurance limit was found to be above 2 X 10° cycles at a stress level of 185 MN/m2 (12T/in2).
2.5.2 Plain carbon steels (>0.25%C and only Si and Mn)
In the group of plain carbon unalloyed steels with over 0.25%C only one reference was found and this was part of work on two unalloyed steels (ref. 265). Cracking and porosity were found but both could be avoided. Plain unalloyed steels with over 0.2%C tend to crack when welded without pre-heat, and both pre-heat and post-weld heat treatments were used in welding steels with up to 0.44%C to avoid cracking. Porosity was reduced by the use of pure iron or aluminium shims and an aluminium sprayed
interlayer was also tried with some success. Post-weld heat treatment was used to restore the mechanical properties of the weld metal and reduce brittleness.
2.5.3 Low carbon alloy steels (0.1 - 0.25%C, up to 5% alloy) No references were found to work in this group.
2.5.4 Medium carbon low alloy steels (0.25 - 0.5%C, up to 5% alloy) In this category mainly sheet material has been welded but a few workers have used plate up to approximately J inch thick. Using sheet material up to | inch thick, a fast welding speed (low heat input) was found to be necessary to avoid grain growth in the H.A.Z. Material that was fully heat treated after welding gave approximately parent material properties (refs. 53, 131, 169, 203, 225, 293). Williams (ref. 315) found that the ultra-high strength steels were readily welded in both the hardened and un-hardened conditions in sheet up to 0.10 inch thick, and that the H.A.Z. contained fine grained structures.
Corrigan et al (ref. 53) attributed the high weld metal strength in 0.04 - 0.08 inch thick sheet to the near absence of micro-flaws, but Randal (ref. 221) in welding 0.10 inch thick air melted and vacuum melted sheet concluded that micro-cracks and micro-shrinkage cavities could and did
cause low load failures, whilst changes in welding procedure did not prevent their formation. In comparing mechanical properties of parent material with both T.I.G, and electron beam welds, specimens were
austenetized and double tempered after welding, and all weld specimens were pre-heated before welding. Unwelded sheet had by far the best bend ductility in both air and vacuum melted materials. Electron beam welds had slightly better bead ductility than T.I.G. welds for vacuum melted steel but the reverse was true for air melted steel.
In an intermediate thickness (0.225 inch) and using tool steel (H-11), Hokanson and Meier (ref. 131) found that annealed plate which was welded and post-weld heat treated to give an ultimate strength of up to 240 k.s.i., failed at similar strength levels in both parent plate and weld metal, but that the failures in the weld metal showed a lowered ductility.
The thickest material reported on was 14mm steel welded in the U.S.S.R. by Makara (ref. 349) in an investigation into the resistance of electron beam welded high strength steels to cold brittleness. Electron beam welds made in 0.42%C steel without filler wire, and in 0.32%C steel with 0.14%C filler wire were compared with manual arc welds. Delayed fracture tests on the 0.42%C steel specimens proved that H.A.Zs in the electron beam welds had four times the strength of arc welded joints with bainitic-pearlitic welds, and twice the strength of austenitic arc welds. There was less difference in the bainitic-pearlitic welds in the 0.32%C steel, but in this material the electron beam welds had even better
resistance to cold cracking than the arc welded joints with austenitic weld metal. The principal reason for the improved resistance to cold cracking
in electron beam welds was shown to be not from reduced hydrogen or lower residual stress level, but from the effect of the thermal cycle. The rapid heating and cooling cycle gave austenitic transformation in the H.A.Z. with subsequent self tempering in the upper martensite range. The absence of overheating in the H.A.Z. gave an additional beneficial
effect in electron beam welding, resulting in very much reduced grain growth compared with arc welding.
McHenry et al (refs. 178, 179, 180, 181) in the U.S.A. comparably welded i inch thick D6-AC steel (0.46%C, 0.55%Ni, l.l%Cr, l.l%Mo). The
initial welds in unheat treated material had adequate tensile and fracture toughness properties but poor fatigue life; micro-cracks initiated at voids caused by the solidification orientation. Transverse beam oscillation was used to disrupt the solidification pattern and eliminate micro'-cracking. Subsequently, heat treated material was welded and post weld stress relieved. The welds showed satisfactory tensile and fatigue properties, but fracture toughness was reduced, which was attributed to micro-segregation in the fusion zone. Williams and Martin (ref. 315) also welding D6-AC, { inch thick, obtained joint efficiencies of 87% in static tensile tests compared with joint efficiencies of 90% for welded sheet 0.10 inch thick in the
same material. Although both Russian and U.S. materials contained
approximately 0.45%C the Russian material contained 1.8%Cr. compared with the 1.1% in D6-AC. In all thicknesses, the workers who compared welding
processes agreed that the electron beam welds gave superior mechanical properties to T.I.G. welds, and Williams and Martin (ref. 315) state that electron beam welding showed the greatest advantage in welding steels in the hardened condition.
2.5.5 Maraging steels (0.01 - 0.1%C)
8
-and are the only representatives of this group to have been the subject of electron beam welding investigations.
Boniszewski and Kenyon (ref. 35), and Roth and Bratkovich (ref, 225) welded sheet material up to 0.185 inch thick. The former obtained joint efficiencies over 90% and found the weld metal to always be the weakest region, no failures ever occurring in the H.A.Z. Shape and size of the weld cross section had no effect on the joint efficiency, and fracture initiated at impurities in the grain boundaries. The impurities were apparently titanium sulphide and titanium carbo-nitride and impurity
segregation was more pronounced in the narrow welds. Tensile ductility was as good as in the parent material in defect-free welds. Roth and Bratkovich welded material in both the solution treated and also the fully age hardened conditions, obtaining defect free welds in both. Micro-fissures were seen in the H.A.Z., and tensile and bend failures occurred at the edge of the fusion zone. Up to 86% of the ultimate strength was retained. Adams and Travis (ref, 1) welded 0,065 to 0,50 inch thick 18% Ni - Cu - Mo maraging steel and obtained joint efficiencies of approximately 90% in transverse welds which were solution treated and aged subsequent to welding. Welds which were only aged subsequent to welding gave slightly lower efficiencies.
In the only reference to fatigue in maraging steels. Baker (ref, 13) purported to show that the treatment sequence to give optimum fatigue resis-tance was solution-anneal, age and then weld, which is at variance with the treatment recommended by Roth and Bratkovich, and Adams and Travis to give the best static tensile strength. In ref, 13 the main cause of failure was from cracks initiated in the weld or weld metal fusion boundary,
2.5.6 High alloy ferritic steels No references were found. 2.5.7 Austenitic steels
The most copmonly used materials in this category are the 18/8 stainless steels. These have been shown to be readily and consistently weld-able by the electron beam process. The good weldability is probably due to the close control of impurities, and many workers have used these materials to assess the effects of variation in electron beam parameters on penetration and weld cross sectional dimensions, and in this way contributed to the assessment of the material weldability.
In butt welding i inch thick unstabilized, titanium stabilized and molybdenum bearing plate, Kenyon (ref. 145) showed that composition was only marginally important, since in tensile failures the weld metal was generally as strong as the parent material, but if optimum strength was required the stabilized material (EN58B) was to be preferred. Failures generally initiated at notches formed by the weld top or bottom beads, but propagation was usually through the parent material. The weld metal was free from cracks and porosity, and the matrix was a mixture of
austenite and ferrite. The amount of retained ferrite was greater than usually found in T.I.G. welds, and the H.A.Z. extended for only a few grain widths into the parent material.
A typical example of the use of 18/8 austenitic stainless steel as a target in assessing fusion zone penetration in electron beam welding was the work of Gunn and King (ref. 96), who used 12mm thick material and showed that at any given combination of welding parameters the penetration will vary according to the focus position with regard to the metal surface, Welds were made over the ranges 2 to 20 ma, 80-150KV and 5-34mm/sec. An empirical relationship was found for the penetration in this material in terms
of beam current, voltage and travel speed.
Fletcher (ref. 82) also used 18/8 stainless steel in assessing distortion in electron beam welding, using material from 3mm to 24mm thick. He showed that below 8mm thick, shrinkage was almost constant at 0,05mm, but above 8mm the shrinkage increased sharply with material thickness,
Shrinkage also increased linearly with joint gap. Angular distortion measurements showed that the effect was greatest on thin sheet,
Distortion on circular section specimens was greatest where the slope out rate was lowest, and least where slope out rate was highest, which might counterbalance metallurgical considerations,
2.5.8 Precipitation hardening stainless steels
In ^ inch thick materials which contained up to 15,5%Cr, 7%Ni and 14%Cr, 8%Ni, fatigue tests (ref, 304) showed that vacuum melted material in 14%Cr - 8%Ni - 0.08%C (PH 14 - 8 - Mo) gave a 30% increase
in life over air melted steel of similar specification, and also increased life over the 14%Cr - 7%Ni - 0.08%C (PH 1 5 - 7 -Mo) manganese bearing material. Specimens were austenitized after welding, then the martensite
transformation obtained by cooling to 38 C and holding for 8 hours before final precipitation hardening. The electron beam welds showed significantly reduced fatigue properties compared with the parent material, and no
endurance limit was reached at 1.5 x 10 cycles. Small variations in hardness in the fusion zone were believed to be connected with premature fatigue failure. It was concluded that fatigue cracks could nucleate and propagate in the softer regions at lower stress levels, thus causing premature failure. Fatigue failure initiated at the junction of the fusion line, and often propagated across the fusion zone. After welding and before heat treat-ment a fairly well defined line of dendrite termination on the weld
centre-line was evident which became less distinct on heat treatment. In McHenry's work (refs. 180, 181) on medium carbon low alloy steels he
attributed fatigue failure to a similar solidification pattern and fatigue cracks initiated and propagated down the weld centre line. Altering the pattern increased fatigue life in that instance, but the initiation and propagation of fatigue cracks in the high chromium martensitic steel was not on the weld centre line and heat treatment gave no improvement in fatigue life,
In the U,S,S.R., Yuschenko (ref. 322) in welding 14%Cr - 5%Ni, and 15%Cr - 5%Ni steels, found that both materials could be welded after strengthening by heat treatment, giving satisfactory strength and ductility without subsequent post-weld heat treatment. Austenite and martensite were found in the weld structure. He reasoned that only a portion of the
austenite in the weld metal was transformed to martensite due to residual stresses, low structural stability and high cooling rate. The volume of weld metal was small and the yield stress of the weld metal lower than that of the parent material, so the martensite transformation was the first to be completed on subsequent application of loads. Because of the formation of deformational martensite the weld metal became strong.
2.5.9 High chromium martensitic steels
Few references are found of investigations into the weldability and material properties of chromium martensitic steels, which generally contain between 12% and 15.5%Cr. Acceptable welds in annealed material were readily obtained in thicknesses ranging from j inch to 4.0 inches
(ref. 220). In the upper thickness range tested, the material contained 13%Cr. and only 0.6% Ni. Care was needed in determining beam parameters when welding all thicknesses since the parameters which produced good weld cross-sectional profile and sound weld structure, gave root defects. Steps taken
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-to minimize root defects tended -to produce severe shrinkage voids in the main body of the weld. The maximum beam intensity needed to be below
the weld halfdepth to avoid the creation of a bottle shaped weld profile which was susceptible to severe shrinkage voids.
2.6 Graphite
Only two references (refs. 246, 308) describe the welding of this material.
2.7 Magnesium
The fully weldable, heat treatable magnesium casting alloy
QE22A has been electron beam welded to determine the effect of welding on the fully heat treated properties (ref. 54). Cast and extruded stock heat
treated both before and after welding were investigated. Although results were somewhat variable, it was found possible to retain full static room temperature strength properties of the alloy heat treated after welding, compared with T.I.G. welding where there was a reversion to an 'as cast' structure. Full post weld heat treatment was found in some cases to be slightly detrimental to static strength properties.
2.8 Molybdenum
Molybdenum sheet containing 0.5% titanium and up to 2mm thick could be easily welded by both high and low voltage electron beams, giving defect free welds. Low voltage systems appeared to give the best weld profile (ref. 258), and the wider resultant welds according to some workers (ref. 312) gave the same strength as stock plate with higher impact
resistance, whilst the very narrow welds gave lower tensile strengths due to metallurgical notches. Other workers (ref. 225) using high voltage equipment found the weld tensile strength to be 50% that of the parent material and 20% greater than T.I.G, welds. Even the very narrow electron beam welds
gave a cast weld metal structure and some recrystallization in the parent metal, but the grain size was much smaller than found in T,I.G. welding, The effect of cold working on the parent material was lost in any welding process, and the ductility of recrystallized molybdenum was zero.
Electron beam welding had the advantage over T.I.G, welding in that there was less risk of contamination with oxygen and nitrogen, both of which raise the transition temperature to well above room temperature,
2.9 Nickel alloys
The nickel alloys which have been electron beam welded fall into two groups, (1) Aging and (2) Non-aging, the former group being by far the larger. Most nickel alloys welded are for service at high temperatures, and both static and fatigue tests at elevated temperatures and after
prolonged "soaking" at elevated temperatures have been made (ref. 71) in a typical precipitation hardenable alloy, containing 19%Cr 14%Co 4,3%Mo -3%Ti and 1,3%A1, Wrought bar in the solution treated condition was welded and subsequently full solution and precipitation heat treated before
testing. Mechanical testing included tensile tests at room temperature and at 427 C, creep tests at 640 C and low and high cycle fatigue tests at
427 C, The possibility of metallurgical instabilities in the joint area was investigated by tensile testing after 1000 hours at elevated temperature under stress. With proper joint design and metallurgical process control, joints that approached or attained base metal integrity were possible in all respects after the removal of the top and bottom beads. Significant reductions in ultrasonic signal resolution were noted due to the high internal scattering effect of the precipitation hardening.
Specimens in i inch thick Nimonic 80A (20%Cr 2.0%Co
-2,0%Ti - 5%Fe - 1%A1) when electron beam welded and subsequently solution treated and aged all failed in the parent material under static tensile load-ing (ref, 80), Similarly treated specimens subjected to cyclic loadload-ing showed only slightly reduced fatigue life compared with the parent
material. Fissuring which is normally found on welding this material can be avoided with careful choice of parameters and a single pass weld.
Rene 41 sheet (19,4%Cr - ll,0%Co - 10.0%Mo - 3.0%Ti - 1.5%A1), 0.062 inch thick, welded in the annealed condition and subjected to various heat treatments after welding (refs. 255, 317) also gave mechanical
properties similar to parent metal values, and were superior when compared with T.I.G, welding,
A lb% chromium bearing alloy containing 5,5% tungsten welded to a 20% chromium alloy(ref, 261) showed that the boron content of either
material needed to be kept below 0,03% to avoid cracking. In material containing above 0,03% boron the joint could be effected by shimming with 0,05 to 1mm thick titanium or steel with little reduction in joint
mechanical properties. It was also found that gaps in the butt joints also increased the tendency to cracking.
Inconel X-750 containing 15%Cr - 2.5%Ti - 6.7%Fe - 0.8%A1 and 0.8%Nb is also an age-hardening alloy, and various process cycles which included the welding stage have been tried to determine the optimum sequence on 2mm thick sheet (ref. 77). The sequence: solution treat, weld, then age produced as good results as the sequence, weld, solution treat and age. The best treatment from a manufacturing viewpoint is solution treat
and age before welding, but this causes a decrease in static strength of the weld zone, but by using high welding speeds (6m/min) with this sequence, the weld zone can be kept so narrow that the decrease in static strength compared to the sequence, solution treat, weld, then age is reduced to 6.5%.
The non-age hardening alloy Inconel 600 containing 15%Cr and
7.2%Fe is readily welded in the thickness range of 2mm to 20mra (ref. 56, 77) and it was found that material properties in static tensile tests could reach values similar to those for the parent material with careful choice of
welding parameters. 2.10 Niobium
Most work in electron beam welding niobium has been done on sheet thickness up to 0.06 inch but a limited amount of work is reported on
i inch thick material (ref. 61). A wide variety of alloys has been welded (refs. 61, 89, 90, 141, 163, 258, 321), with minor additions of carbon, titanium, vanadium, zirconium, and hafnium, whilst some alloys were yttrium modified. The general results show that whilst almost all alloys were
weldable without porosity and any obvious cracking, some alloys were possibly prone to micro-cracking although this was not confirmed (ref. 163). In common with most refractory metals and alloys the ductile/brittle transition temperature was raised by welding in almost every case. The lowest
transition temperature was found (ref. 61) in C-129Y alloy (10%W - 10%Hf + Y ) , which was ductile at -73 C. The transition temperatures were invariably lower for electron beam welds than for T.I.G. welds. Tensile strengths at room temperature and at 1205 C were often comparable to those of the wrought material, but lower values were found (ref. 90) for FS85 alloy (27%Ta
-12%W - 0.6%Zr). In most cases overaging at 1205°C - 1316°C lowered the transition temperature for the weld metal, but post-weld aging for between 72 and 100 hours at temperatures ranging between 805 and 1000 C depending on alloy composition showed a reduction in ductility and embrittlement in bend tests. In investigating the structural stability of two niobium based alloys D-43 (10%W - l%Zr - 0.1%C) and B-66 (5%Mo - 5%V - l%Zr),some workers
12
-welded were related to the dendritic spacings and as the spacings increased the bend transition temperature increased also. Satisfactory ductility at room temperature could be obtained by the smallest dendritic spacing
commensurate with sound crack-free welds. As the spacings increased the rate of age embrittlement and subsequent overaging decreased. It was considered important to maintain a balance in the strengths of the matrix and grain boundaries (ref. 163), where it was seen that alloys with large weld grains (solid solution alloys), low recrystallization temperatures and weak grain boundaries (yttrium modified) had the poorest tensile fracture
characteristics, 2.11 Tantalum
Tantalum alloys have been reported to be easier than niobium alloys to electron beam weld and no special procedures need be adopted (ref, 163). Sheet material up to 0,06 inch thick has been the main subject of study in several alloys and most remarks referring to the transition temperature change and tensile strength of welded niobium sheet apply to tantalum alloy sheet except for the fact that T-111 (8%W - 2%Hf) alloy was ductile down to -196 C (ref. 61). The bend tests which revealed embrittlement in niobium alloys when aged for 100 hours at 805 C to 1000 C showed no such embrittlement for tantalum. Interface porosity of sintered tantalum at commercial purity level was compared with that for vacuum-arc cast and electron beam melted materials (ref, 38) for both electron beam and T.I.G. welds. Porosity was found at the weld fusion zone/base material interface and appeared to be independent of the mean interstitial gas concentration but not independent of the consolidation process in sintered material. Fusion cast tantalum did not appear to be susceptible to this defect. The characteristic features of this porosity suggest that the formation mechanism is one of heterogeneous bubble nucleation of a hydrogen base gas on an unidentified nucleant. The nucleant is apparently soluble or decomposable in sufficiently superheated tantalum, as is the case in both arc casting and electron beam melting, In the sintering process the temperature never exceeds 2400 C (600 C below the melting point), and when welding in a rapid-chill fixture the fusion edge might be superheated only slightly above melting point. Consequently the tantalum is never sufficiently superheated to render any unidentified impurity ineffectual as a gas bubble nucleant,
2.12 Titanium and titanium alloys
Most titanium alloys have proved suitable for electron beam welding and subsequent static loading. The 6%A1 - 6%V - 2%Sn 6 alloy has proved the most difficult to weld (refs. 254, 316),
The alpha alloys 2i%Al - 16%V and 5%A1 - 2j%Sn up to i inch thick welded in the annealed condition and tested without post-weld heat treatment
(ref. 312) showed tensile strengths comparable to the parent material. Fusion zone strengths were shown to be unrelated, over wide limits, to interstitial content, and were essentially independent of joint width.
These strengths were found to be inversely proportional to grain size. _, Weld mechanical properties did not vary with chamber pressures between 10 torr and 10 torr, or with surface condition prior to welding.
More work has been done on electron bean welding the 6%A1-4%V
age hardening alpha-beta alloy than on any other titanium alloy (refs. 312, 316). Electron beam and T.I.G. welds in J inch thick material stress relieved after
welding showed that tensile failures generally occurred in the base metal for the electron beam welds, but that T.I.G. welds failed in the weld or H.A.Z. at joint efficiencies and elongation well below parent material values, probably due to the greater amount of porosity in the T.I.G. welds. For
1.0 inch thick material, tensile strengths were comparable to the parent material properties for both processes, although elongation was reduced
in the T.I.G. welds. Fracture toughness in 1.0 inch thick and 1.75 inch thick 6%A1 - 4%V plate, for various cycles of operation included:
1. Solution and age before welding, and then testing as welded,
2. Solution, age, then weld and age, 3. Anneal, weld, solution, age, 4. Anneal, weld and test as welded, 5. Solution, weld, solution, age;
all gave energy values for both weld and H.A.Z. metal above those for the parent material. Stress rupture at 455°C showed that annealing prior to welding followed by complete solution and ageing treatment gave almost 100% efficiency. Hardness values in 1^ inch thick 6%A1 - 4%V plate showed that beam deflection could be beneficial in reducing H.A.Z. hardness in the middle and bottom sections of the plate. The observations on the fusion zone widths and grain size in welding i inch thick alpha alloys were also applied
to the alpha-beta alloys 6%A1 - 4%V and 4%A1 - 3%Mo - 1%V in the same thickness (ref. 312).
Although the weld profile of 6%A1 - 6%V - 2%Sn beta alloy was deceptively good,the solidification pattern at the weld centre line gave a plane of weakness. Low ductility and poor fatigue properties limit the material's use,especially in thin gauges. Weld line micro-porosity was thought to be the cause of the low material properties and extreme joint cleanliness and multipass welding gave some improvement. The 13%V -ll%Cr - 3%A1 i inch thick beta alloy, (refs, 66, 312, 316) welded in the solution treated condition showed yield strengths comparable to that of
the parent material, with only a slight drop in ultimate tensile strength and elongation in the aged condition. Other work (ref. 225) on all beta alloys
i inch thick showed joint efficiencies of 60% for welds in the cold rolled and aged sheet, and 75% for solution treated material aged after welding. Impact resistance was superior to the base material, and increased with weld width. In 2i inch thick beta processed plate (ref. 316), 100% joint
efficiency was found possible with a slight drop in elongation. The fatigue performance of 2mm thick sheet specimens in
representative materials from each group (a, a-g, and 6 ) , (ref.66) with surface profiles intact was found to be generally disappointing and compared with T.I.G. welded seams. Fractures originated from notches or from weld
spatter defects in the weld root. It was found that fatigue life could be marginally improved by shot peening the weld surface, but fracture then
originated at internal pores. However, the fatigue performance of
6%A1 - 4%V alpha-beta alloy, welded and machined on both faces, was similar to that of the parent material for electron beam welds, although T.I.G. welds gave only 50% parent material life (ref. 316). Electron beam welds machined on one face only had a fatigue life similar to fully machined T.I.G. welds.
2.13 Tungsten
Unalloyed tungsten has been welded by the electron beam process, usually in thicknesses up to i inch (refs. 133, 164, 312, 258) and its
weldability is considered marginal due to the high ductile/brittle transition temperature in the welded or recrystallized condition. The material is susceptible to thermal shock during welding. Very thin fusion zones caused metallurgical notches (ref. 312) and although wider welds gave higher strengths,
14
-impact resistance. The weldability of tungsten alloyed with 25% Re was improved over that of unalloyed material, but had a tendency to hot tearing (ref. 164), Post weld stress relief produced some improvement in the ductile/brittle transition temperature. Tungsten alloyed with 25% Re- 30% Mo showed excellent weldability but was extremely sensitive to oxygen contamination.
2.14 Uranium
Only one reference (ref. 348) reports results on the welding of uranium,
2.15 Zirconium
Extra high purity zirconium, cold rolled with 75% reduction in thickness down to l-2mm thick sheet has been electron beam welded with a lOKV accelerating voltage (ref. 324). Good joints, free from porosity and cracking were obtained with a joint strength equivalent to the annealed metal, and bend ductility equivalent to the rolled and also the annealed sheet.
The main interest lay in the corrosion resistance after welding and it was found that although the weld and H.A.Z. have identical structures, the high corrosion resistance of the weld metal to 20% HCl solution at 175 C was not quite matched in the H.A.Z. The high purity zirconium was not sensitive to intergranular corrosion when subjected to 20% HCl solution at 200 C for 400 hours, but low purity zirconium welds corroded at 115 C in the same solution.
2,16 Dissimilar metal combinations
There is no special difficulty in welding dissimilar metals by the electron beam process if they form a continuous series of solid solutions, or if they have limited solubility and do not form chemical compounds. If the metals have limited solubility and do form chemical compounds (e,g, titanium-aluminium, copper-steel, niobium-steel or nickel alloys), the welding should be carried out with preferential melting of one of the parent metals, or with the use of intermediate metals which make good welds with each of tiie parent metals,
Many metal combinations have been successfully welded using these principles (refs, 91, 148, 215, 282, 313, 346), For example in welding copper to molybdenum or tantalum the copper is melted preferentially and the wettability of the surfaces of the higher melting point materials was found to be good. The two phases were not soluble one in the other and sound joints were formed. In tensile tests of the Cu/Ta joint, failure occurred in the copper and no pores or cracking of the joint face was observed. Failure in tension and bend tests of the Cu/Mo joint occurred in the molybdenum hear the joint. Copper to 18/8 stainless steel joints were also made by preferential melting of the copper, again giving good wetting of the steel with no cracks or pores. Sound butt and lap joints were made by both slowly and rapidly heating the copper to melting point. Bend test angles reached 180 showing no cracks. An interlayer of a solution of copper in steel about 0,02mm wide was seen at the joint. The fact that the copper does not
penetrate the steel along the grain boundaries with the formation of microcracks during surfacing or welding in high vacuum, unlike that which occurs in other
surfacing, manual arc and automatic welding processes may be due to the influence of adsorption effects on variations in the free surface energy or the steel, which could reduce the number of defects in the steel surface,
The joining of ferritic to austenitic stainless steels is sometimes necessary in high pressure, and high temperature applications. It was found (ref. 215) that using Inconel 600 filler rings in i inch thick tubular joint faces gave no satisfactory welds using electron beam welding, but under certain circumstances sound joints could be made by friction and T.I.G. welding. Low voltage electron beam welds using multiruns, joint preparation and filler wire were made successfully using Inconel 921 filler (15%Cr - 2,5%Ti), with no cracks in the weld metal or H.A.Z. The use of Inconel 821 filler wire (20%Cr - 0.75%Ti - 2.5%Nb) gave joints which showed transverse weld metal cracks. Thicker material than i inch was
16 -3. PRODUCTION APPLICATIONS
3.1 Miniature Components
There are very many examples of components electron beam welded in instrument and electronic engineering. In these fields the parts are often fabricated from very thin material requiring an accurate and very localized heat input to effect a joint, and in this respect alone electron beam welding shows an advantage over any other process. Where thin
materials only are to be welded, small power electron beam machines are adequate, and these are comparatively cheap. The fact that the welding must be carried out in vacuum is often an advantage also, since the vacuum is sometimes necess-ary as in bellows assemblies and hermetically sealed cans for electronic
components. The stress levels encountered in most joints in miniature components are usually very low, and because of the thin materials
involved, metallurgical problems of porosity and cracking are less severe than in thicker materials. It is consequently easier to make satisfactory joints in this category, and although a wide variety of such components are electron beam welded they have not been the subject of as much literary attention as more technically difficult problems in thicker materials of all sorts.
Attention to joint configuration is usually more important in thin material to ensure that the beam impinges truly on the joint face, and in many instances an interference fit is necessary, (refs. 86, 252, 284, 295,
298, 3o8, 369, 370).
3.2 Nuclear engineering - fuel cans and reactor elements - isotope encapsulation
The nuclear engineering field was one of the first users of electron beam welding in the fabrication of fuel cans and associated parts made from the more exotic materials (152, 170, 257, 327), More recently
isotope encapsulation has been performed both by T.I,G, and electron beam welding, (refs, 36, 95, 137) but where materials up to { inch thick, such as hasteloy C high temperature alloys are needed for undersea operation, electron beam welding has several advantages including the amenity for remote ultrasonic inspection,
3.3 Aeronautical engineering
In aircraft structures, components have been electron beam welded in a variety of materials including aluminium alloys, titanium alloys and high strength steels, as well as very thin materials for honeycomb panels
(ref. 43). Very large weldments (up to 70ft. long) have been made (ref, 29) using specially adapted machines and sliding seals. Swing-wing pivot joints and undercarriage components are fabricated from maraging steel parts with post weld heat treatment.
In titanium alloys the best example of electron beam welding is the fabrication of large helicopter rotor hubs in material two inches thick, (ref. 177), but other smaller components in titanium are also electron beam welded since fabrication from simple shapes in this expensive material shows a marked saving over machining from forgings or blocks (ref. 66).
Less attention has been paid in aircraft structures to the welding of aluminium alloys than to high strength ferrous materials and titanium alloys. In some cases, with full post-weld heat treatment, joint efficiencies in
will only partially restore the lost material properties in the high strength aluminium alloys so that welding is less attractive in these materials, although electron beam welding gives superior joints to T.I.G. welding. Where possible, joints are positioned so that top and bottom profiles can be machined away to give the best possible fatigue life in all materials welded.
Perhaps the most concentrated use of electron beam welding in any field is found in welding aero-engine parts. In this context turbine blades, rotor shafts, stator casings, gear assemblies and flame tube components have all been successfully fabricated (refs, 40, 132, 165, 226, 329, 335), in various materials. The ability to weld machine finished parts with little distortion is attractive for some parts such as turbine blade assemblies in some high temperature creep resistant nickel alloys, whilst the saving in machining time and post-weld heat treatments is useful in gear assemblies. Weld joint configuration in most instances must be carefully considered since good fatigue performance is essential and top and bottom beads are generally machined away or the material locally thickened to reduce the stress level at the joint. As the need for higher temperature operation to improve
engine performance and efficiency has increased, the search for materials which show good mechanical strength and good creep resistant properties at higher temperatures has also increased. Consequently the welding of refractory alloys has been the subject of study in many more recent investigations, but there have been no reports of any production applications to date of aero-engine components electron beam welded in refractory alloys,
3.4 Spacecraft
Electron beam welding in space technology falls into two definite spheres:
1. The fabrication of spacecraft and equipment on earth for use in space, (refs. 65, 176, 294, 314, 360).
2. The development of welding techniques for the eventual fabrication of space stations and equipment beyond the earth's atmosphere,
(refs. 84, 136, 210).
The first category differs very little from that of conventional
aircraft structures except that high strength/weight ratio is even more important and weld integrity must be of the highest order. A typical example is the
welding of the Apollo lunar module, (ref, 176).
In the second category electron beam welding has been studied along with other joining processes and special mobile guns have been developed for use in space, but so far only reportedly used under simulated conditions on earth. It is possible that a Russian Soyuz flight experimented with welding in some forms in space but no reports on the use of electron beams were made.
3.5 General light and medium engineering
Electron beam welding in light engineering finds its best
representative in the automobile industry, particularly in the U,S.A. where very high volume production shows the process to its best advantage. The complexity of the modern road vehicle in its various forms necessitates many fabricated components, giving scope for the utilization of almost every fab-rication process. Typical examples (refs, 341, 338) are:- flywheels
18
-and counter balance weights are fitted in one operation, collapsible
steering columns produced at 1800 per hour where a smooth inner surface is required (ref. 340), ball joints produced at the rate of 600 per hour in a non-vacuum machine, and the integrated production of road wheels from coiled steel strip. Many other smaller components such as gear clusters,
distribution cams and brake bands are assembled using electron beam welding, (ref. 256, 336). Outside the automobile industry only a few reports deal with the specific production applications of electron beam welding, among which are the manufacture of bi-metallic handsaw blades (ref. 342), circular saw tooth segments (ref. 300) and heat exchangers (ref. 121). Almost all applications of the process are in components where the joining of thick to thin sections, dissimilar metals or complex to plain shapes is required, where the minimum heat input can be tolerated to ensure very limited distortion or shrinkage, and the need for post-weld heat treatment often eliminated. Usually the stress-levels at the joints are low so that porosity and micro-cracking are of marginal significance.
A unique example in butt welding the ends of small tubes (up to 1.0 inch diameter, 1/16 inch thick) is seen in the use of glow discharge welding with a shaped cathode, permitting the tube ends to be joined in a single shot weld (ref. 55). In this work further possible uses of shaped cathode glow discharge welding are considered, but no definitive use has been reported.
The most detailed information on actual production applications of electron beam welding is found in machine manufacturers advertising
literature and not in the more formal reports in technical publications. 3.6 Heavy engineering and pipework
Electron beam welding has not yet penetrated deeply into the field of heavy engineering where thick sections are involved, usually as part of large fabrications. Either very high power machines to allow single pass welds or lower power machines and multipass welds with filler are needed for sections of two inches and greater in steel, and both approaches are under development, (refs. 26, 118, 231). The multipass with filler approach offers several advantages in that the components can be made with a narrow gap where abuttment is made only ^ inch deep at the root. Fusing through this ^ inch material gives good control of root profile and the material
then added in subsequent passes to fill the narrow gap can be modified to give the required metallurgical properties. Also in adding filler the cooling rates are reduced and weld metal hardness can be lowered. The latter attributes give distinct advantages over very deep single pass welds since the usual method of pre-heating to reduce cooling rates will probably not be possible on very large structures under the conditions of vacuum needed for electron beam welding, (ref, 118). Both systems will also require the devel-opment of very large vacuum chambers or local pressure sealing on the
component to be welded. Smaller components used in heavy engineering which can conveniently be welded in existing vacuum chambers and with guns of up to 25KW beam power are typified by reformer tubes in | inch thick 310
stainless steel (ref. 27), for which special seals were devised to enable 40 ft. lengths to be made with end fittings in 2i Cr - 1 Mo, and EN58B steels Ij
inches thick. Other examples (ref. 81) are 3 ft. long turbine blades fitted with erosion shields, high pressure water boxes in 13% chromium - martensitic steel and large camshafts.
Pipeline welding carries the inherent problem of moving the weld-ing head around the pipe to join new sections vertical to the lengths already laid, thereby involving welding in the overhead and downhand positions. A mobile electron beam welding machine has been developed (refs, 27, 339) to do this and test welds have been made using X-60 gas pipeline material 0.312 inch thick. In this machine a vacuum chamber is clamped around the pipes and special plugs inserted into the bores to provide vacuum seals. The gun runs around the inside of the vacuum chamber and the whole assembly is supported by the power supply vehicle. No reports are available on actual service use of the system.
It is doubtful if out-of vacuum electron beam welding (ref, 323) will shortly, if ever, challenge more conventional methods of welding pipelines or large structures such as pressure vessels,
particularly where wall thicknesses are above i inch. High accelerating voltages and beam powers would be needed to compensate for gas scattering of
the beam, and relatively wide welds would result. Radiation effects would be more pronounced with the higher voltages and powers and protection more difficult with no surrounding vacuum chamber. One of the main advantages of electron beam welding, namely the high purity welds due to the absence of atmospheric contamination, would be removed with the introduction of shielding gas.
