Typical Applications of Electron Beam Welding

Electron beam welding is one of the best fusion welding methods for metal materials. Various metals, alloys, intermetallic compounds, etc., can be welded by electron beam welding, and the joint has good mechanical properties.

Steel’s Electron Beam Welding

Welding of Non-alloy Steel (Carbon Steel)

Low carbon steel is easy to weld. Compared with arc welding, electron beam welding results in finer grains in the weld and heat-affected zone. Low carbon boiling steel, due to incomplete deoxidation, may produce intense molten pool reactions during welding, causing spatter and porosity in the weld. Therefore, during welding, a 0.2~0.3mm thick aluminum foil can be clamped at the joint gap to ensure deoxidation.

Sometimes porosity may also occur during the welding of semi-killed steel. Reducing welding speed and widening the molten pool helps to eliminate porosity. Medium carbon steel can also be welded by electron beam welding, but its weldability worsens as the carbon content increases.

When electron beam welding carbon steel with a carbon mass fraction greater than 0.5%, the tendency to crack is lower than when arc welding, but preheating before welding and post-weld heat treatment are required.

Welding of Alloy Steel

When electron beam welding low alloy steel with a carbon mass fraction less than 0.3%, preheating and post-weld heat treatment are not necessary. For workpieces with large thickness and strong structural rigidity, preheating is required to prevent cracking, and the preheating temperature is 250~300℃.

For parts that have undergone quenching and tempering before welding, the post-weld tempering temperature should be slightly lower than the original tempering temperature. For example, gears of light gearboxes are mostly welded by electron beam welding, and the gear material is 20CrMnTi or 16CrMn, the material is in the annealed state before welding, and quenching and surface carburizing treatment is carried out after welding.

High-strength alloy steel with a carbon mass fraction greater than 0.3% can be welded by electron beam, and the weldability is better under annealed or normalized conditions. When the plate thickness is greater than 6mm, preheating before welding and slow cooling after welding should be adopted to avoid cracking.

Welding of Tool Steel

When electron beam welding tool steel, the joint has good performance and high productivity. Compared with other welding methods, tool steel electron beam welding can achieve high-speed welding without annealing and other heat treatments.

For example, the hardness of 4Cr5MoSiV steel with a thickness of 6mm is 50HRC before welding, and after 550℃ normalizing after welding, the hardness of the weld metal can reach 56~57HRC, the hardness of the heat-affected zone drops to 43~46HRC, but its width is only 0.13mm.

Welding of Stainless Steel

Austenitic stainless steel electron beam welding can have higher resistance to intergranular corrosion, as high cooling rates help to suppress carbide precipitation. For martensitic stainless steel, electron beam welding can be performed in any heat treatment state, but the joint area will produce quenched martensite after welding, increasing crack sensitivity.

As the carbon content increases and the cooling rate increases, the hardness and crack sensitivity of martensite also increase. Precipitation hardening stainless steel using electron beam welding can achieve good mechanical properties. The weldability of precipitation-hardening stainless steel with high phosphorus content is poor.

Electron Beam Welding of Non-ferrous Metals and Refractory Metals

Welding of Aluminum and Aluminum Alloys

If pure aluminum and non-heat-treatable aluminum alloys are welded by electron beam welding, the mechanical properties of the joint are close to that of the base material. Appropriate heat treatment can reduce defects and ensure that there is no annealing softening zone in the joint. When electron beam welding aluminum alloys containing more strengthening elements—magnesium and zinc, the choice of welding speed is important.

Too slow a speed will cause a large amount of evaporation of magnesium and zinc; if the welding speed is increased, the weld formation will deteriorate and serious porosity will occur. Zinc-free aluminum alloys should be welded at high speed with high voltage and small beam current.

Before electron beam welding of aluminum and its alloys, the joint area needs to be degreased and the oxide film removed. Control the welding speed during the welding process to prevent porosity and improve weld formation. For aluminum plates with a thickness less than 40mm, the welding speed should be 60~120cm/min; for thick aluminum plates above 40mm, the welding speed should be below 60cm/min.

Aluminum alloys are commonly used to manufacture automotive parts. Non-vacuum electron beam welding of aluminum alloys for automobiles can achieve good joints.

As early as the 1960s, the United States introduced non-vacuum electron beam welding into the mass production of automotive parts, which can reduce costs, improve efficiency, achieve continuous welding on the automotive production line, and at the same time, reduce structural weight, save fuel, and reduce exhaust emissions.

Welding of Titanium and its Alloys

Titanium alloys, with high strength-to-weight ratio, excellent corrosion resistance, and a wide operating temperature range, are widely used in aerospace engine structures. Titanium alloys have strong chemical activity, high melting point, and poor thermal conductivity, making it difficult to obtain quality joints with conventional welding methods, and electron beam welding is the ideal welding method for all industrial titanium and its alloys.

Electron beam welding effectively avoids contamination by harmful gases, and the high energy density of the electron beam and high welding speed prevent the appearance of coarse plate-like α phase in the weld, thus the effective coefficient of the welding joint can reach 100%. During welding, high voltage and small beam current welding parameters should be adopted to prevent grain enlargement.

On aircraft components, there are many instances of electron beam welding of titanium alloys. For example, in order to reduce the weight of the engine, the new engine fan case is often made of titanium alloys, and the fan case outer ring and stator blades are electron beam welded, simplifying the manufacturing process.

Electron beam welding is carried out in a vacuum, completely avoiding the oxidation problem of titanium alloys when welding in the atmosphere; the small heat input and small part deformation of electron beam welding can complete the welding in one time with CNC programming, with high productivity and good welding quality.

Welding of Copper and its Alloys

Electron beam welding is one of the ideal methods for pure copper welding. Under vacuum conditions, pure copper evaporates quite severely when heated, so the energy density of the electron beam should not be too high to prevent excessive evaporation and spattering of pure copper, which weakens the cross-sectional strength of the weld.

Due to the good thermal conductivity of pure copper, the heat from the welding heat source is easily dissipated, so the electron beam power required for welding is larger than that for alloy steel welding. For a 40mm thick copper plate, the heat input required for electron beam welding is 1/7~1/5 of the heat input for submerged arc welding, and the cross-sectional area of the weld is 1/30~1/25 of that for submerged arc welding.

The main defect of copper and its alloys during electron beam welding is porosity. Measures such as increasing assembly gap, preheating before welding, and repeated welding can be used to prevent the occurrence of porosity. Although reducing the welding speed can also prevent porosity, too slow a welding speed will result in poor weld formation and increased voids.

In recent years, the British Welding Institute has used non-vacuum electron beam welding to weld copper nuclear waste containers, achieving good social and economic benefits.

Electron Beam Welding of Refractory Metals

For refractory metals such as molybdenum, tungsten, niobium, and zirconium with melting points above 2000℃, electron beam welding is a relatively ideal welding method, as high power density can use smaller heat input to obtain joints with good performance.

The difficulty in welding molybdenum and tungsten is not only due to their high melting points, but also because melting and recrystallization raise the ductile-brittle transition temperature of these two metals above room temperature. However, during electron beam welding, the short high-temperature dwell time can minimize the effects of grain growth and other reactions that can raise the transition temperature.

When the thickness of the weldment is large and the restraint is intensified, the design can be changed to a flanged joint or a flanged joint to reduce restraint, and a stress relief groove can be set in the near-seam area to adjust its elastoplasticity. In addition, preheating the welding area can reduce the sensitivity to cracking.

Common defects when welding molybdenum are porosity and cracks. Careful cleaning of the weld before welding and preheating are beneficial for eliminating porosity. Adding aluminum, titanium, zirconium, hafnium, thorium, carbon, boron, yttrium, or lanthanum to molybdenum alloys can neutralize the harmful effects of oxygen, nitrogen, and carbon, and improve the toughness of the weld.

When the welding speed is 50~67cm/min, each 1mm thickness of molybdenum requires 1~2kW electron beam power. Tungsten alloys are well adapted to electron beam welding. Preparing and cleaning the joint during welding is very important, and degassing treatment should be carried out after cleaning. Preheating is an effective measure to prevent cold cracks in tungsten joints.

Post-weld annealing can lower the brittle transition temperature of some tungsten alloy welded joints, but it cannot improve the cold brittleness of pure tungsten weld metal. Common defects in niobium alloy welds are porosity and cracks. Preheating the weld with a defocused electron beam at a high vacuum of 1.33×10-2Pa has a cleaning and degassing effect, which is beneficial for eliminating porosity.

Zirconium is very active, and the preparation and cleaning of its joints are crucial to welding quality. Welding should be carried out in a high vacuum of 1.33×10-2Pa or above. Post-weld annealing can improve the joint’s resistance to cold cracking and creep rupture. The annealing conditions are to keep the temperature at 750~850℃ for 1h and cool with the furnace. The heat input used for welding zirconium is similar to that of steel of the same thickness.

Electron Beam Welding of Dissimilar Metals

The adaptability of dissimilar metals to electron beam welding depends on their physical and chemical properties. Dissimilar metals that can form solid solutions have good weldability, while those that easily form intermetallic compounds have poor joint toughness.

However, compared with other fusion welding, it is easier to weld because the high power density of the electron beam can effectively adjust the heat input and precisely control the heating range, thus avoiding the difficulties in welding caused by the poor brittleness of intermetallic compounds and ensuring the compactness and certain mechanical properties of the joint.

With the progress of material science, intermetallic compounds such as Ti3Al and Ni3Al are increasingly used. When electron beam welding two metals that are not soluble in each other, a transition metal that is miscible with both metals can be embedded or preset. During welding, the welding heat input must be strictly controlled, a higher welding speed should be adopted to avoid welding cracks and joint brittleness.

The control of impurities in the material smelting and casting process has a great impact on the welding quality, so the material composition and mechanical properties should be rechecked before welding.

In the electronics and instrument industry, many parts are made of special materials with complex and compact structures, special technical requirements, such as the need to form a vacuum cavity after welding, not to damage temperature-sensitive components, or require precision welding manufacturing.

At this time, vacuum electron beam welding becomes the preferred method. For example, tube strain gauge sensors require the tube to be filled with strain wires and MgO insulating powder. The weld is semi-penetrating, with small deformation. With strict electron beam welding process and suitable tooling, satisfactory welding quality can be achieved, meeting the technical requirements of the product.

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