Vacuum Electron Beam Welding of Rear Axle Shell Axle of Large Passenger Car

The rear axle housings of large passenger cars are generally produced by forging-welding process structures. However, due to the poor welding of materials, especially the middle section of the axle housing and the shaft head are dissimilar steel welding, it is difficult to obtain satisfactory joints by ordinary arc welding methods. quality. The vacuum electron beam welding is characterized by high energy density, strong penetration ability, large weld-width ratio, small weld deformation and high precision, so the vacuum electron beam welding method can obtain satisfactory quality results. The following describes the welding process of the middle section of the rear axle housing and the axle head with the rear axle housing half shaft of a luxury passenger vehicle as an example.

2 Technical test and analysis 2.1 Test conditions As shown, the 16mm-thick large passenger car rear axle axle assembly welding structure. The middle section of the axle housing is made of 16Mn steel; the material of the shaft head is 35CrMoA steel, and the welding state is the quenched and tempered state; the inner lining ring material is 20 steel, which connects the 16Mn steel and the 35CrMoA steel and plays a transitional role to the welding of dissimilar materials. Reduce weld metal sinking and undercuts. The welding method is that no filler material is used, and the welding structure of the axle shaft of the rear axle housing of the parent material passenger vehicle is melted to form a weld seam, and no heat treatment is required after welding.

The process test was carried out on a vacuum electron beam welding machine EBW15/60-701 produced by a German company. The accelerating voltage of the welding machine was 60 kV, the volume of the welding chamber was 701 L, and the maximum welding speed was 50 mm/s. 2.2 Weldability analysis heterogeneous Steel welding, their chemical composition and mechanical properties are shown in Table 1. Dissimilar steel melting welding, in addition to the physical and chemical properties of the metal on the welding effect, the difference in the performance of the two materials will affect the weldability between them to a greater extent, that is, when the 16Mn steel and 35CrMoA steel welding two materials It will certainly produce a transition layer with different chemical composition, organization and performance than the parent material.

Table 1 Chemical composition (%), mechanical properties and heat treatment status of 1 "Mn steel and 35CrMoA steel Grade No. heat treatment state Hot rolling tempering According to the International Welding Society recommended carbon equivalent formula: 16Mn steel carbon equivalent CE quenching tends to be large, point low In the heat-affected zone, it is easy to form a large amount of brittle hard martensite structure, which has a greater sensitivity to cold cracks.35CrMoA steel is also due to its high content of carbon and alloying elements, the weld will easily segregate and cause solidification cracks when solidified. Therefore, it has a large sensitivity to welding hot cracks.In addition, because the 35CrMoA steel is welded in the quenched and tempered state, the reduction in strength caused by the softening of the high temperature tempering zone in the heat affected zone is also considered.

The method of preheating before welding and slow cooling after welding, ie preheating before putting in the oven for preheating 300 grain refinement can improve cold crack resistance and solidification cracking ability because the finer the grain, the strength of the weld. The better the plasticity and toughness, the higher the critical stress for cold cracking. At the same time, the grain size is small, the number of grain boundaries increases, the degree of segregation decreases, and the sensitivity of solidification cracks decreases. The use of heat concentration, energy density and small line energy welding process can reduce the degree of softening.

Because the electron beam welding of the middle section of the axle housing and the shaft head is a later process of the axle housing assembly, the welding deformation is required to be small and the precision is high. Therefore, the two sides of the welding gap should be as parallel as possible, and the splashing range of the metal should be the smallest. It should be accurately positioned to ensure that the part does not bend when it cools.

2.3 Welding process Vacuum electron beam welding is a high-power electron velocity stream formed by a high-pressure acceleration device. It is converged by a magnetic lens to obtain a very small focal point (its power density is 104~109W/cm2), and bombardment is performed in a vacuum chamber. In one piece, the kinetic energy of the electron quickly turns into heat energy, melting the metal to achieve the welding process. Since the focus is small and the electron velocity is high, the generated heat is quite concentrated and has a considerable penetrating power. Therefore, a weld with a relatively large depth and width of the weld seam and a narrow heat affected zone can be obtained. This is very beneficial to refine the weld grain, reduce welding distortion, and control the heat affected zone softening. On the other hand, when welding in a vacuum state, the influence of air on the weld seam can be basically prevented, in particular, hydrogen is prevented from entering the molten pool, and the hydrogen-induced cold cracking tendency is reduced.

Considering that the 35CrMoA steel has a high carbon equivalent and a large difference in the thermal capacity of the joint, a preheating method should be adopted. The preheating temperature is calculated to be 200 to 250. To avoid generating blowholes and reduce the hydrogen content of the weld, weld before welding. Strict cleaning at the place, sanding the oxide film and rust before sanding until the metal luster is exposed, and then using gasoline to remove oil stains. Finally, clean it with acetone and put it in an oven for preheating (200~250 3 test results will be The joints of the welded joints of the test block were cut open and no blowholes, slag inclusions or micro-cracks were found.No weld defects were found in the weld appearances of the welded rear axle housing, as well as ultrasonic and 100% X-ray inspections. After welding, the rear axle housing was loaded with 18.75L, 800,000 fatigue tests were repeated, and no fatigue cracks were found at the welding site. After a period of installation verification, the welded rear axle housing had high strength, stiffness, and toughness. long life.

4 Concluding remarks Through experiments and production, a set of reasonable welding procedures was determined. This shows that the rear axle housing of a large passenger car with vacuum electron beam welding is small in deformation, high in accuracy, and stable in quality. It can meet the requirements of heat treatment and straightness after welding. The need for service.