Research on Laser Welding Process of Automotive Aluminum Filter

【China Aluminum Industry Network】 Energy conservation, consumption reduction and environmental pollution reduction are the pressing issues faced by various countries in the world. To solve this problem, various light alloys (such as aluminum and magnesium alloys) are increasingly used in transportation vehicles. Among them, aluminum alloys have very good physical and mechanical mechanical properties, and they are lightweight. They have been widely used in the automotive industry, among which the filter is one of the more typical applications. Because the aluminum alloy has strong chemical reactivity, the surface is easily formed with an oxide film and has a refractory property. In addition, the aluminum alloy has a high thermal conductivity, and causes no fusion during welding; at the same time, the oxide film can absorb more moisture, and thus Cause the formation of pores in the weld; In addition, the aluminum alloy has a large coefficient of linear expansion, thermal conductivity, strong, easy to produce defects such as undercut, warpage and deformation during welding, and the mechanical properties of joints after welding are reduced. When using conventional argon arc welding (TIG) and inert gas melting-level arc (MIG) methods to weld aluminum alloy, problems such as blowholes, welding cracks, and welding distortion are easily generated, which restricts its application in industry. Compared with conventional welding methods, laser welding is a sophisticated welding method with many functions, strong adaptability, and high reliability, and it is easy to automate. Due to the high power density of the laser, the heat input during welding is low. On the basis of ensuring the penetration depth, the welding heat affected zone is small and the welding deformation is small. The laser welding does not require a vacuum device. Therefore, the laser welding has high quality, high precision and speed. High characteristics. At the same time, with the continuous development of high-power, high-performance laser processing equipment, aluminum alloy laser welding technology has been widely used in the automotive industry.

This article takes the automotive aluminum alloy filter as the research object, analyzes the technical points and related influencing factors of the aluminum alloy filter for automotive welding. The weld of the filter is a girth weld, and the joints are butt joints at the bottom of the lock. The appearance of the weld is uniform and beautiful. The weld width is 2mm or more and the weld depth is 1.5mm or more. Samples are shown in Figure 1.

Figure 1 Samples

1 Equipment, materials and methods

Equipment: Trumpf 3001 laser and welding head (optical configuration: focusing lens focal length 300mm, collimating lens 200mm, fiber core diameter 300μm), as shown in Figure 2;

Figure 2 Trumpf lasers and welding heads

Material: 6 series aluminum alloy;

Method: The laser welding head does not move in the fixed position, the workpiece rotates around the fixed axis to realize the girth welding, and the high purity Ar gas sideshaft protection is used in the welding process.

2 problems with the welding process

1. Problems caused by protection gas blowing: When the shielding gas is blown in the same direction as the rotation direction of the workpiece, that is, after the shielding gas is blown, the shielding gas cannot timely discharge the air at the weld to be welded during welding, which may lead to welding. During the process, the air is mixed in, which makes the weld easily oxidized. The surface of the weld after welding is black and poorly formed (as shown in Figure 3).

Fig. 3 The shape of the weld formed in the same direction as the direction of rotation of the shielding gas

2. The use of a small-diameter trachea results in a narrow protection range and excessive blowing of gas per unit area. For example, when a single copper tube with an inner diameter of 4 mm is used to protect the gas and the sample is placed vertically (as shown in Figure 4) Due to the greater fluidity of the liquid aluminum alloy, the aluminum alloy in the molten pool tends to flow downward in the direction of gravity under the influence of shielding gas blowing force and its own gravity, resulting in the weld seam after welding (as shown in Figure 5). . In addition, the gas blowing area of ​​the small-diameter copper tube is small, and the gas blowing force is large, which may also easily lead to unstable weld formation.

3, Impurity of shielding gas leads to local oxidation of the weld seam, the surface is yellow: Due to the more active chemical properties of the aluminum alloy, it is easy to oxidize at high temperatures, so when the aluminum alloy filter is welded, high purity argon is used to protect the gas (purity 99.99) %), using pure argon (purity 99.9%) protection, due to the intrusion of gas impurities during high-temperature welding, can also lead to local oxidation of the weld, and even poor welding, as shown in Figure 6.

Fig. 6 Bad weld caused by impure gas

4. Bad welding caused by mismatch of process parameters: Laser welding is divided into thermal conduction welding (power density between 105 W/cm2 and 106 W/cm2) and deep penetration welding (power density at 106 W) according to different penetration depths. / cm2 —— between 107 W/cm2). During the thermal conduction welding, the shallow metal is heated to melting by the downward heat conduction after the laser energy is absorbed by the surface. The formed weld is nearly semicircular and the weld has a shallow depth of penetration. . The appearance of small holes in the laser welding process can greatly increase the absorption rate of the laser material, and the small hole as a black body can make the weldment obtain more energy coupling, which is a prerequisite for good welding quality. Aluminum alloys have a very high initial reflectivity for lasers, with reflectivity of up to 96% for C02 laser beams and close to 80% for Nd:YAG laser beams. The thermal conductivity of aluminum alloy is about 3 times that of ordinary medium carbon steel at room temperature. Therefore, in the actual welding of aluminum alloy, it is necessary to ensure sufficient laser power to obtain the required penetration depth. A laser energy density threshold has been found in laser welding of different aluminum alloys. If the laser energy density threshold is lower than this value, only the surface of the weldment will be melted. The welding is performed with a heat conduction type, the penetration is very shallow, and only a laser impact mark is formed on the surface. Once it reaches or exceeds this value, the plasma is generated and holes are induced at the same time, and the penetration depth is greatly increased. Therefore, if the aluminum alloy laser welding is to achieve deep fusion welding effect, it needs to reach a certain power value. However, the power can not reach a large size, easily causing the welding seam to sink due to heat input, and the undercut is serious, as shown in Figure 7a. When the energy is less than the laser energy density threshold, there will be obvious thermal conduction weld morphology, as shown in Figure 7b.

Figure 7 Effect of laser power on weld formation

3 Solutions and Results

1. For the problem that the protective gas blowing force is too large and the blown area is too small to cause the molten pool to be unstable and the weld protection range is too narrow, the protective gas pipe (diameter 9mm) with a larger inner diameter is used instead, as shown in FIG. 8 . The gas pipe can reduce the interference of the gas on the forming of the molten pool under the premise of forming a relatively large protection range for the molten pool.

Figure 8 Large-diameter Trachea Protection

2. In order to meet the requirement of uniform appearance and weld width of 2mm or more on the weld surface, slow and defocus welding is adopted. In addition, the upslope time is 100ms and the downslope time is 300ms in the welding process to reduce the craters formed at the arc excavation.

The parameters of Table 1 are selected as the optimized welding process parameters. The post-weld sample is shown in Figure 9, and the arching profile is shown in Figure 10. The weld surface topography and cross-sectional topography are shown in Figs. 11 and 12, respectively.

It can be seen from Fig. 9, Fig. 10 and Fig. 11 that the surface of the weld seam forms a fine and uniform fish scale appearance, and there are no defects such as surface cracks and air holes, and the arc crater is greatly reduced. It can be detected from Fig. 12 that the weld has a weld width of 2.5 mm, a penetration depth of 1.7 mm, and no internal porosity, cracks, or other defects.