Aluminum alloy welding is very different from the welding of ordinary carbon steel, stainless steel and other materials. It is easy to produce many defects that other materials do not have, and it is necessary to adopt targeted measures to avoid it. Let’s take a look at the problems that are prone to aluminum alloy welding and the requirements for welding technology.
There are many welding methods for aluminum alloys, and various methods have different applications. In addition to traditional welding, resistance welding, and gas welding methods, other welding methods (such as plasma arc welding, electron beam welding, vacuum diffusion welding, etc.) can also easily weld aluminum alloys together.
The characteristics and scope of application of commonly used welding methods for aluminum alloys are shown in Table 1. It should be selected according to the grades of aluminum and aluminum alloys, the thickness of the weldment, the product structure, and the requirements for weldability.
The heat power of the oxygen-acetylene gas welding flame is low and the heat is relatively dispersed, so the weldment is deformed and the productivity is low. Preheating is required when welding thick aluminum weldments with gas welding. The weld metal after welding not only has coarse grains and loose structure, but also is prone to defects such as alumina inclusions, pores and cracks. This method is only used for welding repair of unimportant aluminum structural parts and castings with a thickness ranging from 0.5 to 10 mm.
Argon tungsten arc welding
This method is welded under the protection of argon gas. The heat is relatively concentrated, the arc combustion is stable, the weld metal is dense, and the strength and plasticity of the welded joint are high. The more widely used it is in the industry. Argon tungsten arc welding is a relatively complete welding method for aluminum alloys, but the argon tungsten arc welding equipment is more complicated, and it is not suitable to operate under outdoor conditions.
Melting pole argon arc welding
The arc power of automatic and semi-automatic argon arc welding is large, the heat is concentrated, and the heat affected area is small. The production efficiency can be increased by 2 to 3 times compared with manual argon tungsten arc welding. It can weld pure aluminum and aluminum alloy plates with a thickness of less than 50mm. . For example, it is not necessary to preheat the aluminum plate with a thickness of 30mm, and only the front and back layers can be welded to obtain a smooth surface and good quality weld. Semi-automatic melting electrode argon arc welding is suitable for tack welds, intermittent short welds and weldments with irregular structures. The semi-automatic argon arc welding torch can be used for convenient and flexible welding, but the diameter of semi-automatic welding wire is relatively small. The pore sensitivity of the weld is greater.
Pulse argon arc welding
1) Tungsten pulse argon arc welding
This method can significantly improve the stability of the low-current welding process, and it is convenient to control the arc power and weld shape by adjusting various process parameters. The weldment has small deformation and small heat-affected zone. It is especially suitable for welding of thin plates, all-position welding and other occasions, as well as forging aluminum, duralumin, super duralumin, etc., which are highly sensitive to heat.
2) Molten electrode pulsed argon arc welding
The usable average welding current is small, the parameter adjustment range is large, the deformation of the weldment and the heat-affected zone are small, the productivity is high, the pore resistance and crack resistance are good, and it is suitable for all-position welding of aluminum alloy sheets with a thickness of 2-10mm.
Resistance spot welding, seam welding
It can be used to weld aluminum alloy thin plates with a thickness of less than 4mm. For products with higher quality requirements, DC shock wave spot welding and seam welding can be used. Welding requires more complicated equipment, with high welding current and high productivity. It is especially suitable for mass-produced parts and components.
Friction stir welding
Friction stir welding is a solid state joining technology that can be used for welding various alloy plates. Compared with the traditional fusion welding method, friction stir welding has no spatter, no smoke, no need to add welding wire and shielding gas, and no pores or cracks in the joint. Compared with ordinary friction, it is not restricted by shaft parts and can be welded with straight welds. This welding method has a series of other advantages, such as good mechanical properties of the joint, energy saving, no pollution, and low pre-welding preparation requirements. Due to the low melting point of aluminum and aluminum alloys, friction stir welding is more suitable.
Difficulties in welding aluminum alloy materials
The thermal conductivity of aluminum alloy material is 1 to 3 times greater than that of steel, and it is easy to heat up. However, this kind of material is not resistant to high temperatures, and has a large coefficient of thermal expansion, which is likely to cause welding deformation. Moreover, this material is also prone to cracks and weld penetration during welding, especially the welding of thin aluminum plates is more difficult.
Aluminum alloy welding will produce a certain amount of hydrogen in the molten pool. If these gases are not discharged before the weld is formed, it will cause pores in the weld and affect the quality of the welded parts.
Aluminum is a metal that is easily oxidized, and there is almost no unoxidized aluminum in the air. The surface of the aluminum alloy is directly exposed to the air, and a dense and insoluble aluminum oxide film will be formed on the surface. The oxide film is wear-resistant and high-temperature resistant, and its melting point is as high as 2000 degrees Celsius. Once it is formed, the subsequent processing difficulty will be greatly increased.
Aluminum alloy welding also has problems such as easy softening of the joint and low surface tension in the molten state, which is easy to produce defects.
Aluminum alloy welding process requirements
First of all, from the perspective of welding equipment, if a MIG/MAG welding machine is used, it must have pulse functions such as single pulse or double pulse. The best double pulse function. Double pulse is the superposition of high-frequency pulse and low-frequency pulse, and low-frequency pulse is used to modulate the high-frequency pulse. In this way, the double pulse current is fixed to periodically switch between the peak current and the base current at the frequency of the low-frequency pulse, so that the weld forms a regular fish-scale pattern.
To change the forming effect of the weld, you can adjust the frequency and peak value of the low-frequency pulse. Adjusting the low-frequency pulse frequency will affect the switching speed between the peak value and the base value of the double pulse current, which will change the size of the seam fish-scale spacing. The greater the switching speed, the smaller the distance between the fish-scale patterns. Adjusting the peak value of the low-frequency pulse can change the stirring effect on the molten pool, thereby changing the welding depth. Choosing an appropriate peak value has obvious effects on reducing the generation of pores, reducing heat input, preventing expansion and deformation, and improving the strength of the weld.
Furthermore, from the point of view of the welding process, the following matters should be noted:
One is to clean the surface of the aluminum alloy before welding, and all dust and oil must be removed. Acetone can be used to clean the surface of the aluminum alloy welds. For thick aluminum alloys, first use a wire brush to clean it, and then clean it with acetone.
The second is that the welding wire material used should be as close as possible to the base material. Whether to choose aluminum-silicon welding wire or aluminum-magnesium welding wire should be determined according to the requirements of the welding seam. In addition, aluminum-magnesium welding wire can only be used to weld aluminum-magnesium materials, while aluminum-silicon welding wire has a wider range of uses, which can weld aluminum-silicon materials as well as aluminum-magnesium materials.
Third, when the thickness of the plate is large, the plate should be preheated in advance, otherwise it is prone to impermeable welding. When the arc is closed, the arc should be closed and filled with a small current.
Fourth, when performing argon tungsten arc welding, a DC argon arc welding machine should be used, and the forward and reverse AC and DC are alternately used. The forward direct current is used to clean the aluminum surface oxidation mold, and the reverse direct current is used for welding.
Also note that the welding specifications should be set according to the thickness of the plate and the requirements of the weld; MIG welding should use special aluminum wire feed wheels, and use Teflon guide wire tube, otherwise it will produce aluminum shavings; the length of the welding torch cable should not be too long, aluminum The welding wire is soft, and the welding torch cable is too long, which will affect the stability of the wire feeding.
Looking at the development prospects of laser welding technology through aluminum alloy welding
With the development of laser technology, laser welding technology has become mature, and even three-dimensional laser welding and non-metal laser welding have achieved a higher level of technology. In the field of metal welding, aluminum alloy welding accounts for a large part of the proportion, because aluminum alloy has high specific strength, high fatigue strength, good fracture toughness and low crack growth rate, and also has excellent forming process and good resistance. Corrosive, has been widely used in aviation, aerospace, automobile, machinery manufacturing, shipbuilding and chemical industries. The wide application of aluminum alloy has promoted the development of aluminum alloy welding technology, and the development of welding technology has promoted the research of laser welding technology. At the same time, the development of welding technology has expanded the application field of aluminum alloy.
However, the characteristics of aluminum alloy itself make its related welding technology face some urgent problems to be solved: surface refractory oxide film, joint softening, prone to porosity, prone to thermal deformation, and excessive thermal conductivity. Traditional aluminum alloy welding generally adopts TIG welding or MIG welding process. Although these two welding methods have high energy density and can obtain good joints when welding aluminum alloy, they still have poor penetration ability, large welding deformation, and low production efficiency. And other shortcomings, so people began to seek new welding methods.
Laser technology has become the main development direction of aluminum alloy welding technology in the future. Laser welding has the advantages of high power density, low welding heat input, small welding heat-affected zone and small welding deformation, so that it has received special attention in the field of aluminum alloy welding. Therefore, in the aspect of aluminum alloy welding, research and use of high-power lasers for deep penetration welding of large thicknesses will be the inevitable trend of future development. Deep penetration welding with large thickness has more prominent the pinhole phenomenon and the influence on weld pores. Therefore, the formation mechanism and control of pinholes have become more and more. It will definitely become a hot issue of common concern and research in the industry.
Improving the stability of the laser welding process and welding seam formation, and improving the welding quality are the goals that people pursue. Therefore, new technologies such as laser-arc composite technology, wire-filled laser welding, preset powder laser welding, dual focus technology and beam shaping will be further improved and developed.
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