Common Defects in Aluminum Alloy Laser Welding

Common Defects in Aluminum Alloy Laser Welding

Whether laser autogenous welding or laser-arc hybrid welding is used for aluminum alloys, there are some common technical issues, i.e., defects may occur if the process parameters and welding conditions are  metallurgicimproper. Theal defects in aluminum alloy joints mainly include two types: weld porosity and welding hot cracks. In addition to porosity and hot cracks, defects such as undercut and poor backside formation also exist in laser welding of aluminum alloys. Compared with weld porosity, the probability of welding cracks (visible to the naked eye or under low magnification) is not high. However, because cracks are more dangerous, JIS Z 3105 stipulates that once a crack is detected in a weld, the weld shall be judged as Class IV. Undercut, poor backside formation and other defects are mostly serious defects caused by improper speed control or mismatched process parameters. Such defects generally appear in the stage of process exploration and debugging, and rarely occur in normal actual production operations. Therefore, porosity is a type of defect that is more harmful in laser welding of aluminum alloys and in the service of welded structures, and it is difficult to fundamentally eliminate.

1. Porosity

Porosity is the most common and major volume defect in laser welding of aluminum alloys, with sizes ranging from hundreds of microns to several millimeters. Its formation mechanism is not yet fully clear. Porosity not only weakens the effective working section of the weld, but also causes stress concentration, reducing the dynamic strength and fatigue performance of the welded joint.

When aluminum alloy melts in a hydrogen-containing environment, its internal hydrogen content can reach more than 0.69 ml/100g, but after the alloy solidifies, its hydrogen solubility in equilibrium is at most 0.036 ml/100g. It is generally believed that during the cooling process of laser welding, the solubility of hydrogen drops sharply, and the precipitation of supersaturated hydrogen will form hydrogen porosity. The evaporation of low-melting-point and high-vapor-pressure alloying elements may also lead to porosity, which is called metallurgical porosity. In addition, the disturbance of the laser beam and the instability of the keyhole can also form porosity, but such porosity has an irregular shape and can be called process-induced porosity. Due to the high chemical activity of aluminum alloys, an oxide film is easily formed on the surface. During welding, the crystal water and combined water decomposed from the oxide film on the aluminum alloy surface, together with the moisture in the air and protective gas, directly decompose to produce hydrogen in the high-temperature area under the action of the laser. These hydrogen gases may either precipitate during the cooling and solidification of the molten pool to form bubbles or directly generate bubbles on the incompletely melted oxide film. Due to the low specific gravity of aluminum alloys, the rising speed of bubbles in the molten pool is slow. In addition, aluminum alloys have strong thermal conductivity, and the cooling and solidification speed of the molten pool is extremely fast. Some bubbles cannot escape in time and remain in the weld, thus forming metallurgical porosity. Studies have shown that the main gas in the porosity of aluminum alloy welds is hydrogen, so the porosity in aluminum alloy welds is sometimes called hydrogen porosity. When observing the fracture of porosity under a scanning electron microscope, the porosity mostly presents a spherical morphology with tightly arranged dendrite ends of dendritic crystals, and the inner wall is smooth, clean and free of oxidation traces. The existence of porosity not only reduces the compactness of the weld and the bearing capacity of the joint, but also reduces the strength and plasticity of the joint to varying degrees.

2. Hot Cracks

Hot cracks (including solidification cracks and liquation cracks) form during the solidification process of molten pool metal and are one of the common defect types in laser welding of aluminum alloys. The most obvious feature of the fracture morphology of solidification cracks is that the fracture surface is composed of a large area of smooth but uneven granular cobblestone or potato-like structures, and the surface often retains intergranular low-melting-point eutectics or liquid film folds, as well as traces of brittle fracture of dendrites. The fracture morphology of liquation cracks is similar to that of solidification cracks, but it has the characteristics of high-temperature intergranular fracture or solidification fracture. In the fatigue fracture of fusion-welded joints under fatigue loading, fatigue crack sources caused by such hot cracks are also common. The causes of hot cracks in laser welding of aluminum alloys are mainly related to their own characteristics and welding processes. Aluminum alloys have a large shrinkage rate during solidification (up to 5%), resulting in large welding stress and deformation; in addition, low-melting-point eutectic structures are formed along the grain boundaries during the solidification of weld metal, which weakens the bonding force of grain boundaries, thus forming hot cracks under the action of tensile stress. In addition, the crack morphologies in laser welding of aluminum alloys can be summarized into the following categories: weld center cracks; weld fusion line cracks; intergranular cracks in welds; heat-affected zone liquation cracks; cracks caused by oxide films; and intergranular microcracks.

In addition, poor protection during welding causes the weld metal to react with gases in the air, and the formed inclusions are also potential crack sources. The type and quantity of alloying elements have a great influence on the hot cracking tendency during aluminum alloy welding. Generally, Al-Si and Al-Mn series aluminum alloys have good weldability and are not easy to produce hot cracks; while Al-Mg, Al-Cu and Al-Zn series aluminum alloys have relatively high hot cracking tendencies. The hot cracking tendency can be reduced by adjusting the welding process parameters to control the heating and cooling rates. Generally speaking, the hot cracking tendency of laser-arc hybrid welding is better than that of laser filler wire welding, and the hot cracking tendency of laser filler wire welding is better than that of laser autogenous welding.

3. Undercut and Burn-Through

Aluminum alloys have low ionization energy, and photo-induced plasma is prone to overheating and expansion during welding, resulting in unstable welding processes. In addition, liquid aluminum alloys have good fluidity and low surface tension. To improve penetration, a larger protective gas flow rate and laser output power are often required, which deteriorates the stability of the welding process, causing the molten pool to fluctuate violently under pressure and easily leading to defects such as undercut and burn-through. The backside formability of laser-welded aluminum alloy plates can be effectively improved by installing a water-cooled copper plate on the back of the weld.

4. Slag Inclusion

Another type of defect often occurring in car body welding is weld slag inclusion. Studies have shown that slag inclusion mainly comes from oxides on the surface of weldments and welding wires, as well as unstable processes in the localization of aluminum alloy materials. Therefore, aluminum alloy material manufacturers should strengthen technological innovation and improve casting processes to minimize the content of impurities and hydrogen in raw materials and enhance the quality stability of products.