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2023.7.17
Welding aluminum can be a rewarding skill to develop, opening up possibilities for various projects in industries such as automotive, aerospace, and construction. This comprehensive guide is designed to help beginners understand the essentials of welding aluminum and get started with this versatile metal.
The chemical properties of aluminum and aluminum alloys are very active, and the surface is prone to the formation of refractory oxide films (such as Al2O3 with a melting point of about 2050 ℃ and MgO with a melting point of about 2500 ℃). Additionally, aluminum and aluminum alloys have strong thermal conductivity, and welding heat input is prone to rapid loss to the base material, which can easily cause incomplete fusion defects in aluminum and aluminum alloys. The main problems in welding production of aluminum and aluminum alloys are as follows:
1. Aluminum has large Specific heat capacity and thermal conductivity
The Specific heat capacity and thermal conductivity of aluminum are larger than that of steel, so the heat input in the welding process is lost due to the rapid conduction to the base metal. Therefore, when welding by fusion welding, a highly concentrated heat source is required. In order to obtain high-quality welding joints, sometimes preheat technical measures are required to realize the fusion welding process; When Electric resistance welding is used to weld aluminum and aluminum alloy, it is necessary to use super power source for welding.
2. Large linear expansion coefficient
The linear expansion coefficient of aluminum and aluminum alloys is relatively large, about twice that of steel, and the volume shrinkage rate during solidification is about 6.5%. Therefore, weldments are prone to significant welding deformation.
3. High affinity between aluminum and oxygen
Aluminum has a high affinity for oxygen and is highly susceptible to oxidation. During the welding process, aluminum and its alloys oxidize and form a high-density (3.85g/cm3) oxide film (Al2O3) on the welding surface, with a melting point of up to 2050 ℃. This oxide film hinders the good bonding of the molten metal during the welding process and is prone to slag inclusion.
4. Easy to produce pores
The most common defect in the welding process of aluminum and aluminum alloys is hydrogen porosity. This is because there is always a certain amount of water present in the space of the welding arc column, especially in humid seasons or areas with high humidity. Hydrogen, which is decomposed from the water in the arc column atmosphere, dissolves into the overheated molten pool metal. During low-temperature solidification, the solubility of hydrogen will undergo significant changes and sharply decrease, If it cannot precipitate before solidification in the weld pool, hydrogen pores will form in the weld.
Secondly, the moisture adsorbed in the oxide film of welding wires and weldments is also an important reason for the formation of pores. The oxide film of Al Mg alloy is not dense and has strong water absorption. Therefore, Al Mg alloy has a greater tendency to produce pores than pure aluminum with dense oxide film.
5. No color change during melting of aluminum and aluminum alloys
When aluminum and aluminum alloys change from solid to liquid during the welding process, there is no significant color change, making it difficult for welders to control the heating temperature. In addition, due to the low strength of aluminum and its alloys at high temperatures (aluminum only has a strength of 10MPa at 370 ℃), it is easy to cause the weld pool to collapse or the metal in the pool to leak. Therefore, a backing plate needs to be added to the back of the weld seam during welding.
6. Welding hot cracks
During the welding process of aluminum and aluminum alloys, hot cracks appear in the weld metal and near the seam area, mainly metal solidification cracks, and liquefaction cracks can also be seen in the near seam area. The existence of fusible Cocrystal is an important reason for solidification cracks in aluminum and aluminum alloy welds. The coefficient of linear expansion of aluminum and aluminum alloys is twice that of steel. When welding under constrained conditions, significant welding stress is generated, which is also one of the reasons why aluminum and aluminum alloys have a greater tendency to crack.
7. Equal strength of welded joints
Aluminum alloys that can be age strengthened, except for Al-Zn-Mg alloys, have lower welding strength than the base metal whether they are welded in an annealed state or an aging state without heat treatment after welding. Non aging strengthened aluminum alloys, such as A-Mg alloys, have equal strength between the welded joint and the base metal when welded in an annealed state; When welding under cold working stress state, the strength of the welded joint is lower than that of the base metal.
The unequal strength performance of aluminum and aluminum alloy welding indicates that the welded joint has undergone some degree of softening or there is a weak link in performance. The weak link in the performance of this type of joint can exist in any region of the weld seam, fusion zone, or heat affected zone.
(1) Due to its casting structure, the strength difference between the weld seam area and the base metal may not be significant, but the plasticity of the weld seam is generally not as good as that of the base metal. At the same time, the greater the welding heat input, the greater the trend of decreasing the performance of the weld seam.
(2) The main problem with non aging strengthened aluminum alloys in the fusion zone is the reduction of plasticity due to grain coarsening; When aging strengthened aluminum alloy is welded, not only does the grain coarsening occur, but cracks may also occur due to grain boundary liquefaction. So, the main problem in the fusion zone of the weld is the deterioration of plasticity.
(3) The performance of non aging strengthened aluminum alloys and aging strengthened aluminum alloys in the heat affected zone after welding is mainly due to the softening of the weld metal.
8. Corrosion resistance of welded joints
After welding of aluminum and aluminum alloys, the corrosion resistance of welded joints is generally lower than that of the base metal. The main reasons that affect the corrosion resistance of welded joints are as follows:
1) Due to the non-uniformity of the structure of the welded joint, the electrode potential at various parts of the welded joint is uneven. Therefore, the heat treatment before and after welding will have an impact on the corrosion resistance of the joint.
2) Excessive impurities, coarse grains, and the precipitation of brittle phases can significantly reduce corrosion resistance. Therefore, the purity and density of cold wind metals are one of the reasons that affect the corrosion resistance of joints.
3) Welding stress can also affect corrosion resistance.
Safety Precautions:
Before beginning any welding project, prioritize safety. Ensure you have proper protective gear, including a welding helmet, gloves, and clothing, to shield yourself from UV radiation, sparks, and heat. Additionally, work in a well-ventilated area to avoid exposure to harmful fumes.
Selecting the Right Welding Process:
Aluminum can be welded using several welding methods, including TIG (Tungsten Inert Gas) welding, MIG (Metal Inert Gas) welding, and oxy-acetylene welding. For beginners, TIG welding is recommended due to its versatility, control, and ability to produce high-quality welds.
Equipment and Materials Needed:
To weld aluminum, you’ll need a TIG welding machine, a TIG torch with a suitable electrode, argon gas for shielding, filler rods specific to aluminum, and a clean stainless steel wire brush for cleaning.
Cleaning and Preparation:
Aluminum must be thoroughly cleaned before welding to ensure a strong bond. Use a stainless steel wire brush to remove any oxide layer, grease, or dirt from the surface. If necessary, use a dedicated aluminum cleaner or solvent.
Setting Up TIG Welding Machine:
Adjust the TIG welding machine settings according to the thickness of the aluminum and the desired welding current. Set the gas flow rate between 15-20 cubic feet per hour (CFH), and ensure proper tungsten electrode selection based on the project’s requirements.
Technique for TIG Welding Aluminum:
Maintaining proper technique is crucial for successful aluminum welding. When TIG welding, hold the torch at a slight angle (10-15 degrees), and use a slow, steady motion to create a puddle. Control the heat input to prevent burn-through or distortion, and work on a clean, consistent joint.
Welding Joint Types:
The most common joint types for aluminum welding are butt joints, lap joints, T-joints, and corner joints. Each requires specific preparation and welding techniques, so choose the appropriate joint type for your project.
Filler Rod Selection and Usage:
Select the appropriate aluminum filler rod that matches the base metal. Clean the filler rod using a wire brush and keep it close to the weld puddle while maintaining a consistent arc length. Control the filler rod’s feed rate to achieve proper heat distribution and fusion.
Post-Welding Finishing:
After completing the weld, examine it for any defects or porosity. Use a wire brush or suitable cleaning method to remove any slag or residues. Consider post-weld treatments like grinding or sanding to achieve a smooth, finished appearance if required.
Practice and Patience:
Mastering aluminum welding takes practice and patience. Start with simple projects and gradually progress to more complex ones. Take the time to analyze your welds, identify areas for improvement, and refine your technique through continuous practice.