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⑴Strong oxidizing capacity: Aluminum readily combines with oxygen in the air to form a dense Al2O3 film with a thickness of about 0.1 μm. The melting point of Al2O3 is as high as 2050°C, far exceeding that of aluminum and its alloys (around 660°C), and it also has a higher volume density, approximately 1.4 times that of aluminum. During welding, the alumina film hinders good bonding between metals and can easily lead to slag inclusions. The oxide film also absorbs moisture, which can cause pores to form in the weld during welding. Therefore, it is necessary to thoroughly remove the oxides from the surface of the weldment before welding, and to enhance the protection of the welding area. ⑵Higher thermal conductivity and specific heat capacity: The thermal conductivity and specific heat capacity of aluminum and aluminum alloys are approximately twice those of steel, which allows a large amount of heat to be rapidly conducted into the base metal during welding. Therefore, welding aluminum and aluminum alloys requires more heat than welding steel, and preheating and other process measures are often necessary before welding. ⑶It has a high tendency to develop thermal cracks. Its linear expansion coefficient is about twice that of steel, and its volume contraction rate during solidification is around 6.5%. Therefore, when welding certain aluminum alloys, thermal cracks often occur due to excessive internal stresses. In production, adjusting the composition of the welding wire is commonly used to prevent the formation of hot cracks, such as by using the HS311 welding wire. ⑷Pores are easily formed; the gas that causes pore formation is hydrogen. The solubility of hydrogen in liquid aluminum is 0.7 mL/100g, but at the solidification temperature of 660°C, this solubility drops sharply to 0.04 mL/100g, causing a large amount of hydrogen that was previously dissolved in the liquid aluminum to precipitate out and form bubbles. At the same time, aluminum and aluminum alloys have low density, so bubbles rise slowly in the molten pool. Additionally, due to aluminum’s high thermal conductivity, the molten pool cools rapidly; as a result, the rising bubbles often do not have time to escape and remain in the weld as pores. The moisture in the arc column atmosphere, as well as the moisture adsorbed on the surface oxide films of the welding materials and base metal, are all major sources of hydrogen; therefore, it is essential to thoroughly clean the surface of the workpiece before welding. ⑸Unequal strength at the joint: The heat-affected zone of aluminum and aluminum alloys softens due to heating, resulting in a decrease in strength; as a result, the joint does not achieve the same strength as the base material. The strength of joints made of pure aluminum and non-heat-treated strengthened aluminum alloys is approximately 75% to 100% of that of the base material ； The joint strength of heat-treated strengthened aluminum alloys is low, at only 40% to 50% of that of the base material. ⑹Through-welding: When aluminum and its alloys transition from a solid state to a liquid state, there is no noticeable change in color, making it difficult to determine the temperature of the base material. During welding, excessive temperature can go unnoticed, leading to through-welding. What preparations are required before welding aluminum and aluminum alloys? ⑴Pre-welding cleaning: The purpose of cleaning is to remove the oxide film and oil stains from the surface of the weldment, which is an important measure to prevent the formation of pores and slag inclusions. a Chemical cleaning: it offers high efficiency and stable quality, and is suitable for cleaning welding wires as well as weldments of small size that are produced in batches. Chemical cleaning is divided into two methods: dipping and scrubbing. As for cleaning agents and processes, mechanical cleaning involves first using organic solvents (**, rosin, or gasoline) to wipe away the oil from the surface of the welded parts, then using a fine copper wire brush to bring out a metallic shine on the surface, or using a scraper to clean it. The cleaned weldment should be welded within 4 hours; otherwise, it must be cleaned again. ⑵Shims: To ensure full penetration and prevent the welded joint from being penetrated or collapsing, shims can be placed under the joint before welding. The backing plate material can be graphite, stainless steel, or carbon steel, with a circular arc groove on its surface to ensure proper formation of the weld on the reverse side. ⑶Preheating is generally not necessary for thin and small weldments. When welding joints with a thickness of over 5 mm, in order to achieve the required temperature in the vicinity of the joint, the workpiece must be preheated before welding, with a preheating temperature of 100–300°C. What cleaning tasks should be carried out after welding aluminum and aluminum alloys? The residual flux and slag remaining in the weld and its vicinity after welding must be cleaned up promptly; otherwise, under the influence of air and moisture, these residual substances can destroy the alumina film that provides corrosion protection, leading to severe corrosion of the welded part. Therefore, the residual dirt on the welded parts must be strictly removed immediately after welding. Common methods and steps for removing slag: 1) Thoroughly scrub the weld joint with a bristled brush in hot water. 2) Immerse the welded part in an aqueous solution of chromic anhydride or potassium dichromate at a temperature of 60–80°C and with a mass fraction of 2%–3% for about 5–10 minutes, and then scrub it carefully with a bristled brush. Alternatively, the welded parts can be immersed in a nitric acid solution with a mass fraction of 10% at 15–20°C for 10–20 minutes. 3) Rinse and wash the welded parts in hot water. 4) Dry the welded parts with hot air or in a drying oven at 100°C. Discuss the welding process of tungsten inert gas welding for aluminum and aluminum alloys. ⑴Selection of welding wire: The grades of welding wires for aluminum and aluminum alloys are shown in Table 2. Among them, HS311 is a general-purpose welding wire; when using this wire for welding, the metal exhibits good fluidity, high resistance to thermal cracking, and ensures a certain level of strength. However, when welding aluminum-magnesium alloys, the brittle compound Mg2Si forms in the weld, reducing the ductility and corrosion resistance of the joint. HS331 should be used when welding aluminum-magnesium alloys. ⑵Welded joint and groove type: The groove types are shown in Table 3. ⑶An AC power source should be used for the welding power supply. Discuss the gas welding process for aluminum and aluminum alloys. ⑴Joint types: For gas welding of aluminum and aluminum alloys, lap joints and T-joints should not be used, as these joints tend to retain gas welding flux and slag, making it difficult to clean them after welding. ⑵Flame and nozzle number: For gas welding, a neutral flame (or a slightly carburizing flame) can be used. The nozzle number is selected based on the thickness of the aluminum sheet. Nozzle and wire angles: When welding thin sheets, the nozzle angle is around 30°–45°, while the wire angle is around 40°–50° ； When welding thick plates, the nozzle angle should be around 50°, while the wire angle should be between 40° and 50°.