5083 aluminum alloy is a high magnesium alloy with good strength, corrosion resistance and machinability among non-heat treatable alloys. The main alloying elements in the 5083 alloy are Mg and Mn. Due to the high content of Mn, the mechanical properties of the aluminum alloy can be improved without causing the corrosion resistance of the alloy to decrease, and the welding performance of the 5083 aluminum alloy can be improved at the same time. At the same time, the addition of Mn element can make the phase distribution of Mg-containing elements uniform, and improve the strength and corrosion resistance. Excellent corrosion resistance enables 5083 aluminum alloy to be widely used in navigation, aviation, transportation, petroleum and other fields.
Causes of surface cracks of 5083 welded aluminum tubes
The 5083 aluminum alloy was welded by manual argon arc welding to form a cylindrical product with a size of 324mm × 12mm. A crack was found at the circumferential weld of the pipe body. The welding material is ER5183H18 (welding wire), the workpiece is pretreated at room temperature before welding, the welding current is AC, the current is 180-220A, the arc voltage is 16-20V, the welding speed is 10-14cm/min, and the tungsten electrode is 4.0mm. Cerium tungsten, argon shielding gas is used during welding, and no heat treatment process is performed after welding.
(1) Influence of welding stress
The thermal conductivity and specific heat capacity of aluminum alloys are very large, and a heat source with concentrated energy and high power must be used during welding. Due to the influence of welding heat, the mechanical properties of the base metal near the seam will deteriorate. At the same time, the linear expansion coefficient of 5083 aluminum alloy is about 23.4 μm/(m·k), and the volume shrinkage rate during solidification is about 6.5%. Therefore, there is often a large internal stress in and near the aluminum alloy weld. This internal stress will cause cracks in some parts with poor toughness and toughness, and the aluminum alloy parts have a serious influence on the degree of restraint during welding. Due to the action of the welding heat source, the heated metal expands, and the cold metal around the welding hinders its expansion, so that the hot metal is subjected to compressive stress, and during the cooling and solidification process, it is subjected to tensile stress due to volume shrinkage, and these internal stresses are difficult to release. It may lead to the formation of welding hot cracks in the welding area.
(2) Influence of non-metallic inclusions
Through metallographic microscope and fracture morphology analysis, it can be seen that there are a large number of non-metallic inclusions in the base metal structure and crack source area of the sample. The existence of the non-metallic inclusions may cause stress concentration and induce the formation of cracks; secondly, the non-metallic inclusions are mostly brittle phases, which makes the structure of this area relatively fragile, and may induce cracks under the action of force. The third is to deteriorate the combination of surrounding tissues, making the local tissues more fragile, and the performance degradation may become the source of cracks.
To sum up, on the one hand, there is welding stress in the sample during the welding process, on the other hand, there are a large number of non-metallic inclusions in the base metal of the sample and the microstructure of the crack source area. These two factors work together to cause the formation of cracks.