Research On Detecting Technique Of Aluminum Alloy Welding Hot Cracking

In recent years, with the rapid development of high speed railway, it is urgent problem to reduce the body weight effectively on the premise of security is guaranteed. During the research and manufacturing of the material for high-speed vehicle, some domestic material have greater tendency of the welding hot cracking, The traditional method of welding hot cracking detection is not only inefficient but also impressionable due to human factors. In this paper, appropriative technology and a device to detect welding hot cracking sensitivity are developed, meanwhile welding hot cracking of relevant materials are detected quickly and accurately.In this paper,the characteristics and disadvantages of existing detection method are summarized, and a novel equipment for welding hot cracking detection is developed. The equipment magnifies the slight difference among different materials, and detects the hot cracking susceptibility efficiently, rapidly and accurately.The strain rate in the Brittle Temperature Range (BTR) is a important factor for welding hot cracking, but it is difficult to measure directly at different loading speed. Hence, the relationship between loading speed and strain rate is developed by finite element method, thereby strain rate could be replaced by loading speed as a parameter for the result analysis.Experiments of welding hot cracking susceptibility are carried out for three 6N01 aluminium alloy (Jk,N7,K4) by the equipment. The results show there are some extents of limitations to evaluate the cracking degree by quantity and length of crack in used in traditional method. To solve the problem, in this paper, on the basis of traditional method, the width of crack and the proportion of crack area to the molten pool are also employed to make a comprehensive evaluation for the cracking degree. By the new evaluating method, for the three aluminium alloy, Japanese aluminium alloy Jk displays the lowest hot cracking tendency at different loading speed, while the domestic alloy K4 has the highest hot cracking tendency in the three alloys, which is superior to the alloy N7. The results of matching experiments on wires and base metal show wires 5356 and 4047 , which are better than 4043 and Pure aluminum wires, reduce hot cracking tendency effectively for the three alloy.The distribution and content of alloy elements is the root of the difference of the welding hot cracking susceptibility. The observation of microstructure shows, for the three alloys, Mg2Si is the main strengthening phase in 6N01 aluminium alloy. Eutectic reaction occurs between Mg2Si and the matrix of the alloy in local area during the thermal cycle, and result in liquid-film which causes hot crack under tensile strain. Fe, Cr and other elements react with Al and Si forming coarse impurity phases thereby the stress concentration and decrease in the ductility are caused. If alloy stoikiometric ratio in the weld bead is kept in a certain range by filler metal, then fluidity of the bead is improved during solidification, result in decrease of the welding hot crack tendency.