Study On Formation Machanism And Influence Factor Of Weld Crack In ZM6 Alloy Thin-walled Casting By TIG Welding

Magnesium alloy has been widely concerned for its good performance in different fields. But in the casting process, magnesium alloys are prone to porosity, slag inclusion, cracks etc. That leaded to castings rejection. Repair welding is an effective method for repairing the casting defects and widely used in industrial production. But in the welding process of magnesium alloy thinwalled parts, welding defects such as cracks easily occurs. In this paper, the thinwalled components of ZM6 magnesium alloys as the research object, through Xray diffraction phase analysis(XRD), optical microscope(OM), scanning electron microscopy(SEM), energy dispersive spectroscopy(EDS), backscattered electron image analysis(BSE), microhardness test and tensile testing method, studied the welding cracks formation mechanism and welding current, casting wall thickness on the influence of microstructure and properties welding joints.Based welding crack analysis of the ZM6 thin-walled casting, welding crack is divided into two types. One is the liquation cracks, the other one is the solidification cracks. In the grain boundary, the low melting-point eutectic phase formed by segregation of Nd atoms that resulted liquation crack. Nd and Zn segregation around solidification crack.Under the experimental conditions, with the increase of welding current, grain size of weld zone decreased. When the welding current was 240 A, the grain size was the smallest, only 11.7 μm. But in the weld zone of this specimen, there were solidification cracks. Microhardness measurements shown that with the increase of the welding current, the weld zone hardness increased. When the welding current was 240 A, its hardness maximum was 64. The tensile test results shown that with the increase of welding current, the tensile strength of welded joints increased first and then decreased. When the welding current was 220 A, the maximum tensile strength was 137 MPa. The specimen fracture location in the heat affected zone.With the increase of the plate thickness, the tread of the grain size was increased first and then decreased. When the plate thickness was 6 mm, maximum grain diameter in weld zone was 21.0 μm. When the plate thickness was more than 6 mm, there were liquation cracks in the heat affected zone. Microhardness measurements shown that with the increase of the thickness, the microhardness decreased first and then increased. When the thickness was 6 mm, the minimum hardness was 51, when the wall thickness was 10 mm, the hardness reached the maximum value of 65. When the plate thickness was 8 mm, the maximum tensile strength was 130 MPa.