Simulation And Experimental Study On Hot Crack Of Die Casting Of Magnesium Alloy Gas Nail Gun

The air nailer body is a kind of pressure vessel. It’s made from aluminum or magnesium alloy by die casting process. Due to its complex structure and anomalous wall thickness, the crack defect was serious during production. The simulation was employed combined with experiment to fix a more reasonable die casting process and reduce the hot tearing defect of nail gun body; The mechanism of hot tearing was also discussed; The effect of solution and aging treatment on microstructure of die casting magnesium alloy defect band and mechanical properties of the matrix were researched by using the optical microscope (OM), scanning electron (SEM), energy spectrum analysis (EDS), micro hardness tester and X-ray diffraction (XRD).The top of the main cylinder、the screw connection of the main cylinder、the side of the main cylinder、the connection between the main cylinder and handle、the end of the handle and the internal of the main cylinder were the main crack sites.3D laser scanning and microstructure observation showed that cracks were mainly hot tearing. According to the flow field, there was a slight air entrapment in the main cylinder because of the metal liquid impact the core of the main cylinder directly during pouring. The temperature field showed that the feeding of the nail gun body was limited since the gate closed early. There were some isolated melting pools on the side of the main cylinder. The hot tearing tendency of the casting was studied by using the hot tearing prediction module in ProCAST combined with orthogonal test design. The results showed that the positions of the defect in numerical simulation were basically consistent with the actual production. The mold temperature was the most significant factor, pouring temperature took the second place and injection speed was the least. The hot tearing tendency of the body was relatively small when the pouring temperature is 690 ℃, the die casting speed is 5m/s and the mold temperature is 230 ℃, which is 0.0900. After the air tightness test, the qualified rate reached 72% and the average leakage rate of the qualified nail gun was 21.6ml/min. Compared with the solid fraction and equivalent stress of the nine groups, equivalent stress occured when the solid fraction was greater than 50%. And when the solid fraction was greater than 90%, the tensile stress increased rapidly while hot tearing was most likely to happen at the same time.The numerical simulation of the optimized gating system was carried out. According to the results, the new scheme avoids impacting the core of the main cylinder. The hot tearing tendency of the body was reduced to 0.0807 owing to the better filling and solidification process. The experimental results showed that the hot tearing of the body was less and the hot tearing in the main cylinder was not obviously. The pass rate reached 84% and the average leakage rate of the qualified body was 16.3ml/min after air tightness test.Microstructure observation indicated that the defect bands in the main cylinder of the body which formed because of the accumulation of the reticular brittle phase were disappeared after solution. The β-Mg17Al12 was precipitated in the form of fine strip shape and short rod shape from matrix after aging treatment. According to the XRD test, the p-phase gradually disappeared with the extension of the solid solution time and it was reappeared after aging. EDS test showed that the distribution of Mg and Al elements in the die casting was very uneven. There were different degrees of segregation. The alloying elements were fully diffused, and the distribution of Al and Mg elements in the matrix was uniform after solution treatment. After solution and aging treatment,the tensile strength and elongation of the matrix were 203.3Mpa and 3.71%, and improved 25.03% and 57.20% respectively compared with the squeeze cast. The hardness of the defect bands has fallen from 160.8HV to 76.3HV, which is equivalent to the matrix organization.