Effect Of Reverse Pressure On The Microstructure And Mechanical Properties Of ZL205A Alloy

In this paper,the influence of the reverse synchronic pressurization casting technique after low pressure filling on the microstructure and properties of ZL205 A alloy large thin--walled castings was studied.In this paper,the gas path and tooling of reverse pressure processing were designed to ensure the air tightness.In this paper,under low pressure casting conditions,when the holding pressure was 60 KPa,a thin-walled aluminum alloy casting with a height of 720 mm was cast.Under the reverse pressure conditions,the holding pressure was unified at 60 KPa,and the reverse pressures of 20 KPa,30 KPa,and 40 KPa were applied respectively,and thin-walled aluminum alloy castings of the same size were cast.Under conditions of low pressure and reverse pressure of 20 KPa,30 KPa,40 KPa,the average tensile properties of the casting was 309.2 MPa,369 MPa,408.5 MPa,429.2 MPa respectively.Under conditions of reverse pressure of 20 KPa,30 KPa and 40 KPa,the average tensile properties of the casting was increased by 19.3%,32.1%,and 38.8% respectively,compared with that under low pressure.Under the reverse pressure of 20 KPa,30 KPa,and 40 KPa,the tensile strength of the top of the casting has been greatly improved.This proved that the reverse pressure is very effective for improving the tensile properties of the top of the large thin-walled ZL205 A alloy castings.With the increase of the reverse pressure,the tensile properties of the casting became more uniform from the top to the bottom,eliminating the location sensitivity under low pressure casting.Under conditions of low pressure,reverse pressure of 20 KPa,30 KPa and 40 KPa,the maximum difference of tensile strength from top to bottom was 158.6 MPa,123.5 MPa,139.5 MPa,42.9 MPa.Under reverse pressure of 20 KPa,30 KPa and 40 KPa,the maximum difference of castings were reduced by 22.1%,12% and 73% respectively,compared with that under low pressure casting.Under the condition of low pressure casting,the fracture was mainly composed of intergranular fracture,few cleavage fracture and few dimples.Under the reverse pressure condition,the grain size of the fracture face is obviously reduced,and the fracture form of the intergranular fracture was still the main fracture form,but the number of dimple is increased.In this paper,on the basis of Darcy's law,the formula of influencing factors of hydrogen gas hole in ZL205 A alloy under low pressure casting was derived,and the factors affecting the production of hydrogen gas hole under low pressure were analyzed.The existence of the Al₁₁Cu₅Mn₃ phase in the heat-treated state ZL205 A alloy was found by diffraction spot calibration,and the following orientation relationship existed with the matrix:?3?6?_<sub>Al₁₁Cu₅Mn₃//?200?_?-Al,[001]_?-Al//[20?]_Al11Cu5Mn3.At the same time,there was the following mathematical relationship between the interplanar spacing:d?3?6?_Al11Cu5Mn3=d?200?_?-Al=0.202nm.With the increase of pressure,the interface heat transfer coefficient increased and the grain size decreased.The Al₂ Cu phases existing at the grain boundaries of reverse pressure heat treatment state castings were more diffusely and evenly distributed than the condition of low pressure casting.The bulk distribution of the Al₂ Cu phase at the grain boundaries under the condition of low pressure casting was improved.