Research On Fabricating And Thixoforming Of Semisolid Billet Via Semisolid Insothermal Treatment Of Hot Rolled 2A12 Aluminum Alloy

With the increasing requirement of lightweight,aluminum alloys are widely used in aerospace,transportation and other fields,due to its good lightweight metal material.At present,the traditional manufacturing methods of aluminum alloys mainly include casting and forging.The casting forming process is simple,and the shape of the parts is not limited.But the performance is not easy to meet the requirements for use.In addition,the forging process has a high performance but it has a long process flow and high cost.Under this background,the semi-solid thixoforming technology inherits the comprehensive advantages of the casting and forging processes,which is the precise and advanced nearnet shape manufacturing technology.In this paper,the 2A12 hot rolled aluminum alloy sheet is used as the raw material to prepare the relay box forgings.The microstructure observation and mechanical properties analysis were conducted to explore the influence of the process parameters on their microstructure and properties,which provides a technical support to thixoforging the relay box-shaped structural parts.The isothermal treatment of the hot-rolled 2A12 aluminum alloy was performed in the semi-solid temperature range,and the microstructure was observed using the optical microscope.The results show that with increasing isothermal temperature and holding time,the difference of grain size decreases first and then increases.The average grain size gradually increases,and the roundness decreases first and then increases.The microstructure of the hot-rolled 2A12 aluminum alloy sample was observed,which was gradually heated from room temperature to semi-solid state.It was found that a large amount of recrystallized grains appeared around the grains of the rolled sample when the temperature reached 520°C.Until the semisolid temperature range,the recrystallization process continues,the fine grain is merged and grown,and the boundary of the crystallized grain is partially melted,finally obtaining the semi-solid billet of spherical grains.The numerical simulation of the thixoforming process of the relay box-shaped structural parts performed via the DEFORM-3D software to get the stress,strain and temperature fields of the material during thixotropic process.Effect of the process parameters such as isothermal temperature,mold temperature,friction coefficient,loading speed on the stress,strain and temperature fields was investigated.The results show that the proper increases of the isothermal temperature and the die temperature,and the reduction of the friction coefficient are favorable for filling the cavity of the semisolid billet.The relay box forgings are manufactured through the thixoforming experiments to obtain the best production process parameters.The result shows that the parts have high requirements for the forming process parameters.The isothermal temperature is 620 °C,the holding time is 30 min,and the male die temperature should be controlled in a range from 320 °C to 340 °C.After solid solution and natural aging treatment,the mechanical properties of the thixoforged parts are significantly improved.Among them,the mechanical properties of the heat-treated structural parts with the solution temperature of 492 °C and the solution time of 2.5 h are the best.Its tensile strength can reach 414.2 MPa,which increases by 20%,compared with the highest tensile strength of 21.9% before the heat treatment.Its elongation rate can reach 25.1%.Compared with the highest elongation of 21.9% before the heat treatment,the structural elongation increases by 14.6%.