In this paper, a novel method of pressurized metallurgy technology was proposed to improve cast structure of M42 high speed steel (HSS). The effect of solidification pressure (0.1, 1 and 2 MPa) on the cast structure of M42 HSS was investigated by means of experimental analysis and calculation of Thermo-Calc and DICTRA software. Increasing solidification pressure can obviously enhance cooling rate by improving interfacial heat transfer coefficient, which results in some remarkable improvement of the cast structure of M42 HSS. Firstly, the primary/secondary dendrite arm spacing and the average thickness of eutectic ledeburite reduce, which means dendrite structure is refined and eutectic ledeburite more homogeneously distributes with smaller size. Secondly, increasing solidification pressure, the volume fraction of M 6 C carbides decreases obviously and that of M 2 C increases correspondingly. And the morphology of M 2 C carbide changes from larger size lamellar and straight-rod shape into smaller size curved-rod morphology under higher solidification pressure due to larger nucleation number and overgrowth of γ, indicating that carbides are refined and distribute more uniformly. At last, higher solidification pressure is beneficial to reduce the lamellar spacing of M 2 C carbide and make compositions distribute more homogeneously.

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The effect of solidification pressure (0.5, 0.85 and 1.2 MPa) on heat transfer between ingot and mould was investigated with the measurement of cooling curves and calculation of heat transfer coefficient. Combined with cooling rate, temperature gradient and local solidification time (LST), the influence of pressure on solidification structure of 19Cr14Mn0.9N was revealed by macrostructure observation. The calculation results of heat transfer coefficient, obtained by the Beck-Nonlinear estimation technique, indicate that increasing solidification pressure obviously enhances heat transfer at the ingot/mould interface. And higher solidification pressure is benefit to increase cooling rate and temperature gradient of ingot. Meanwhile, increasing solidification pressure considerably suppresses nitrogen gas pore, and reduces the whole area of dispersing porosity and shrinkage, which is favorable to obtain a sound ingot. With the solidification pressure increasing from 0.5 to 1.2 MPa, the columnar zone is lengthened, the columnar-toequiaxed transition (CET) position gradually moves to the ingot center, and both dendritic arm spacing (λ 1 and λ 2 ) and local solidification time (LST) gradually decrease. The solidification structure is significantly refined and compressed under higher solidification pressure.

Self‐piercing riveting (SPR) has been used to joint steel to aluminum alloy sheets in multimaterial car body. It is a challenge to joint high strength steel to aluminum alloy by SPR process, especially for materials with high strength and large thickness. Herein, the SPR process for 1500 MPa hot stamped steel and aluminum alloy sheets is studied by experiment and simulation method. The laser heat treatment process is applied to the hot stamped steel sheet before joining. The relationship between heat treatment process and hardness distribution is studied and the softening zone with hardness less than 300 HV is obtained. Furthermore, the local softening zone is joined to an aluminum alloy sheet by SPR process. In the process, the rivets with high hardness are used to pierce the steel sheet. The failure mode and maximum tensile shear force of the joint are studied. At the same time, the joints without laser heat treatment are compared. The results show that the interlock and the maximum shear force for laser heat treatment increase by 91% and 26%, respectively.

Unexpected pressure rise may occur in the end-burning grain solid rocket motor. It is generally believed that this phenomenon is caused by the nonparallel layer combustion of the burning surface, resulting in the increase of burning rate along the inhibitor. In order to explain the cause of this phenomenon, the experimental investigation on four different end configurations were carried out. Based on the X-ray real-time radiography (RTR) technique, a new method for determining the dynamic burning rate of propellant and obtaining the real-time end-burning profile was developed. From the real-time images of the burning surface, it is found that there was a phenomenon of nonuniform burning surface displacement in the end-burning grain solid rocket motor. Through image processing, the real-time burning rate of grain center line and the real-time cone angle are obtained. Based on the analysis of the real-time burning rate at different positions of the end surface, the end face cone burning process in the motor working process is obtained. The closer to the shell, the higher the burning rate of the propellant. Considering the actual structure of this end-burning grain motor, it is speculated that the main cause of the cone burning of the grain may be due to the heat conduction of the metal wall. By adjusting the initial shape of the grain end surface, the operating pressure of the combustion chamber can be basically unchanged, so as to meet the mission requirements. The results show that the method can measure the burning rate of solid propellant in real time and provide support for the study of nonuniform combustion of solid propellant.