The hot tearing susceptibility (HTS) of binary Mg-Gd alloys is tested from 1.0 to 8.0 wt% using a constrained rod casting (CRC) mold. It is found that the curve of the HTS versus Gd content follows a typical "Λ" curve: HTS increases firstly, reaches a maximum at 1.5% Gd, and then decreases, which is in good agreement with the results predicted by Clyne-Davies model. Microstructure observations and thermodynamic calculations show that grain structure (size and morphology), the susceptible freezing range, the volume fraction of eutectic liquid, and the amount of intermetallics (Mg 5 Gd) are the main factors influencing the HTS of Mg-Gd experimental alloys. The highest HTS observed in Mg-1.5Gd alloy is attributed to its coarse columnar grains, the high freezing range, and the thin and continuous liquid film as well as the hard and brittle intermetallics precipitated along the grain boundaries. In contrast, the lowest HTS observed in Mg-8Gd alloy is mainly related to the fine equiaxed grains, which can effectively accommodate the strain during solidification, and the tear healing by eutectic liquid. Furthermore, the effect of grain refinement by Zr addition on the HTS is also studied, and the results show that grain refinement can significantly reduce the HTS.

Considering the safety of the charge filled in the chambers and the operational reliability, a low temperature eutectic alloy bonding was preferred for the final assembling process of MEMS-based solid propellant microthrusters (SPM) array with top-side igniters. The optimum conditions of the alloy coating are the pH value of the solution is 0.5, the base metal layer is composed of Ti (60nm)/Cu (500nm) deposited by magnetron sputtering. The bonding process was conducted in an oven with air and the bonding temperature is 70°C. To predict the performance of bonding layer, the temperature distributions of eutectic alloy layer were simulated by ANSYS software when the chamber is full of high temperature reactants. The simulation results demonstrated the bonding strength wouldn’t cut down since the heat transfer induced by the combustion of charge. The assembled MEMS-SPM array was tested under constant voltage, the results indicated that the eutectic solder bonding procedure meet the requirements of the MEMS-SPM assembling.