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The toughening effect of SiC particles was applied to inhibition of the cracks in Al2O3 ceramic for Laser Direct Forming(LDF).The experiments of coaxial laser cladding Al2O3 ceramic added SiC particles were performed on the Ti-6Al-4V alloy substrate to investigate the feasibility of Al2O3+SiC composite ceramic by LDF and explore the factors effecting on the crack susceptivity.The cracks,microstructures and the combining of the two compositions were observed by an optical microscopy,and the phase transitions were detected by X-ray diffraction.The results show that some complete SiC particles contained in the Al2O3 ceramic can inhibit the cracks,and the Al2O3+SiC composite ceramic without serious chemical reactions or combining defects can be prepared.The crack susceptivity is significantly affected by the proportion of SiC(f),laser power(P),scanning velocity(v) and the powder flow rate(n).Finally,a thin-wall Al2O3+SiC composite ceramic block(17 mm×6 mm×2 mm)without the obvious defects was formed with the parameters of f,P,v and n in 10%(weight proportion),186 W,300 mm/min and 1.78 g/min,respectively.

A three-dimensional thermal-mechanical Finite Element Model(FEM) was established by the ANSYS code to reveal the influence of fixture restraint distance on Pulsed Laser Welding(PLW) distortion of Hastelloy C-276 thin sheet with a thickness of 0.5 mm.The influence laws of the restraint distance on welding longitudinal deflection and transverse shrinkage distortion were investigated by using the displacement restriction assumption instead of the actual restraint condition.Experiments were carried out to measure the welding thermal cycle curves and residual distortions.The simulation results are in good agreement with that of the corresponding experiment,which validates the reliability of the FEM.It shows that the welding longitudinal deflection and transverse shrinkage distortion increase linearly with the restraint distance increasing from 8 mm to 20 mm,especially the welding transverse shrinkage distortion increases by nearly 3.6 times.Furthermore,the welding residual distortions can be controlled by reducing the restraint distance.Results dennonstrate that the FEM can simulate accurately the temperature field and the residual distortion of PLW and can provide theoretical and experimental basis for suppression of the welding distortion of thin sheets.