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Pores are common defects in the process of directed laser deposition (DLD) which not only greatly reduce the fracture toughness of ceramic materials, but also lead to the failure of shaped parts. In this paper, the formation mechanism of pores was analyzed and the effects of laser power, feeding rate, scanning speed and ultrasonic power on pores were investigated. Transmission electron microscope, scanning electron microscopy observation and X-ray diffraction analysis were carried out for sample microstructure and phase composition respectively. The relative density of samples was measured by the progressive focused ion beam and the porosity was calculated by image processing software Image. The results show that the pores are divided into gas holes and shrinkage cavities. The appearance of circular gas holes with smooth inner walls are caused by the feeding method by gas forced blowing, the gas mixed with powder itself, and the gas in the molten pool formed by gasification of low-melting impurities and alumina/zirconia during laser processing. The gas holes are evenly distributed in the cross-section of the thin-walled specimen parallel to the scanning speed. As the temperature changes drastically, the material around the melt solidifies first, the melt will be attached to the solidified material to shrink, so that the melt can not be filled as a solid and finally the shrinkage cavities are formed. Generally the shrinkage cavities are irregular and the pore wall is relatively rough, mainly concentrated on the top of thin-walled samples. The laser power has the greatest influence on the pores, which has the greatest effect on the porosity but little effect on the shrinkage cavities. After attaching the ultrasound, the gas holes are mainly distributed on the top of the sample, and the shrinkage cavities almost disappear due to acoustic streaming effect of ultrasonic. When the ultrasonic power is 180 W, the porosity reaches a minimum of 0.1±0.05% and the relative density is 99.9±0.1%.

To evaluate the effects of pulsed laser welding on the microstructure characteristics and mechanical properties of nickel-based superalloy (C-276) and austenitic stainless steel (304) lap-welded joint, the analyses of morphology, microstructure, phase structure, element distribution, tensile–shear strength and microhardness were carried out. The results indicated the unmixed zone was found at the interface between the weld metal (WM) and 304. A small number of inter-dendritic precipitates were also found, and the precipitates were most probably por μphases. Compared with the phase structure in C-276, the dominated phase structure in the WM was also austenite phase, however, the lattice constant was reduced due to the decrease of Ni, Mo and W content. The macro-element distribution was inhomogeneous in the WM due to the rapid cooling rate in pulsed laser welding, and the element transition region adjacent to C-276 was wider than that adjacent to 304. The fracture mode of lap-welded joint in tensile–shear test was tensile fracture, and the tensile strength was decreased by 17% compared with that of C-276. The microhardness value in the WM was not lower than that in the base metals.

Achieving high strength in the precipitation-strengthened Al-Zn-Mg-Cu alloys produced via additive manufacturing (AM) remains challenging. This work focused on an innovatively developed pulsed laser-arc hybrid process for Al-Zn-Mg-Cu alloy, and further heat treatment for improving the properties was proposed. The as-deposited sample displayed an alternating microstructure featuring coarse columnar grains in the arc zone (AZ) and equiaxed grains in the laser zone (LZ). By simulating the thermal history, it was determined that the formation mechanism of the microstructure was mostly due to the sharp temperature gradient and solidification rate during the pulsed laser duration, as well as the accelerated fluid flow. The stable Zn- and Mg-enriched phases were dispersed within the grains in the LZ, whereas aggregated as the network in the AZ. The two-stage solid solution and aging treatments promoted the generation of numerous disk-like η′ nanoprecipitates and minor needle-like GP-II zone. Moreover, investigations revealed that heat treatment not only contributed to simultaneously increasing the strength and elongation, but also significantly decreasing the anisotropy. This results in a favorable strength (ultimate tensile strength of 602.3 ± 7.6 MPa), which was superior to that of conventional wrought 7075 aluminum alloy. Yield strength modeling analysis demonstrated that the enhanced properties were chiefly attributable to the synergistic effect of Orowan strengthening by the η′ precipitates and dislocation shearing given by the coherent GP-II zone. Our approach of laser-arc hybrid will be applicable to other alloy systems where AM is difficult to perform, and it will have a wide range of applications in the aerospace industry.

•Corresponding finite element analysis model of A/AT OMGCs-LDED was developed.•Relationship between microstructure, stress level and cracking behavior was revealed.•Preferred energy input process window with better shaping quality was determined.