High-performance engineering alloys, such as 7000 series aluminum alloys, suffer poor printability in laser powder bed fusion (LPBF) additive manufacturing. An enormous challenge lies in the suppression of solidification cracks caused by solidification shrinkage and thermal stresses. Porosity formation, as one of the main concerns for LPBF application, should also be avoided at the same time. In this study, aluminum alloy (AA) 7075 with and without Zr modification was additively manufactured by LPBF. Processing parameters of laser power and scanning speed, resulting in various volumetric energy density (VED), were experimentally determined to produce crack-free components with tailored microstructure. Optical microscopy was used to reveal how the crack density and porosity vary with VED. Scanning electron microscopy and transmission electron microscopy uncovered the detailed microstructure in the molten pool and the evolution of the elemental Zr addition. The results indicate that 1 w.t.% addition of elemental Zr in AA7075 led to lower crack density compared with 0.3 w.t.% addition. In 1 w.t.% Zr-modified AA7075, crack-free components were obtained under high VED. Fine equiaxed grains, instead of large columnar grains, were formed at the bottom of the molten pool boundary due to the existence of Al3Zr compound, which favored the nucleation of aluminum grains and elimination of cracks. The phenomenon of silicon segregation near cracks remained in Zr modified alloys, although its effects on cracking were suppressed. Spherical pores in the Zr-modified AA7075 increased due to the deterioration of fluidity by unmelted particles, which distracted the Marangoni flow as well. Sufficient laser energy input can increase the viscosity and ease the pores escaping. By optimizing parameters, crack-free AA7075 parts with low porosity can be manufactured through LPBF with Zr addition.

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Global Navigation Satellite System–Acoustic (GNSS-A) positioning technique is an important tool for monitoring the submarine tectonic movement and seismic. The submarine positioning accuracy however is seriously affected by the sound speed variation. This contribution investigates the influence of sound speed variation on the seafloor geodetic positioning and proposes diversified acoustic delay models, including two five-parameter models and two three parameter models. It shows that the five-parameter zenith delay model can be degenerated into a three-parameter zenith delay model in the single-point positioning case or under the single-layer sound speed field (SSF) assumption. The proposed zenith delay models are verified by the Japanese opened seafloor geodesy observation-array (SGO-A) data and the sound speed gradient relative to the reference sound speed profile (SSP) is obtained. Experimental tests show that, both the proposed five-parameter zenith delay and three-parameter zenith delay models can achieve a three-dimensional positioning precision at centimeter-level and they can be used to produce a more stable long-term horizontal coordinate time series relative to the GNSS-A ranging combined positioning solver (GARPOS V1.0.0).