To solve the problem of insufficient strength of thick aluminum alloy welded joints, this study introduces a pioneering technique known as variable-polarity plasma arc keyhole-tungsten inert gas cover hybrid welding (VPPA keyhole-TIG covering welding), which is designed for welding large-thickness aluminum alloys without any weld groove. Using 13-mm-thick 2319 aluminum alloy plates as experimental materials, plasma arc butt welding experiments were conducted by varying the plasma gas flow rate and application of cover welding. The research findings underscored that the use of cover welding, coupled with an increase in the plasma gas flow rate, markedly improved the overall performance of the weld. An average tensile strength of 299.0±4.0MPa and an elongation rate of 4.5±0.3% were achieved for the welded joint. A microscopic analysis revealed that the heightened plasma gas flow rate did not precipitate the θ' phase but instead resulted in undesired grain size enlargement, leading to reduced joint hardness. The secondary thermal cycle effect of covering welding played a pivotal role in facilitating the nucleation of needle-like θ' strengthening phases within the matrix, ranging in size from 50 to 150nm. This phenomenon has emerged as a principal factor contributing to the substantial enhancement of the mechanical properties of welded joints.

Directed Energy Deposition-Arc (DED-Arc) was utilized to fabricate Inconel-copper bimetallic structures, with the aim of improving manufacturing modes and reducing costs for rocket engine thrust chambers. In this paper, bimetallic Inconel superalloy (GH4169) and copper alloy (C18150) structures were deposited using different deposition strategies. The formation mechanisms of grain-size gradients and heterogeneous interfacial microstructural evolution were comprehensive investigated and further discussed. Both strategies produced high-quality bimetallic samples with tensile strength exceeding 260.2 MPa. Fractures occurred within the C18150 metal rather than at the interface, indicating successful interface strengthening of the bimetallic samples. Gradual gradients in grain size distribution and nano-hardness were observed across the interface. The crystal structures of C18150 were unaffected by GH4169, and no new phases were generated in the interfacial region. The crystallographic orientation relationship between Ni and Cu was determined to be [011]Cu//[011]Ni and (111)Cu//(111)Ni, and interfaces were strengthened by grain boundary and dislocation strengthening through. This work demonstrates an approach for fabricating large, high-performance bimetallic structures with tailored grain-size gradients and heterogeneous microstructures, suitable for rocket engine thrust chamber applications.

The variable-polarity plasma arc (VPPA) keyhole welding technology offers an efficient solution for producing defect-free aluminum alloys, but a keyhole is left at the end of the welding process. Traditional manual TIG welding, which involves filling keyholes, often lacks productivity and can result in inconsistent weld quality, hindering the production of high-strength and high-precision parts. In this study, a keyhole self-closing method was proposed, and the microstructure and mechanical properties of keyholes closed by the proposed method were compared to those closed by manual TIG welding. The results showed that the self-closed keyhole had a smaller fusion line wide, and the weld zone had a uniform distribution of equiaxed crystal, with fewer pore defects. While the manual TIG welding-filled keyhole had a higher density of dislocations, which leads to an increase in the microhardness. The presence of pores in the manual TIG welding-filled keyhole reduces its tensile strength and elongation, which were lower than those of the self-closed keyhole by 61.5 MPa and 5.8 %, respectively. The self-closing method can effectively address the keyhole retention issue in VPPA keyhole welding. By using this method, higher-quality welds with better microstructure and mechanical properties can be produced, which can have important implications for various industrial applications, and in applications where mechanical properties are critical.

Arc oscillation is an important process parameter that can significantly improve the quality of the deposition. However, the mechanism and characteristics of arc oscillation on the microstructure and mechanical properties are unclear during wire arc additive manufacturing (WAAM) process. In this study, the microstructure evolution and mechanical properties of GH4169 superalloy fabricated by WAAM with arc oscillation were investigated and compared with the corresponding as deposited samples. Results showed that the tensile properties (ultimate tensile strength and yield strength) of the deposition with arc oscillation were improved with retention in elongation above 45%. With the arc oscillation, Laves phase was significantly suppressed and the content of Laves phase decreased. Further, arc oscillation induced texture randomization with lower texture intensity (10.15 mud), achieving a prominent reduction by 36.7%. From the present study, it is evident that arc oscillation during WAAM is a potential manufacturing route that can effectively tailor the microstructure.

Interfacial bonding reliability is a critical issue in bimetallic structures. To understand the effect of deposition sequence on interfacial characteristics, bimetallic structures of Inconel and copper alloys were fabricated by directed energy deposition arc (DED-Arc). The interfacial element diffusion behaviors and formation mechanisms were investigated in detail. The bimetallic structure of C18150 on GH4169 exhibits a sound metallurgical bonding interface with a wide interfacial region (similar to 400 mu m), which is attributed to Marangoni convection and intense elemental interdiffusion. The study highlights the feasibility of fabricating large Inconel-copper functional bimetallic structures with sound metallurgical bonding using DED-Arc and it provides a theoretical reference for design and fabrication of bimetallic structures with material-property mismatches.

Adding heterogeneous nucleation refiners is an effective route to obtain fine grained microstructure. To investigate effect of silicon carbide (SiC) particles on mechanical properties of Al–Mg alloy, metallic matrix composites (MMCs) were fabricated by an innovative powder and wire cold metal transfer (CMT) process with enhanced mechanical properties. Microstructure evolution, grain orientation and mechanical properties of CMT processed MMCs reinforced by SiC particles at different concentrations were discussed. The mechanical properties of Al–Mg alloy reached the highest tensile strength of 325.6 ± 16 MPa by addition of SiC particles with a volume fraction of 8% and an average grain diameter of 49.1 μm. This research provided a novel, time-saving, low-cost approach to fabricate Al–Mg composites and expands application of Al–Mg alloy.