Carrying capacity is the core indicator of the value of a rocket. The weight of a rocket and its carrying capacity are trade-offs. Reducing the dead weight of a rocket and improving its structural efficiency are the keys to obtain a high carrying capacity. As an aerospace structural material, aluminum (Al) alloys are widely developed as aviation structural materials with a low melting point and high specific strength. Al alloys used as fuselage materials have become an important direction for future civil passenger aircraft material selection, and the laser melting deposition (LMD) technology needs to be developed urgently. The alloys have problems such as large hot cracking tendency and microstructural inhomogeneity that influence the properties of Al2024. In order to solve the problem of low strength of Al2024 alloys in additive manufacturing, the emerging friction stirring technology has been widely studied in which the Al2024 alloys are used. First, a two-layer gradient structure covered with a laser melting deposition layer and a stirring layer is fabricated on the substrate surface by combining the friction stirring technique. The resulting surface is then laser deposited. Friction stirring combining with laser cladding results in ultrafast laser hybrid fabrication. This gradient structure exhibits high strength, high toughness, and is resistant to fracture risk.
The diameter of the stirring head is 10 mm, and the friction stirring process parameters are the rotation speed of 800 r/min and the feeding speed of 50 mm/min. First, using the friction stirring technology, laser fusion deposited 2024 aluminum alloys repaired by different processes are prepared, and the microstructures and mechanical properties of Al alloy samples prepared by this composite process are studied. Next, the microstructures and mechanical properties of the composite-fabricated Al2024 alloys are studied with an optical microscope and a Vickers hardness tester to analyze the relationship between the process and their microstructures and properties.
The results show that the microstructures of Al2024 alloys stirred with an overlap ratio of 80% are refined and present a gradient distribution compared with those by LMD. Through the orthogonal experiment of laser fusion deposition and the single-layer multi-channel experiment, the evolution law of penetration depth and width of an Al alloy under different process conditions and the evolution law of microstructures are obtained. The spot diameter of 2.0 mm, the scanning speed of 600 mm/s, and the overlap rate of 50% are chosen as the final composite process parameters (Figs. 4 and 5). Through the microstructural analysis, it is found that the friction stirring treatment can significantly refine the grains, homogenize the structure, and reduce metallurgical defects. On the other hand, crystal grains with a directional gradient are deposited in the stirring zone, the growth direction of which is perpendicular to the substrate, and the size of the crystal grains tends to decrease. As the number of deposited layers increases, the microstructure presents fine and uniform grains without obvious directionality. Along the gradient direction, the transition layers are well combined, and the obvious gradient structure characteristics are observed as a whole (Fig. 6). After the aging treatment, the hardness of the substrate region is in the range of 135-140 HV, which is significantly improved. And with the addition of the stirring zone, the hardness after secondary deposition can reach 160 HV. In addition, as the number of depositions increases, a gradient distribution of hardness appears.
In the present study, friction stirring welding is carried out by the heat generated by the metal through friction. The difference from traditional friction welding is that it not only rubs the metal surface, but also uses a stirring rod to insert into the metal for friction (stirring), so that the depth of welding and the depth of bonding are greatly improved. Friction stirring welding does not need welding wire and has a small heat-affected range, which is especially suitable for occasions that are prone to cracking and deformation. At present, the scope of application of this technology is limited, mainly in the field of aerospace exploration and application. In this study, a novel gradient-structured aluminum alloy is successfully fabricated by the laser + stirring technology, and the surface is composed of periodic deposition layers covered with a stirring layer. After aging treatment, the hardness after secondary deposition can reach 160 HV, which indicates that LMD can also achieve ultra-high hardness. Our study shows that gradient alloys with comprehensive mechanical properties can be obtained through a rational design.
