TC4 titanium alloy is widely used in the aerospace industry due to its high specific strength and good heat and corrosion resistance. Laser additive manufacturing has the advantages of digital design and manufacturing integration, which can greatly improves the utilization rate of raw materials and is particularly suitable for manufacturing the structural parts of titanium alloy. It has become one of the core technologies to enhance the design and manufacturing capability of high-performance complex components. However, during laser additive manufacturing of titanium alloy, there exists a large temperature gradient between the melt pool and the substrate, which results in the poor comprehensive mechanical properties of formed parts. Therefore, it is very important to find a suitable method to improve the comprehensive mechanical properties and extend the service life. Laser shock peening (LSP) is an important method for the post-treatment of metal parts, which uses a high-power short-pulse laser on the surface of the material to produce compressive residual stresses and work hardening layers, and thus it possesses the outstanding advantages of controllability and significant strengthening effect. In this paper, the effect mechanism of LSP on the microstructure and properties of laser additive manufactured TC4 (LAM-TC4) titanium alloy is systematically investigated by adopting the LSP surface treatment. It is expected to first improve the mechanical properties of the surface layer of the LAM-TC4 titanium alloy, then improve the comprehensive mechanical properties, and finally provide an experimental basis for optimizing the microstructure and mechanical properties of LAM-TC4 titanium alloy.

In this work, LSP is first applied to the surface of the LAM-TC4 titanium alloy. Then, the physical phases of block samples before and after LSP are analyzed by the X-ray diffractometer (XRD), and the microstructures of block samples before and after LSP are observed by the optical microscope (OM). The block samples before and after LSP are manually ground to 50 μm thick ones with different types of sandpapers and punched into ?3 mm discs. Then they are thinned by the Gatan 691 ion thinning instrument, and the microstructure is further investigated using the JEM-2010 transmission electron microscope (TEM). Finally, the mechanical properties and fracture morphologies of the samples before and after LSP are characterized by the X-ray stress analyzer, the microhardness tester, the tensile testing machine, and the scanning electron microscope (SEM).

The original microstructure of the LAM-TC4 titanium alloy consists of a large number of thick α laths and a certain volume fraction of inter-lath β phases [Fig. 4(a)]. After the LSP treatment, the microstructure of the surface layer is broken and refined by the action of high-energy shock waves [Fig. 5(b)], and a large number of dislocations [Fig. 5(bd)] and deformation twins [Fig. 5(d)(f)] are formed. The LSP treatment changes the tensile residual stress of the LAM-TC4 titanium alloy into the compressive residual stress (Fig. 7). After the LSP treatment, the surface of the LAM-TC4 titanium alloy has the maximum compressive residual stress (-190 MPa) (Fig. 7), the microhardness is increased by 16.5% (Fig. 8), and the microhardness decreases with the increase of depth. In addition, after the LSP treatment, the yield strength and tensile strength of LAM-TC4 titanium alloy are increased by 46.3% and 32.3%, respectively, compared with those of the original sample, and the plasticity remains basically unchanged (Fig. 9). The fracture morphologies of the LAM-TC4 titanium alloy before and after LSP are mainly composed of deep equiaxed dimples, and the fracture mechanism is typical of ductile fracture (Fig.10).

The effect of LSP on the microstructural evolution and properties of the LAM-TC4 titanium alloy is systematically investigated by the surface treatment based on LSP. It is shown that the LSP treatment causes severe plastic deformation in the surface layer of the LAM-TC4 titanium alloy, generates a large number of deformation twins in the surface matrix, the dislocation density increases significantly, and the interaction among various dislocation structures develops to form (sub)grains resulting in grain refinement. The maximum compressive residual stress (-190 MPa) exists on the surface of the LAM-TC4 titanium alloy after the LSP treatment, and the surface microhardness value of the specimen increases significantly, reaching the maximum value of 380.7 HV. Both the residual stress and microhardness decrease with the increase of depth, and the corresponding values decrease with the increase of distance from the surface layer. The yield strength and tensile strength of the LAM-TC4 titanium alloy after the LSP treatment are increased by 46.3% and 32.3%, respectively, compared with those of the original sample, while the plasticity remains basically unchanged. The fracture morphology is still dominated by deep equiaxial dimples, and the alloy obtains a better match between strength and plasticity.