Metal additive manufacturing (MAM) technology has great application potential in the aerospace, medical and energy fields with its high material utilization efficiency to achieve the manufacturing of metal parts of any shape. However, the extreme thermal, mechanical, and metallurgical coupling in MAM process leads to large residual stresses in the manufactured samples. Residual stress has a significant effect on the dimensional stability, corrosion resistance, crack growth resistance and mechanical properties of MAM samples. As a result, residual stress can be regarded as a key factor in controlling costs, enhancing product efficiency and quality. To help researchers and engineers attain up-to-date information and knowledge about residual stress in MAM, the current paper provides a comprehensive review in this field, especially the formation mechanisms, the influence of process parameters, prediction and control methods. 

Review on residual stresses in metal additive manufacturing: formation mechanisms, parameter dependencies, prediction and control approaches

Grain refinement and arrangement is an effective strategy to enhance tensile and fatigue properties of key metallic components. Laser shock peening (LSP) is one of surface severe plastic deformation methods in extreme conditions, with four distinctive features, namely, high pressure (1 GPa–1 TPa), high energy (more than 1 GW), ultra-fast (no more than nanosecond scale), and ultra-high strain rate (more than 106 s−1), and generates a deeper compressive residual stress (CRS) field and the formation of a gradient nanostructure in the surface layer to prevent the crack initiation of metallic materials and alloys, which is widely used in aerospace, overload vehicle, ocean engineering, and nuclear power. Despite some investigations of LSP on surface integrity, microstructural evolution, and mechanical properties of metallic materials and alloys, there is a lack of a comprehensive perspective of LSP-induced microstructural evolution, mechanical properties for metallic materials and alloys in the last two decades. Furthermore, the relationship between the mechanical properties of metallic materials and alloys and the LSP processing parameters is presented. In particular, LSP-induced featured microstructure and grain refinement mechanisms in three kinds of crystal structures, for instance, face-centred cubic, body-centred cubic, and hexagonal close-packed metals, are present and summarised for the first time. In addition, some new emerging hybrid LSP technologies and typical industrial applications as important chapters are shown. Finally, the faced challenges and future trends in the next 10–20 years are listed and discussed. Results to date indicate that LSP, as an emerging and novel surface modification technology, has been increasingly used to surface layer of metallic components. These topics discussed could provide some important insights on researchers and engineers in the fields of surface modification and advanced laser manufacturing. 

Progressive developments, challenges and future trends in laser shock peening of metallic materials and alloys: A comprehensive review

Achieving high strength and ductility in selective laser melting Ti-6Al-4V alloy by laser shock peening

Ultrasonic surface rolling process (USRP) is a novel surface severe plastic deformation (SPD) method that integrates ultrasonic impact peening (UIP) and deep rolling (DR) to enhance the surface integrity and surface mechanical properties of engineering materials. USRP can induce gradient nanostructured surface (GNS) layers on the substrate, providing superior mechanical properties, thus preventing premature material failure. Herein, a comprehensive overview of current-state-of-the art USRP is provided. More specifically, the effect of the USRP on a broad range of materials exclusively used for aerospace, automotive, nuclear, and chemical industries is explained. Furthermore, the effect of USRP on different mechanical properties, such as hardness, tensile, fatigue, wear resistance, residual stress, corrosion resistance, and surface roughness are summarized. In addition, the effect of USRP on grain refinement and the formation of gradient microstructure is discussed. Finally, this study elucidates the application and recent advances of the USRP process.

Ultrasonic Surface Rolling Process: Properties, Characterization, and Applications

In the recent decades, the laser-directed energy deposition (L-DED) of aluminium alloy has encountered significant challenges owing to the defects and tensile residual stress. In this study, the in-situ laser shock peening method is employed for the first time to improve the surface quality and mechanical properties of L-DED-fabricated AlSi10Mg alloy. The physical mechanism of the compressive pressure on the pore defects is accounted for using the finite element method. The numerical results indicate that severe plastic deformation driven by the shock wave contributes to defect healing, and the maximum pressure plays a crucial role. The effects of surface laser shock peening (SLSP) and layer-by-layer laser shock peening (LLSP) on the densification, surface quality, including the roughness and residual stress, microstructure, and mechanical properties of the samples are experimentally studied. After the LLSP treatment, the surface quality and mechanical properties of the samples exceed those of common aluminium alloys fabricated by laser powder bed fusion (L-PBF). After the SLSP treatment, a compressive residual stress with a maximum value of 30 ± 14 MPa is formed on the upper surface, and the LLSP treatment overcome the depth limitation of the affected zone. Furthermore, the LLSP treatment addresses the strength–ductility trade-off, and realizes an excellent combination of strength and ductility through grain refinement, dislocation strengthening, and compressive residual stress. Therefore, this work establishes the viability of in-situ laser shock peening for the rapid additive manufacturing of large-size aluminium alloys. The data used to support the findings of this study are available from the corresponding author upon request. 

In-situ laser shock peening for improved surface quality and mechanical properties of laser-directed energy-deposited AlSi10Mg alloy

The enhancement of the mechanical performance of materials is a continuous pursuit in engineering. Laser shock peening (LSP) is an advanced manufacturing method employing extreme conditions, which effectively improves the mechanical properties of the material surface. Herein, LSP was used to treat aircraft 5A06 aluminum alloy, leading to significant enhancement of properties. In the LSP process, the pulse shock wave caused severe plastic deformation of the metal surface, where the strain rate reached 107 s−1. The dislocations multiplied in large numbers and then evolved into large-angle grain boundaries and other structures to achieve grain refinement. Also, LSP significantly improved the residual stress of metals, causing a series of mechanical improvement effects. Under the impact of fine-grain strengthening, the resultant 5A06 aluminum alloy's micro-hardness was increased to 140 HV, a value 65% higher than that of bare aluminum. After the LSP process, the maximum plastic deformation layer depth (PDLD) reached 800 μm. On this basis, the ultimate tensile strength of aluminum alloy was increased by 27%, and the ultimate tensile displacement was also increased. The LSP treatment increased the strength of aluminum alloy without losing ductility. The resultant sample's friction coefficient was reduced by 34%, and the overall surface-displayed uniform wear. This work reveals the structure-activity relationships between the surface residual stress and metal mechanical properties and clarifies the LSP process's strengthening mechanism.

Laser shock peening regulating aluminum alloy surface residual stresses for enhancing the mechanical properties: Roles of shock number and energy

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Laser shock peening regulating aluminum alloy surface residual stresses for enhancing the mechanical properties: Roles of shock number and energy