Fengze Dai

Effects of Laser Shock Processing on Mechanical Properties of Laser Welded ANSI 304 Stainless Steel Joint

With the rapid development of engineering component with integration, high-speed and multi-parameter, traditional techniques haven’t met practical needs in extreme service environment. Laser welding, a new welding technology, has been widely used. However, it would generate the drop of mechanical properties for laser welded joint due to its thermal effect. Laser shock processing (LSP) is one of the most effective methods to improve the mechanical properties of laser welded ANSI 304 stainless steel joint. In this paper, the effects of LSP on the mechanical properties of laser welded ANSI 304 stainless steel joint have been investigated. The welded joint on the front of the tensile samples is treated by LSP impacts, and the overlapping rate of the laser spot is 50%. The tensile test of the laser welded joint with and without LSP impacts is carried out, and the fracture morphology of the tensile samples is analyzed by scanning electron microscope (SEM). Compared with the yield strength of 11.70 kN, the tensile strength of 37.66 kN, the yield-to-tensile strength ratio of 0.310 7, the elongation of 25.20%, the area reduction of 32.68% and the elastic modulus of 13 063.876 MPa, the corresponding values after LSP impacts are 14.25 kN, 38.74 kN, 0.367 8, 26.58%, 42.29% and 14 754.394 MPa, respectively. Through LSP impacts, the increasing ratio of the yield strength and tensile strength are 121.79% and 102.87%, respectively; the elongation and area reduction are improved by 5.48% and 29.38%, respectively. By comparing with coarse fracture surface of the welded joint, the delamination splitting with some cracks in the sharp corner of the welded joint and asymmetric dimples, LSP can cause brighter fracture surface, and finer and more uniform dimples. Finally, the schematic illustration of dimple formation with LSP is clearly described. The proposed research ensures that the LSP technology can clearly improve the yield strength, tensile strength, yield-to-tensile strength ratio, elongation, area reduction and elastic modulus of the welded joint. The enhancement mechanism of LSP on laser welded ANSI 304 stainless steel joint is mainly due to the fact that the refined and uniform dimples effectively delay the fracture of laser welded joints.

Effect of submicron SiC particle on friction and wear properties of copper matrix composites under oil-lubricated condition

Submicron SiCp/Cu composite was fabricated by means of powder metallurgy route followed by hot extrusion, and then its morphology and mechanical properties were investigated. The results showed that densification of the composite was achieved by this fabricating process when the content of submicron SiC particle was below 5 vol.%. Furthermore, the friction and wear properties of all the submicron SiCp/Cu samples were carefully investigated with lubricant of 20# machinery oil. It was found that 5 vol.% SiCp/Cu samples exhibit excellent wear properties. This phenomenon can be explained by the following fact that a new kind of tribolayer is formed, in which submicron SiC particles are well dispersed during the wear test. In addition, the wear mechanism under lubricated condition was also discussed.

Effects of Laser Shock Processing on Morphologies and Mechanical Properties of ANSI 304 Stainless Steel Weldments Subjected to Cavitation Erosion

Effects of laser shock processing (LSP) on the cavitation erosion resistance of laser weldments were investigated by optical microscope (OM), scanning electron microscope (SEM) observations, roughness tester, micro hardness tester, and X-ray diffraction (XRD) technology. The morphological microstructures were characterized. Cumulative mass loss, incubation period, erosion rate, and damaged surface areas were monitored during cavitation erosion. Surface roughness, micro-hardness, and residual stress were measured in different zones. Results showed that LSP could improve the damage of morphological microstructures and mechanical properties after cavitation erosion. The compressive residual stresses were generated during the process of LSP, which was an effective guarantee for the improvement of the above mentioned properties.

ANALYSIS OF SURFACE ROUGHNESS AT OVERLAPPING LASER SHOCK PEENING

The overlapping effects on surface roughness are studied when samples are treated by laser shock peening (LSP). Surface roughness of overlapped circular laser spot is calculated by ISO 25178 height parameters. The usually used overlapping styles namely isosceles-right-triangle-style (AAP) and equilateral-triangle-style (AAA) are carefully investigated when the overlapping degree in x-axis (ηx) is below 50%. Surface roughness of isosceles-right-triangle-style attains its minimum value at ηx of 29.3%, and attains its maximum value at ηx of 43.6%. Surface roughness of equilateral-triangle-style attains its minimum value at ηx of 42.3%, and attains its maximum value at ηx of 32%. Experimental results are well consistent with theoretical analysis.

Micro-dent arrays fabricated by a novel net mask laser shock processing on the surface of LY2 aluminum alloy

A novel technology called net-mask laser shock processing (NMLSP) was introduced to fabricate micro-dent arrays on the surface of LY2 aluminum alloy. Experimental results showed that the as-fabricated micro-dents whose diameter and depth were about 230-250 μm and 9.3 μm, respectively, was closed to be circular although the original shape of the net mask was square. The height of upwarped area around micro-dent was about 4 μm. Moreover, the interference of neighboring surface shock waves would affect the topography of micro-dents. A dynamic analysis performed by ABAQUS/Explicit code exhibited that the dynamic formation process of micro-dents fabricated by NMLSP, and the simulation results were mostly consistent with experiment results.

Effects of processing parameters on fatigue properties of LY2 Al alloy subjected to laser shock processing

We address the effects of processing parameters on residual stresses and fatigue properties of LY2 Al alloy by laser shock processing (LSP). Results show that compressive residual stresses are generated near the surface of samples due to LSP. The maximum compressive residual stress at the surface by two LSP impacts on one side is higher than that by one LSP impact. The maximum value of tensile residual stress is found at the mid-plane of samples subjected to two-sided LSP. Compared with fatigue lives of samples treated by single-sided LSP, lives of those treated by two-sided LSP are lower. However, these are higher than untreated ones.

High temperature wear performance of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy subjected to laser shock peening

ABSTRACT This paper investigates the influence of laser shock peening (LSP) parameters on surface integrities and high temperature wear performances of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy. Surface integrities include surface roughness, residual stress and micro-hardness are measured. High temperature wear performance of TC11 alloy with and without LSP are investigated. The results indicate that multiple LSP impacts can improve surface micro-hardness and induce compressive residual stress in the surface layer. Moreover, the friction coefficient and wear rate of TC11 alloy significantly decrease after LSP with optimised parameters. The improved wear performance is ascribed to the low surface roughness, higher surface micro-hardness and compressive residual stress induced by multiple LSP impacts in the surface layer.