激光冲击对不锈钢焊接接头残余应力场的影响

针对不锈钢焊接接头存在残余应力且分布不均匀、容易发生应力腐蚀的问题,采用激光冲击强化对其进行处理,探究激光功率密度和冲击次数对表面残余应力状态的优化作用,并通过应力腐蚀试验验证优化效果。结果表明：随着功率密度增加,表面残余应力明显下降,但下降幅度逐渐减小,功率密度4.24 GW/cm2与2.83 GW/cm2冲击产生的残余应力相差不大,熔合区还存在残余拉应力,说明高功率密度不足以消除表面残余拉应力;随着冲击次数增加,残余拉应力显著降低,2.83 GW/cm2冲击3次之后,残余拉应力完全消除,局部最高应力梯度从54.7 MPa/mm下降到11.7 MPa/mm,获得了高数值、分布均匀的残余压应力层。激光冲击强化后,焊接试样的应力腐蚀断裂时间提高了33.48%,激光冲击强化产生的残余压应力是其应力腐蚀抗性提高的重要原因。     

不锈钢焊接件不等强度激光冲击与应力腐蚀试验研究

针对不锈钢焊接接头应力及组织分布不均匀,容易导致应力腐蚀开裂的问题,采用不等强度激光冲击波对316奥氏体不锈钢焊接接头进行处理。通过应力腐蚀试验、残余应力测试及微观组织分析,研究了激光冲击强化对焊接接头应力腐蚀抗性的影响及其作用机理。试验结果表明：激光冲击强化将焊接件的应力腐蚀断裂时间提高了33.48%。激光冲击波的作用,在焊接接头部位引入了高数值的残余压应力,一方面消除了热影响导致的残余拉应力,同时抵消了拉伸工作载荷的作用,降低局部应力梯度,从而延缓表面钝化膜的破裂;另一方面,激光冲击使焊接接头不同区域之间的微观组织均匀和细化,提高了微裂纹萌生的条件,降低了金属发生阳极溶解的可能性。两种因素的共同作用,使得不锈钢焊接接头的抗应力腐蚀性能显著增强。     

不锈钢焊接件不等强度激光冲击与应力腐蚀试验研究

针对不锈钢焊接接头应力及组织分布不均匀,容易导致应力腐蚀开裂的问题,采用不等强度激光冲击波对316奥氏体不锈钢焊接接头进行处理。通过应力腐蚀试验、残余应力测试及微观组织分析,研究了激光冲击强化对焊接接头应力腐蚀抗性的影响及其作用机理。试验结果表明:激光冲击强化将焊接件的应力腐蚀断裂时间提高了33.48%。激光冲击波的作用,在焊接接头部位引入了高数值的残余压应力,一方面消除了热影响导致的残余拉应力,同时抵消了拉伸工作载荷的作用,降低局部应力梯度,从而延缓表面钝化膜的破裂;另一方面,激光冲击使焊接接头不同区域之间的微观组织均匀和细化,提高了微裂纹萌生的条件,降低了金属发生阳极溶解的可能性。两种因素的共同作用,使得不锈钢焊接接头的抗应力腐蚀性能显著增强。     

激光冲击7075-T6铝合金焊缝的残余应力场数值模拟

针对激光冲击强化铝合金焊缝过程中存在多场耦合导致残余应力变化幅度大、实时表征难的问题,采用ABAQUS非线性有限元软件建立了激光冲击强化7075-T6铝合金板材焊接件的有限元模型,模拟计算焊缝在激光冲击强化前后的残余应力场分布,重点分析了激光冲击强化对焊缝的横、纵向残余应力场的影响规律,并对焊缝的激光冲击强化工艺参数进行优化。研究结果表明,焊缝表面为残余拉应力场并且分布不均,激光冲击强化使得焊缝的拉应力状态转变为高幅压应力状态;优化激光冲击强化工艺参数(如激光能量、激光光斑尺寸、搭接率),可以明显改善焊缝区域和热影响区的残余应力分布;当激光光斑搭接率为50%至70%时,残余应力分布趋于均匀,可以有效消除"残余应力洞"现象。     

激光冲击强化“残余应力洞”的形成机制

利用ABAQUS有限元软件进行了单个圆形高斯光斑的激光冲击强化数值模拟,分析材料表面光斑中心区域形成的"残余应力洞"现象,并通过分析材料的动态力学响应特征揭示了"残余应力洞"的形成机制。结果表明:在冲击波加载时,光斑边界处会产生很强的剪切应力,形成向四周传播的表面稀疏波和向材料内部传播的剪切波。当稀疏波同时传播到光斑中心,发生相遇、汇聚,使材料产生急剧的上下位移过程,造成冲击波加载塑性变形后的二次塑性变形。二次塑性变形中形成了较大的剪切塑性应变,并降低了冲击波加载阶段产生的轴向和径向塑性应变,使残余压应力降低,从而形成"残余应力洞"。     

2024铝合金激光多次冲击诱导残余应力状态分布规律

采用高功率激光器多次冲击2024铝合金,用X射线衍射技术分析了冲击区域的残余应力,研究了冲击残余应力状态分布规律,并用其评价激光冲击强化效果。研究表明,随着冲击次数增加,塑变量及塑性应变梯度逐渐减小,测点是双向压应力状态,而4次冲击时,塑性应变梯度增大,光斑中心是单向压应力状态,其他点是双向压应力状态。当激光功率密度为2.8 GW/cm2时,3次冲击强化效果最佳,材料是二向压应力状态,残余最大主应力及应力强度的均值最大,方差最小,分布基本均匀,塑性应变梯度较小。     

2024铝合金激光多次冲击诱导残余应力状态分布规律

采用高功率激光器多次冲击2024铝合金,用X射线衍射技术分析了冲击区域的残余应力,研究了冲击残余应力状态分布规律,并用其评价激光冲击强化效果。研究表明,随着冲击次数增加,塑变量及塑性应变梯度逐渐减小,测点是双向压应力状态,而4次冲击时,塑性应变梯度增大,光斑中心是单向压应力状态,其他点是双向压应力状态。当激光功率密度为2.8 GW/cm2时,3次冲击强化效果最佳,材料是二向压应力状态,残余最大主应力及应力强度的均值最大,方差最小,分布基本均匀,塑性应变梯度较小。     

激光冲击诱导弹簧钢丝残余应力状态分析

利用高功率Nd:YAG激光对不同工艺处理的SWOSC-V弹簧钢丝进行单点冲击处理,用X射线应力分析仪测量弹簧内外侧、侧表面的残余应力并计算出残余主应力,建立了激光冲击SWOSC-V弹簧钢丝表面残余应力的产生模型,并利用该模型分析了弹簧钢丝表面残余应力产生的原因。结果表明:弹簧钢丝在经激光冲击处理的表面强化区产生残余压应力,钢丝退火后直接激光冲击处理与经喷丸强化的钢丝激光冲击处理的表面残余应力变化不同,喷丸强化所引起的材料硬化是激光冲击处理弹簧钢丝残余应力变化不同的原因。     

激光冲击诱导弹簧钢丝残余应力状态分析

利用高功率Nd：YAG激光对不同工艺处理的SWOSC-V弹簧钢丝进行单点冲击处理,用X射线应力分析仪测量弹簧内外侧、侧表面的残余应力并计算出残余主应力,建立了激光冲击SWOSC-V弹簧钢丝表面残余应力的产生模型,并利用该模型分析了弹簧钢丝表面残余应力产生的原因。结果表明:弹簧钢丝在经激光冲击处理的表面强化区产生残余压应力,钢丝退火后直接激光冲击处理与经喷丸强化的钢丝激光冲击处理的表面残余应力变化不同,喷丸强化所引起的材料硬化是激光冲击处理弹簧钢丝残余应力变化不同的原因。     

大耕深旋耕刀激光冲击强化残余应力研究

研究了激光冲击强化对回转半径为350mm大耕深旋耕刀残余应力的影响,利用ANSYS对旋耕刀进行理论应力分析,发现应力集中在刀柄外弯角处,同时进行了激光冲击强化模拟,得到理论残余应力引入值。利用X射线衍射法对激光冲击强化前后旋耕刀表面进行了残余应力测试。结果表明,ANSYS模拟激光冲击强化引入的残余应力值与试验实测的结果吻合得较好,相比未经处理的旋耕刀,激光冲击强化后刀具表面残余应力明显增大,最大残余压应力达412.25 MPa,增幅达166%,残余压应力的引入可消除旋耕刀工作的应力集中,提高了刀具使用寿命。研究结果为进一步提高和优化大耕深旋耕刀制造工艺提供了新的参考依据。     

Effect of an absorbent overlay on the residual stress field induced by laser shock processing on aluminum samples

Laser shock processing (LSP) or laser shock peening is a new technique for strengthening metals. This process induces a compressive residual stress field, which increases fatigue crack initiation life and reduces fatigue crack growth rate. Specimens of 6061-T6 aluminum alloy are used in this investigation. A convergent lens is used to deliver 2.5 J, 8 ns laser pulses by a Q-switch Nd:YAG laser, operating at 10 Hz. The pulses are focused to a diameter of 1.5 mm onto aluminum samples. Density of 2500 pulses/cm² with infrared (1064 nm) radiation was used. The effect of an absorbent overlay on the residual stress field using this LSP setup and this energy level is evaluated. Residual stress distribution as a function of depth is assessed by the hole drilling method. It is observed that the overlay makes the compressive residual stress profile move to the surface. This effect is explained on the basis of the vaporization of the coat layer suppressing thermal effects on the metallic substrate. The effect of coating the specimen surface before LSP treatment may have advantages on improving wear and contact fatigue properties of this aluminum alloy.     

High level compressive residual stresses produced in aluminum alloys by laser shock processing

Laser shock processing (LSP) has been proposed as a competitive alternative technology to classical treatments for improving fatigue and wear resistance of metals. We present a configuration and results for metal surface treatments in underwater laser irradiation at 1064 nm. A convergent lens is used to deliver 1.2 J/cm² in a 8 ns laser FWHM pulse produced by 10 Hz Q-switched Nd:YAG, two laser spot diameters were used: 0.8 and 1.5 mm.Results using pulse densities of 2500 pulses/cm² in 6061-T6 aluminum samples and 5000 pulses/cm² in 2024 aluminum samples are presented. High level of compressive residual stresses are produced −1600 MPa for 6061-T6 Al alloy, and −1400 MPa for 2024 Al alloy. It has been shown that surface residual stress level is higher than that achieved by conventional shot peening and with greater depths. This method can be applied to surface treatment of final metal products.     

Characterization of Kovar-to-Kovar laser welded joints and its mechanical strength

For the packaging of a pump laser in butterfly package, the most crucial assembly step is the fiber-to-laser diode coupling and attachment. The use of laser welding as the joining method offers several advantages if compared with the adhesive joints: strong joining strength, short process time and less contamination. This paper reports on laser welding process characteristics; weld strength and its fracture mode. The penetration depth and melt area of laser spot welds were found to be complicated functions of laser pulse energy, intensity, and beam diameter. Effects of pulse width, input power and size of the focal spot on the rate of energy input to the workpieces and consequently, the weld strength were reported. The weld strength was found to be dependent on the overlapping area between the two joining materials. Surface roughness, R_a, has influence on the fraction of energy absorbed, A, and therefore, affecting the penetration depth. Thermal analysis was carried out on the laser-welded joints and its heat-affected zone (HAZ) induced by various power densities was examined. These data are important in order to optimize and utilize the laser welding process as an effective manufacturing tool for fabrication of reliable pump laser.