Preheating-assisted solid-state friction stir repair of Al-Mg-Si alloy plate at different rotational speeds

Additive friction stir deposition(AFSD)is a novel structural repair and manufacturing technology has become a research hotspot at home and abroad in the past five years.In this work,the microstructural evolution and mechanical performance of the Al-Mg-Si alloy plate repaired by the preheating-assisted AFSD process were investigated.To evaluate the tool rotation speed and substrate preheating for repair quality,the AFSD technique was used to additively repair 5 mm depth blind holes on 6061 aluminum alloy substrates.The results showed that preheat-assisted AFSD repair significantly improved joint bonding and joint strength compared to the control non-preheat substrate condition.Moreover,increasing rotation speed was also beneficial to improve the metallurgical bonding of the interface and avoid volume defects.Under preheating conditions,the UTS and elongation were positively correlated with rotation speed.Under the process parameters of preheated substrate and tool rotation speed of 1000 r/min,defect-free specimens could be obtained accompanied by tensile fracture occurring in the substrate rather than the repaired zone.The UTS and elongation reached the maximum values of 164.2MPa and 13.4%,which are equivalent to 99.4%and 140%of the heated substrate,respectively.

Stress-corrosion behavior and characteristics of the friction stir welding of an AA2198-T34 alloy

To better understand the stress-corrosion behavior of friction stir welding(FSW),the effects of the microstructure on the stress-corrosion behavior of the FSW in a 2198-T34 aluminum alloy were investigated.The experimental results show that the low-angle grain boundary(LABs)of the stir zone(SZ)of FSW is significantly less than that of heated affected zone(HAZ),thermo-mechanically affected zone(TMAZ),and parent materials(PM),but the grain boundary precipitates(GBPs)T1(Al2CuLi)were less,which has a slight effect on the stress corrosion.The dislocation density in SZ was greater than that in other regions.The residual stress in SZ was+67 MPa,which is greater than that in the TMAZ.The residual stress in the HAZ and PM is-8 MPa and-32 MPa,respectively,and both compressive stresses.The corrosion potential in SZ is obviously less than that in other regions.However,micro-cracks were formed in the SZ at low strain rate,which indicates that the grain boundary characters and GBPs have no significant effect on the crack initiation in the stress-corrosion process of the AA2198-T34.Nevertheless,the residual tensile stress has significant effect on the crack initiation during the stress-corrosion process.

Influence of precipitation on recrystallization texture of AA3104 aluminum alloy

The development of the recrystallization texture of the alloy AA3104 was investigated by analysis of orientation distribution functions determined by X-ray diffraction, supported by EBSD local texture analysis. A typical β-fiber with nearly 20% Bs orientation {011}<211> was detected in the final hot rolled sheet. At the beginning of annealing at 350 °C, the cube component {001}<100> got a sharp increase. TEM results show that the growth of both number and size of precipitation appears to inhibit the advantage of Cube orientation {001}<100> notably after annealing at 350 °C for 15 min. Finally, it comes out to be a random distributed orientation by full recrystallization.

Effect of quenching cooling rate on residual stress and microstructure evolution of 6061 aluminum alloy

In this study,the cooling rate was manipulated by quenching with water of different temperatures(30,60 and 100℃).Surface and internal residual stresses in the quenched 6061 aluminum alloy samples were measured using hole-drilling and crack compliance methods,respectively.Then,the processability of the quenched samples was evaluated at cryogenic temperatures.The mechanical properties of the as-aged samples were assessed,and microstructure evolution was analyzed.The surface residual stresses of samples W30℃,W60℃and W100℃is−178.7,−161.7 and−117.2 MPa,respectively along x-direction,respectively;and−191.2,−172.1 and−126.2 MPa,respectively along y-direction.The sample quenched in boiling water displaying the lowest residual stress(~34%and~60%reduction in the surface and core).The generation and distribution of quenching residual stress could be attributed to the lattice distortion gradient.Desirable plasticity was also exhibited in the samples with relatively low quenching cooling rates at cryogenic temperatures.The strengthes of the as-aged samples are 291.2 to 270.1 MPa as the quenching water temperature increase from 30℃to 100℃.Fine and homogeneous β"phases were observed in the as-aged sample quenched with boiling water due to the clusters and Guinier-Preston zones(GP zones)premature precipitated during quenching process.

An improved process for grain refinement of large 2219 Al alloy rings and its influence on mechanical properties

The large 2219 Al alloy rings used to connect propellant tank components of a satellite launch vehicle to each other are conventionally manufactured by radial-axial ring rolling at 460°C with 50% deformation,but often suffer from coarse elongated grain and low ductility. An improved process(hot ring rolling at460°C with 30% deformation, then air cooling to 240°C, followed by ring rolling at 240°C with 20% deformation) was tested for ring manufacturing. The corresponding microstructure evolution and mechanical properties of the produced rings were studied. The results show that the improved process can successfully be applied to manufacture the large 2219 Al alloy rings without formation of macroscopic defects,resulting in a product with fine and uniform grains after heat treatment. The fracture mechanism of both rings was mainly intergranular fracture. With the resulting grain size refinement due to the improved process, more homogeneous slip occured and the crack propagation path became more tortuous during the tensile testing process. Thus, the elongation in all three orthogonal directions was greatly improved,and the axial elongation increased from 3.5% to 10.0%.

采用优化参数的裂纹柔度法测量铝铜合金锻件内部残余应力

准确测量铝合金锻件内部残余应力对于控制合金后续机加工变形具有重要作用。本文采用裂纹柔度法计算了铝铜合金锻件内部残余应力,结合实验和仿真的方法研究了裂纹柔度法模型参数对残余应力计算精度的影响。结果表明,随着引入裂纹深度的增加,计算精度先逐渐增加再逐渐降低。机加工过程中的应变数据波动导致残余应力计算精度降低,最优的裂纹深度范围为锻件厚度的71%。采用低阶插值阶数会导致残余应力曲线过于平滑,而高阶插值阶数会使残余应力曲线出现扭曲。吉洪诺夫正则法可有效抑制应变数据波动导致残余应力曲线计算失真,从而大幅提高计算精度。采用优化的裂纹柔度法测量了不同淬火方式铝铜合金锻件内部残余应力,计算结果表明使用该方法可以准确地测量合金内部残余应力。

不同应力水平下铝铜合金搅拌摩擦焊接头的蠕变时效行为

蠕变时效成形(CAF)在航空航天领域有着广泛的应用。但是,蠕变时效过程中复杂的应力状态对搅拌摩擦焊(FSW)制造的超大型尺寸构件成形是一个巨大的挑战。为了更好地制定并优化这类构件的蠕变时效成形工艺,需要系统地研究应力对焊缝组织性能演变的作用规律。本文系统地研究了不同应力水平下厚板铝铜合金搅拌摩擦焊接头的蠕变行为和微观组织演变规律。结果表明,随着外加应力的增大,焊缝的蠕变行为变化较母材更敏感。当应力增加到接近初始焊缝的平均屈服强度时(140 MPa),焊缝的蠕变量与强度都得到了显著提高。同时,在厚度方向上焊缝的蠕变行为存在差异,这与焊核区内部的不均匀梯度组织有着密不可分的联系。研究结果显示,焊核区沿厚度方向从上到下晶粒尺寸逐渐减小,而局部应变和位错密度逐渐增加,这使得焊缝底层在蠕变时效过程中表现出较高的蠕变量和稳态蠕变速率。随着外加应力的提高,焊缝蠕变时效后各区域内析出相的尺寸逐渐减小。此外,在相同的应力水平下,蠕变时效后焊核区内部的析出相形貌和尺寸分布自上而下发生了变化。其中,焊核区的上层与中层分布着大量的细小θ"相,而较大尺寸的θ'相分布在焊核区底层。因此,可以通过寻求合适的预处理工艺来增加焊缝的初始位错密度,进而提高接头的蠕变性能。

Interstitial concentration effects on incipient plasticity and dislocation behaviors of face-centered cubic FeNiCr multicomponent alloys based on nanoindentation

Interstitial atoms that commonly occupy the octahedral or tetrahedral interstices of face-centered cubic(FCC)lattice,can significantly affect the dislocation behaviors on deformation.Recently,interstitial doping has been applied to tune the mechanical properties of the emerging multicomponent,often termed high-entropy alloys(HEAs)or medium-entropy alloys(MEAs).However,the fundamental mechanisms of the dislocation nucleation and the onset of plasticity of interstitial multicomponent alloys governed by the concentration of interstitial atoms are still unclear.Therefore,in the present work,an instrumented nanoindentation was employed to investigate the interstitial concentration effects of carbon atoms on single FCC-phase equiatomic FeNiCr MEAs during loading.The results show that the pop-in events that denote the onset of incipient plasticity are triggered by the sudden heterogeneous dislocation nucleation via the primary atoms-vacancy exchange with the instant stress field,regardless of the interstitial concentration.Moreover,the measured activation volumes for dislocation nucleation of the FeNiCr MEAs are determined to be increased with the interstitial concentration,which definitely suggests the participation of interstitial atoms in the nucleation process.Besides,it is also found that the average value measured in statistics of the maximum shear stress corresponding to the first pop-in is enhanced with the interstitial concentration.Such scenario can be attributed to the improved local change transfer and lattice cohesion caused by the interstitial atoms with higher concentrations.Furthermore,the significant drag effect of interstitial carbon atoms hinders the mobile dislocations before exhaustion,which severely suppresses the subsequent occurrence of pop-in events in the carbon-doped specimens.The work gives a microscale view of interstitial effects on the mechanical properties of multicomponent alloys,which can further help to develop new interstitial strengthening strategies for structural materials with remarkable performance.