Surface grain refinement mechanism of SMA490BW steel cross joints by ultrasonic impact treatment

Ultrasonic impact treatment (UIT) is a postweld technique for improving the fatigue strength of welded joints. This technique makes use of ultrasonic vibration to impact and plastically deform a weld toe and can achieve surface grain refinement of the weld toe, which is considered as the main reason for the improvement of fatigue strength. In this paper, the microstructure of the surface of a treated weld toe was observed by metallographic microscopy and transmission electron microscopy (TEM). The results show that UIT could produce severe plastic deformation on the surface layer of the weld toe and the maximum depth of plastic deformation extended to approximately 260 μm beneath the treated surface. Repeated processing could exacerbate the plastic deformation on the surface layer, resulting in finer grains. We can conclude that the surface grain refinement mechanism of SMA490BW welded joints is related to the high density of dislocation tangles and dislocation walls.     

Effects of size reduction on deformation,microstructure, and surface roughness of micro components for micro metal injection molding

The fabrication of 17-4PH micro spool mandrils by micro metal injection molding was described here. The effects of size reduction on deformation, microstructure and surface roughness were studied by comparing a φ500 μm micro post and a φ1.7 mm cylinder after debinding and sintering. Experimental results show that slumping of the micro posts occurred due to a dramatic increase in outlet vapor pressure initiated at the thermal degradation onset temperature and the moment of gravity. Asymmetrical stress distribution within the micro component formed during the cooling stage may cause warping. Prior solvent debinding and adjustment in a thermal debinding scheme were useful for preventing the deformation of the micro components. Smaller grain size and higher micro hardness due to impeded grain growth were observed for the micro posts compared with the φ1.7 mm cylinder. Surface roughness increased with distance from the gate of the micro spool mandril due to melt front advancement during mold filling and the ensuing pressure distribution. At each position, surface roughness was dictated by injection molding and increased slightly after sintering.     

纳米化处理超声金属表面

采用超声表面加工处理方法在45钢上制备出纳米结构表层．TEM（透射电子显微镜）、XRD（X射线衍射）、粗糙度和显微硬度试验结果表明：经超声处理后，样品表层的晶粒可细化至纳米量级，平均晶粒尺寸为50nm，且在某些区域出现纳米晶与非晶嵌套结构，表面形成残余压应力，表面粗糙度值降为原始样品的1／100，与样品心部相比，表层硬度显著提高．     

低碳钢表面机械研磨处理过程的有限元分析

运用有限元分析方法,对低碳钢表面机械研磨处理(SMAT)过程进行了数值模拟,分析计算了材料表面层的应力和应变分布情况,初步探讨了应变量和应变速率对材料塑性变形机制的影响.结果表明,经SMAT后样品表面发生了明显的塑性变形,其表面层的应变、应变速率和应力沿厚度方向均逐渐减小,这与其微观组织相对应;塑性变形时的应变量和应变速率对于样品晶粒细化和处理后最终晶粒尺寸的大小起重要作用.在系统激振频率为50 Hz时,SMAT过程中样品最表面层的应变速率最大可达681 s-1.     

表面机械研磨316L不锈钢诱导表层纳米化

采用表面机械研磨处理方法在316L不锈钢上制备出纳米结构表层,用X射线衍射和透射电镜研究横截面组织结构的演变过程,通过测定表层到内部的硬度变化研究纳米化对硬度的影响·结果表明:经过60min表面机械研磨处理后,在距样品表面约100μm深度形成高密度位错;随着应变量和应变速率的增加,在距样品表面约50μm深度诱发了机械孪生,形成了片层状孪晶,并相互交割细化组织;最终在样品表面形成了厚度约为20μm的纳米层,表层显微组织由晶粒尺寸为10～30nm的双相组织(马氏体和奥氏体)组成·表面纳米化是通过位错 孪晶及孪晶 孪晶相互作用实现晶粒细化·表面纳米化后,表层硬度显著提高·     

振动研磨工艺对QAl10-4-4合金的表面组织及性能的影响

研究了振动研磨工艺对涡轮用QAl10-4-4合金组织和表面性能的影响.结果表明：振动研磨处理后,QAl10-4-4合金产生了从研磨表面至心部硬度逐渐降低的硬化层,表面硬度提高约20%;相对于未研磨处理的QAl10-4-4合金,经振动研磨处理后,合金处于表面压应力状态,表面残余应力提高约300 MPa,表面粗糙度明显降低,表面摩擦因数显著下降,耐磨性显著提高.工程实际应用结果表明：使用振动研磨工艺处理QAl10-4-4合金蜗轮提高整机的传动效率超过20%.振动研磨后,材料表面综合性能的提高与表层组织细化有直接关系.     

Microstructure,mechanical properties and deformation mechanisms of an Al-Mg alloy processed by the cyclical continuous expanded extrusion and drawing approach

Al-Mg alloys are an important class of non-heat treatable alloys in which Mg solute and grain size play essential role in their mechanical properties and plastic deformation behaviors.In this work,a cyclical continuous expanded extrusion and drawing(CCEED)process was proposed and implemented on an Al-3Mg alloy to introduce large plastic deformation.The results showed that the continuous expanded extrusion mainly improved the ductility,while the cold drawing enhanced the strength of the alloy.With the increased processing CCEED passes,the multi-pass cross shear deformation mechanism progressively improved the homogeneity of the hardness distributions and refined grain size.Continuous dynamic recrystallization played an important role in the grain refinement of the processed Al-3Mg alloy rods.Besides,the microstructural evolution was basically influenced by the special thermomechanical deformation conditions during the CCEED process.     

铜/钢反向凝固及加工变形的结合机理

介绍了铜/钢反向凝固及加工变形复合的工艺过程,利用光学显微镜和扫描电镜等手段分析了铜和钢界面形态、组织结构·在Instron拉伸机上测定了铜/钢界面的结合强度·结果表明,经预处理的钢丝表面呈现细微的凸凹不平状态,特别有利于液态铜在钢丝表面异质形核、凝固、结晶,使铜和钢的晶粒在界面上相互镶嵌,达到良好结合·此外,CS线再经加工变形后,伴随着晶粒的拉长和界面的扩展,导致晶粒间相互插入,机械咬合能力加强,使铜/钢界面结合强度进一步提高·     

Characteristic investigation of trilayered Cu/Al8011/Al1060 composite: Interface morphology,microstructure, and in-situ tensile deformation

In this work, Cu/Al8011/Al1060 trilayered composite with an ultrathin copper layer was fabricated successfully by the hybrid process of cast-rolling and hot-roll-bonding. The effects of heat treatment were investigated on the microstructure and mechanical behavior of the composite. The results showed that annealing temperatures above 300 ​°C led to the formation of new intermetallic compounds that joined their initial counterparts. Also, the interface growth was fast above 340 ​°C. It was found that the annealing temperature of 320 ​°C led to the optimal tensile properties of the composite. Through rolling, hard-brittle IMCs imposed intense plastic strains on the surrounding regions, resulting in further grain refinement of both metals in the vicinity of the interface. Although the kernel average misorientation of the trilayered composite was almost identical with the Cu/Al8011 bimetal, its amount was considerably decreased after heat treatment, as the distorted structure changed into a nearly strain-free one using the restoration phenomenon. The SEM-based in-situ tensile test showed that crack initiation occurred within the IMCs and propagated into the Cu strip, leading to its early fracture. However, the strength-displacement curve during in-situ tensile deformation did not decrease significantly due to the ultrathin copper layer.     

Precipitation behavior and grain refinement of burnishing Al-Zn-Mg alloy

Burnishing is a unique strengthening approach to improve the strength of surface layer and remains the ductility of the interior of metallic materials. In this work, burnishing treatment was employed to improve the surface microstructure of naturally aged Al-Zn-Mg alloys after solid solution. Transmission electron microscopy, highresolution transmission electron microscopy, X-ray diffraction and nano-indentation were used to characterize the effects of the burnishing on the microstructures of surface layer and Guinier-Preston(GP) zones. It was indicated that GP zones uniformly distributed and dispersed in the matrix before burnishing, and the amount of GP zones decreased dramatically after burnishing processing. Additionally, the grains in the surficial layer were refined into nano-crystals with an average grain size of 78 nm. Burnishing treatment not only led to formation of large number of dislocation substructures in the sub-surface and near-matrix surface, but also promoted the precipitation of metastable η' phase at grain boundaries. The synergistic effects of the grain refinement, dislocation multiplication and the precipitation of η' phase strengthen the burnished layer of Al-Zn-Mg alloy.     

Microstructure refinement and work hardening in a machined surface layer induced by turning Inconel 718 super alloy

The microstructural changes in the machined surface layer of Ni-based super alloys essentially determine the final performance of the structural components of aerospace engines in which these alloys are used. In this work, multiscale metallurgical observations using scanning electron microscopy, electron-backscatter diffraction microscopy, and transmission electron microscopy were conducted to quantitatively characterize the microstructure of the machined subsurface. Next, to elucidate the factors that affect the formation of the refinement microstructure, the distributions of the deformation parameters (strain, strain rate, and temperature) in the machined subsurface were analyzed. A dislocation–twin interaction dynamic recrystallization mechanism for grain refinement during machining of Inconel 718 is proposed. Furthermore, microhardness evolution induced by grain refinement in the machined surface is evaluated. The results suggest that the gradient microstructure and the work hardening can be optimized by controlling the cutting parameters during turning of Inconel 718.     

搅拌摩擦加工IF钢的组织性能

对3 mm厚的DC04冷轧IF钢板进行搅拌摩擦加工,研究加工区域的微观组织与力学性能. 在旋转速度为950 r· min-1 ,加工速度为60 mm·min-1时,采用加工后强制冷却技术可获得光滑平整且没有缺陷的加工表面. 搅拌摩擦加工后组织显著细化,加工中心的平均显微硬度约为HV 135. 6,是母材硬度的1. 4倍,表面细晶层硬度最高可达到HV 312. 8,细晶层和过渡层的抗拉强度分别比母材的抗拉强度提高50. 9%和47. 6%,加工前后试样的拉伸断口均呈微孔聚合韧性断裂特征. 细晶强化对材料抗拉强度的提高起主要作用.     

含钒超细晶双相钢的细化机制

利用铁素体+马氏体+贝氏体的初始显微组织结合冷轧和连续退火的方法达到了细化晶粒的目的,通过这种方式制备的双相钢中有63.8%的铁素体晶粒尺寸分布于0.5～1μm,有53%的马氏体晶粒尺寸分布于0.5～1μm.针对该现象研究了基于铁素体+马氏体+贝氏体初始显微组织含钒超细晶双相钢的晶粒细化机制.分析认为,细化机制主要有三个方面:第一是形变对显微组织的细化,包括为了得到铁素体+马氏体+贝氏体的初始显微组织而进行的热轧和冷轧;第二是冷轧态显微组织的再结晶和快速奥氏体化;第三是钒的析出物阻碍奥氏体的长大.     

基于微晶刚玉砂轮的20 CrMnTi齿轮成型磨削表面完整性

为研究微晶刚玉砂轮成型磨削20CrMnTi齿轮的表面完整性,开展了20CrMnTi齿轮成型磨削试验,分析了砂轮线速度、轴向进给速度及径向进给量对齿面粗糙度、表层/次表层显微硬度、微观组织和残余应力的影响规律,探讨了由磨削引起的磨削烧伤、微观裂纹等损伤缺陷的形成机理,结果表明:径向进给量对表面粗糙度的影响最显著,砂轮线速度次之,轴向进给速度最不显著;磨削温度过高会导致磨削烧伤,淬火烧伤使得表面硬度提高5%~20%,回火烧伤则导致表面硬度不同程度地下降;表层组织从外至内分别为白层、暗层和基体组织,白层主要由致密的马氏体+碳化物+残余奥氏体组成;砂轮线速度和径向进给量的增大使得由磨削引起的残余拉应力增大,表面残余压应力下降并逐渐向拉应力转变,当表面最终残余拉应力大于材料的断裂强度时,表面产生微观裂纹.     

Effect of continuous induction annealing on the microstructure and mechanical properties of copper-clad aluminum flat bars

Copper-clad aluminum (CCA) flat bars produced by the continuous casting–rolling process were subjected to continuous induction heating annealing (CIHA), and the effects of induction heating temperature and holding time on the microstructure, interface, and mechanical properties of the flat bars were investigated. The results showed that complete recrystallization of the copper sheath occurred under CIHA at 460°C for 5 s, 480°C for 3 s, or 500°C for 1 s and that the average grain size in the copper sheath was approximately 10.0 μm. In the case of specimens subjected to CIHA at 460–500°C for longer than 1 s, complete recrystallization occurred in the aluminum core. In the case of CIHA at 460–500°C for 1–5 s, a continuous interfacial layer with a thickness of 2.5–5.5 μm formed and the thickness mainly increased with increasing annealing temperature. After CIHA, the interfacial layer consisted primarily of a Cu₉Al₄ layer and a CuAl₂ layer; the average interface shear strength of the CCA flat bars treated by CIHA at 460–500°C for 1–5 s was 45–52 MPa. After full softening annealing, the hardness values of the copper sheath and the aluminum core were HV 65 and HV 24, respectively, and the hardness along the cross section of the CCA flat bar was uniform.     

Ti-7333合金β锻动态再结晶行为

 利用Gleeble3800热模拟试验机研究了在温度870～970℃和应变速率0.001～10s^-1范围内,近β钛合金Ti-7333 β锻热变形的组织演化规律及动态再结晶行为.实验结果表明,Ti-7333钛合金在温度较高、应变速率较低的情况下变形时,表现出典型的动态再结晶行为,动态再结晶晶粒尺寸和再结晶体积分数均随变形温度升高和变形速率降低而增大,而应变速率对再结晶晶粒尺寸的影响较显著.在变形速率较高(>0.1s^-1)且变形温度较低(<870℃)时,晶粒严重变形拉长,但动态再结晶将很难发生.因子Z决定着动态再结晶晶粒尺寸,二者之间为幂指数关系.通过回归分析方法得出动态再结晶晶粒尺寸的数学表达式为：lnD_r=8.50949-0.31411lnZ.采用该表达式可以对一定变形条件的动态再结晶晶粒尺寸进行精确预测,从而为Ti-7333钛合金热变形条件下的组织控制提供可靠依据.不适当的热变形工艺会造成组织粗大或者不均匀,进而使材料性能恶化.因此,应该从材料组织均匀性和晶粒细化角度选择最佳的热变形参数.     

Microstructure and M echanical Properties of Magnesium Alloy Sheet by Friction Heating Single Point Incremental Forming

By friction heating single point incremental forming,truncated square pyramid parts with different draw angles of a magnesium alloy AZ31 B were formed at room temperature.Metallurgical,tensile and micro-hardness tests were carried out to obtain the effects of wall angle on microstructure and mechanical properties. The results show that grain in side wall of the formed parts becomes refined significantly. Furthermore,with the increase of draw angle,grain size increases,but strength,hardness and plasticity decrease. In addition, surface roughness tests were performed on the formed surface to determine the influence of speed of forming tool. The results show that surface roughness has a little increase with the increase of tool rotational speed.     

随机抽样在粗糙表面接触力学行为分析中的应用

为研究表面粗糙度对接触表面力学行为的影响规律,结合表面粗糙度的加工参数和随机抽样方法,对服从正态分布和预设粗糙度的表面轮廓曲线进行了模拟.根据统计得到的模拟轮廓曲线几何形态的共性特征,建立了基于圆锥与平面接触的三维粗糙表面接触力学模型,推导出平面几何压力与粗糙表面微凸体变形间的定量关系.分析结果表明:服从正态分布的轮廓高度曲线中的峰角顶点数目约为样本容量的1/3;圆锥压入平面时平均压力/材料纯剪应力的比值与其半角呈二次方关系;表面粗糙度越大,轮廓曲线的起伏越大,两接触面相互嵌合越容易;表面粗糙度越大,材料屈服强度对接触微凸体变形的影响也越明显.     

退火对表面纳米化纯钛热稳定性的影响

采用表面机械研磨技术实现纯钛材料表面纳米化,并研究了纳米化处理后的材料表层组织结构,详细分析了样品表层纳米晶组织在不同温度、不同时间退火后的热稳定性.结果表明:纯钛TA2经过表面机械研磨处理后可以在表面形成等轴且取向随机的纳米晶层,晶粒尺寸约为12nm.对表面纳米化样品退火后发现,表层纳米晶组织在773 K以下温度退火后具有良好的热稳定性,晶粒尺寸没有明显增大;在773 K温度退火150min后晶粒尺寸稍有增大,而在773 K温度退火240min后晶粒尺寸明显增大,且横截面显微硬度也比表面纳米化后未退火样品显著下降,良好的热稳定性消失.     

Tubular pure copper grain refining by tube cyclic extrusion–compression (TCEC) as a severe plastic deformation technique

Tube Cyclic Extrusion–Compression(TCEC) method is a novel severe plastic deformation technique developed for grain refining of cylindrical tubes to ultrafine grained(UFG)/nanostructured ones. In this method, tubes are fully constrained and deformed between chamber and mandrel with a small neck zone. The principle of TCEC technique which was adopted to impose severe plastic strains to the tubular materials was explained. Also, the deformation and grain fragmentation mechanism during TCEC was analyzed. The material deformation characteristics during TCEC were numerically simulated by FE code of ABAQUS/Explicit. The FEM results demonstrated that TCEC technique was able to impose extremely high plastic strains. The TCEC method was successfully applied to a commercially pure copper(99.99%) and significant grain refinement was achieved. TEM observation demonstrated the refinement of grains from the initial size of 45 μm to 200–350 nm after four processing cycles of TCEC. Microhardness measurements were carried out across the thickness of the initial and processed tubes. The results show good homogeneity of hardness distribution and an increase to 102 Hv from initial value of 55 Hv after four TCEC cycles. Mechanical properties of the specimens were extracted from tensile tests. The obtained results documented notable increase in the yield and ultimate strengths, whereas the uniform and total elongations decreased. Fracture surfaces after tensile tests were investigated by scanning electron microscopy(SEM), and the observed morphology indicates ductile fracture mode after four cycles of TCEC.