Formation and Evolution of Non-dendritic Microstructures of Semi-solid Alloys Prepared by Shearing/Cooling Roll Process

The shearing/cooling roll （SCR） process was adopted to prepare semi-solid A2017 alloy. The formation and evolution of non-dendritic microstructures in semi-solid A2017 alloy were studied. It is shown that the microstructures of semi-solid billets transform from coarse dendrites into fine equiaxed grains as the pouring temperature of molten alloy decreases o.r roll-shoe cavity height is reduced. From the inlet to the exit of roll-shoe cavity, microstructure of semi-solid slurry near the shoe surface is in the order of coarse dendrites, degenerated dendrites or equiaxed grains, but fine equiaxed grains are near the roll surface. Microstructural evolution of semi-solid slurry prepared by SCR process is that the molten alloy nucleates and grows into dendrite firstly on the roll and shoe＇s surface. Under the shearing and stirring given by the rotating roll, the dendrites crush off and disperse into the melt. Under the shearing and stirring on semi-solid slurry with high volume fraction of solid, the dendrite arms fracture and form equiaxed grain microstructures.     

Effect of Equal Channel Angular Extrusion on the Microstructures and Properties of Two Extruded Al-Mg-Si Alloys

1.IntroductionAl-Mg-Si(6xxx)alloys make up nearly90%of alu-minium extrusion alloys[1].Chemical composition andheat treatment technique play important roles in gov-erning microstructure,extrudability and the mechani-cal properties of extrusions[2~12].A complete sequen-tial heat-treatment and thermo-mechanical processing(TMP)for6xxx series extrusion alloy is comprised ofhomogenisation,solution treatment,extrusion and age-ing.T5(see below)is a widely accepted post-extrusiontreatment in industry…     

Microstructure of Al-Mg-Si Weld Joints Produced by Pulse TIG Welding

Al-Mg-Si alloys (6000 series) are heat-treatable aluminum alloys and are extensively used in various sectors such as aerospace, automobiles, and general construction. Fabrication of these alloys often requires welding. Pulse TIG offers many advantages over conventional TIG welding as far as control of microstructure is concerned. This research investigated the effect of pulsed TIG welding parameters such as pulse duration, peak current, pulse frequency on the microstructure of heat-affected zone, and fusion line and weld metal of Al-0.5%Mg-0.5%Si weld joints. It was observed that a significant change in microstructure takes place in moving from the base metal to weld centerline. Dissolution of phases present along the grain boundary occurs as the fusion line is approached. Grain structure of heat-affected zone, fusion line, and weld metal appears to have tangible relationship with pulse parameters.     

7075铝合金CREM铸锭的微观组织

实验研究了低频交变电磁场作用下，7075合金凝固壳高度以及微观组织的变化规律。结果表明，随磁场强度增大，凝固壳高度以及熔体与结晶 的接触高度减小，铸锭表面质量提高；相应地铸坯中近球形组织增加，蔷薇形组织减少，整体组织变得细小和均匀。     

时效早期Al-Mg-Si合金的组织和析出相的演变

用显微硬度测试、差示扫描量热法(DSC)和高分辨透射电镜(HRTEM)观察等手段研究了Al-Mg-Si合金人工时效过程中的硬化、组织变化以及早期析出相的演变。结果表明：在170℃时效的合金具有更高的峰值硬度。在时效初期晶内析出高数量密度的溶质原子团簇和GP区,合金的硬度显著提高。在170℃处理4 h后合金的硬度达到峰值,此时晶内析出相以针状β″相为主,β″相与Al基体界面三维共格应变是合金强化的主要原因。同时,晶界析出相呈断续分布状态。随着时效时间的增加β″相开始粗化,晶界析出相的连续程度降低。在过时效阶段晶内析出相的严重粗化和数量密度的降低,使合金的硬度剧烈降低。在时效的初始阶段,合金的析出序列为过饱和固溶体→球形原子团簇→针状GP区→针状β″相。     

Al-Mg-Si基合金方板对角拉伸临界失稳及应力应变场模拟

基于弹塑性板壳失稳理论,采用虚功原理和非线性弹塑性有限元方法,模拟了Al-Mg-Si基合金方板对角拉伸的皱曲特性,得出了方板临界失稳的物理判据和临界状态下的应力、应变场.结果表明:Uz和εx急剧变化的突发点为起皱的失稳点,拉伸失稳的临界载荷为7780牛顿.拉伸载荷达临界载荷前表现为面内变形,达临界载倚时方板发生分叉失稳.继续拉伸,演变成起皱带.临界状态下横向应变和横向应力的分布不均匀,最大值都发生在方板中心.可把失稳状态下方板中心点的横向压应力作为描述YBT模型皱曲失稳的特征值.     

多道次ECAE动态成型Al-Mg-Si合金导线组织与性能

借助S4800扫描电子显微镜、Philips DM420透射电子显微镜、SANS CMT5105电子万能材料试验机和QJ48双臂直流电桥,研究了多道次ECAE动态成型Al-Mg-Si合金导线的组织与性能。结果表明:4道次ECAE动态成型可制备平均尺寸在10μm左右甚至更小的Al-Mg-Si合金导线晶粒。随着Mg、Si含量的增加,合金导线的抗拉强度增大,伸长率与等效导电率降低。经160~170℃/7h时效处理后,Al-0.59%Mg-0.59%Si合金导线的抗拉强度、伸长率和等效导电率分别为305.71~309.63 MPa,4.7%~5.4%和55.18%IACS~56.33%IACS,与目前国产Al-Mg-Si合金导线(295 MPa,52.5%IACS)相比,导电性能显著提高。     

Microstructure,mechanical properties and deformation characteristics of Al-Mg-Si alloys processed by a continuous expansion extrusion approach

In the present work,a double-pass continuous expansion extrusion forming (CEEF) process was proposed for an Al-Mg-Si alloy,in which the diameter of rods was gradually expanded.The microstruc-tural evolution,mechanical properties and deformation characteristics were investigated by utilizing microstructural observations,mechanical testing and a finite element method coupled with a cellular automata model.The results showed that the strength and ductility of the double-pass CEEF processed Al-Mg-Si alloys were improved synchronously,especially in artificially aged alloys.The grain size of the processed Al-Mg-Si alloy rods was refined remarkably by continuous dynamic recrystallization (CDRX)and geometric dynamic recrystallization (GDRX),and the homogeneity of microstructure was gradu-ally improved with increasing number of processing passes.The artificially aged alloy processed with double-pass CEEF and water quenching contained fine (sub)grains and high-density dislocations,which resulted in more needle-shaped β" precipitates and a larger precipitate aspect ratio than the as-received and air-cooled CEEF alloys owing to the different precipitation kinetics.The severe cumulate strain and microshear bands were found to accelerate CDRX and GDRX for grain refinement between adjacent posi-tions of the parabolic metal flow due to the special temperature characteristics and large shear straining during the CEEF process.     

Effects of Heat Treatment on Hardness and Dry Wear Properties of a Semi-Solid Processed Fe-27 wt pct Cr-2.9 wt pct C Cast Iron

EfFects of heat treatments on hardness and dry wear properties of a semi-solid processed Fe-26.96 wt pct Cr- 2.91 wt pct C cast iron were studied. Heat treatments included tempering at 500℃, destabilisation at 1075℃ and destabilisation at 1075℃ plus tempering at 500℃, all followed by air cooling. Electron microscopy revealed that, in the as-cast condition, the primary proeutectic austenite was round in shape while the eutectic M7C3 carbide was found as radiating clusters mixed with directional clusters. Tempering did not change the microstructure significantly when observed by scanning or transmission electron microscopy. Destabilisation followed by air cooling led to a precipitation of secondary M23C6 carbide and a transformation of the primary austenite to martensite. Precipitation behaviour is comparable to that observed in the conventionally cast iron. Tempering after destabilisation resulted in a higher amount of secondary carbide precipitation within the tempered martensite in the eutectic structure. Vickers macrohardness and microhardness in the proeutectic zones were measured. Dry wear properties were tested by using a pin-on-disc method. The maximum hardness and the lowest dry wear rate were obtained from the destabilisation-plus-tempering heat treatment due to the precipitation of secondary carbides within the martensite matrix and a possible reduction in the retained austenite.     

Fe－Mo二元合金显微组织的SEM观察

用扫描电镜观察分析了Ｍｏ含量为２０～５７ｗｔ％的Ｆｅ－Ｍｏ二元合金的显微组织，发现合金的相组成与相平衡图有显著差异，探讨了差异出现的原因。     

热机械循环对Fe-Mn-Si-Cr-Ni合金形状记忆效应的影响

以开发石油井管管接头材料为背景，主要研究了热机械循环中循环训练次数、变形量、中间退火温度对Ｆｅ－Ｍｎ－Ｓｉ－Ｃｒ－Ｎｉ合金的形状记忆效应（ＳＭＥ）的影响。结果表明训练２～３次，合金的ＳＭＥ有大幅度提高。继续训练，则提高不大。随着变形量增加，形状恢复率下降，但可恢复变形量增加。而最佳中间退火温度受合金成分的影响。本文通过金相观察、分析，对其机制进行了探讨。     

镁合金表面转化膜预处理工艺的研究进展

本文阐述了镁合金腐蚀的原因、机理、类型及其腐蚀特点,总结了磷酸盐-高锰酸盐转化膜、稀土金属盐转化膜、氟锆酸盐转化膜和钒酸盐转化膜等转化膜预处理工艺的主要优缺点,重点评述了转化膜预处理的耐蚀性能和环境友好性能,并对其发展进行了展望。     

Fe-Cr-B-C堆焊合金的组织与耐磨性

采用药芯焊丝气体保护堆焊方法制备Fe-Cr-B-C堆焊合金,利用金相、SEM、XRD等方法,分析了不同硼含量对堆焊合金组织及硼化物形貌的影响。结果表明:Fe-12Cr-xB-0.1C合金的显微组织由铁素体+奥氏体+(Fe,Cr)2B+(Fe,Cr)23(B,C)6组成。当硼含量<3%(质量分数,下同)时,随着硼含量增加,硼化物形态逐渐由断续网状转变为网状;当硼含量≥3%时,随着硼含量增加,初生块状(Fe,Cr)2B数量逐渐增加,其尺寸和分布更均匀,硼化物主要呈块状、条状、鱼骨状、蜂窝状及菊花状分布。初晶(Fe,Cr)2B近似呈四边形柱状体,趋向垂直于堆焊层表面生长。当硼含量≤4%时,硼的增加能显著提高Fe-Cr-B-C堆焊合金的硬度及耐磨性。     

铜对孪晶诱发塑性合金组织和性能的影响

研究了铜对Fe-22.5/30 Mn-3Al-3Si TWIP钢的显微组织和力学性能的影响规律。结果表明,随着铜含量的增加,TWIP钢中奥氏体平均晶粒尺寸减小。铜含量超过0.5wt%后,TWIP钢的显微硬度明显提高。TEM观察显示TWIP钢未形变时组织中存在许多层错群和规则排列的位错列,形变后出现大量密集排列的形变孪晶和被形变孪晶分割的位错。     

半固态非枝晶组织合金的制备及形成机理

结合作者的研究方向--固液混合铸造制备非枝晶微晶合金,介绍了半固态非枝晶组织材料制备的发展与现状,分析了其非枝晶组织的形成机理.重点介绍了固液混合铸造技术的工艺及其非枝晶组织的形成机理.     

Microstructures and Mechanical Properties of Al/Mg Alloy Multilayered Composites Produced by Accumulative Roll Bonding

Al/Mg alloy multilayered composites were produced successfully at the lower temperature(280 C) by accumulative roll bonding(ARB) processing technique.The microstructures of Al and Mg alloy layers were characterized by scanning electron microscopy and transmission electron microscopy.Vickers hardness and three-point bending tests were conducted to investigate mechanical properties of the composites.It is found that Vickers hardness,bending strength and stiffness modulus of the Al/Mg alloy multilayered composite increase with increasing the ARB pass.Delamination and crack propagation along the interface are the two main failure modes of the multilayered composite subjected to bending load.Strengthening and fracture mechanisms of the composite are analyzed.     

Microstructures and Mechanical Properties of Rapidly Solidified Mg-Al-Zn-MM Alloys

Mg-Al-Zn-M M （misch metal） alloy powders were manufactured by inert gas atomization and the characteristics of alloy powders were investigated.In spite of the low fluidity and easy oxidation of the magnesium melt,the spherical powder was made successfully with the improved three piece nozzle systems of gas atomization unit. It was found that most of the solidified powders with particles size of less than 50μm in diameter were single crystal and the solidification structure of rapidly solidified powders showed a typical dendritic morphology because of supercooling prior to nucleation.The spacing of secondary denrite arms was deceasing as the size of powders was decreasing.The rapidly solidified powders were consolidated by vacuum hot extrusion and the effects of misch metal addition to AZ91 on mechanical properties of extruded bars were also examined.During extrusion of the rapidly solidified powders,their dendritic structure was broken into fragments and remained as grains of about 3μm in size.The Mg-Al-Ce intermetallic compounds formed in the interdendritic regions of powders were finely broken,too.The tensile strength and ductility obtained in as-extruded Mg-9 wt pct Al-1 wt pct Zn-3 wt pct MM alloy wereσ-（T.S.） =383 MPa andε=10.6%,respectively.All of these improvements on mechanical properties were resulted from the refined microstructure and second-phase dispersions.