Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： To reduce the weight of car body, Al-Mg-Si-Cu alloys are becoming increasingly attractive as a candidate for material substitution used to produce the outer body panels of automobiles because of their favorable bake-hardening response. However, the formability still needs to be further improved compared to steels. In this work, the effect of the thermomechanical processing on the mechanical properties and microstructure of Al-Mg-Si-Cu alloy is studied through tensile test, OM, SEM and TEM observation, as well as EBSD characterization. The results reveal that there is almost no change in both strengths and strain-hardening exponent n of the sheets in T4P condition after different thermomechanical processing, but the average plasticity strain ratio r, planar anisotropy Δr and elongations in the three directions show obvious differences. The sheet undergone hot rolling, cold rolling, intermediate annealing, cold rolling and solution (processing II) has a better formability (r= 0.6187) and a weaker planar anisotropy than that subjected to hot rolling, intermediate annealing and then cold rolling before solution treatment (processing I). Although the particle stimulated nucleation (PSN) effect of processing I is remarkable during solution treatment, due to the appropriate controlling cold deformation and distribution of second-phase particles with different sizes in processing II, most of the recrystallization grains are equiaxial and the recrystallization texture is only consisted of CubeND, Cube and H with a low intensity. At last, according to the relationship between the microstructure and the thermomechanical processing, the microstructure evolution model during different thermomechanical processes is established.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： To reduce the weight of car body, Al-Mg-Si-Cu alloys have been widely used to produce outer body panels of automobiles due to their favorable high strength-to-weight ratio, corrosion resistance, weldability and good formability. Al- Mg- Si- Cu alloys belong to age- hardenable aluminium alloys, whose strength derives mainly from the matrix precipitation during aging treatments. However, their bake hardening response still need to be further improved to enhance their dent resistance. A novel thermo-mechanical treatment consisting of conventional pre-aging, pre-deformation and re-aging was developed to enhance the tensile properties and bake hardening increment of Al-Mg-Si-Cu alloys. In this work, the effect of pre-deformation on the precipitation behaviors of Al-Mg-Si-Cu alloy was studied by DSC, mechanical property measurement and TEM. The results show that, the GPzone dissolution rate decreases with increasing pre-deformation during the slow heating up process for the pre-aged alloy, the corresponding activation energies of 0, 5% and 15% pre-deformed alloy calculated by Avrami-Johnson-Mehl method are 137.1, 189.5 and 141.3 kJ/mol, respectively. If the pre-deformed alloys are directly bake hardened at 185 ℃ for 20 min, precipitation and bake hardening increment can be greatly improved by pre-deformation (the highest bake hardening increment is 160 MPa), but the bake hardening increment rate gradually decreases if the pre- deformation is above 10%. In addition, the GP zone dissolution rates of pre- deformed alloys after bake hardening treatment are much lower when the heat treatment temperatures are below one certain value, but if the treatment temperatures above it, the corresponding GP zone dissolution rates are higher than that of alloy without pre-deformation, finally, the activation energy changes from high value to low value even can be observed in the ln[(dY/dT)f/f(Y)]-1/T curve. For the β″ precipitation in the alloys, with increasing pre-deformation, its activation energy gradually decreases, corresponding gradually increase of precipitation rate.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： To reduce the weight of car body, Al-Mg-Si-Cu alloys have been used to produce outer body panels of automobiles due to their relatively good formability in the solution treated condition and high strength in the age hardened condition. However, their formability is significantly poor compared to that of steels, which are the major drawbacks to wide-scale application of aluminum in the automotive industry. The microstructural characteristics developed during recrystallization, most notably grain size and crystallographic texture, play a dominant role in controlling the mechanical properties and formability of sheet in the T4 condition. In this work, the effect of particles with different sizes on the mechanical properties, microstructure and texture of Al-Mg-Si-Cu alloys was studied through tensile test, OM, SEM, TEM and EBSD measurement. The results reveal that with increase of solute concentration, the average plastic strain ratio rˉ, yield strength and ultimate tensile strength increase, but the elongation decreases and with different extents in the three directions. In addition, the number of observed particles with different sizes in the alloy matrix such as Mg2Si, Al15Mn3Si2 and a-Al(Fe, Mn)Si phases also increases. When the size and concentration of the se particles are controlled appropriately, lots of finer recrystallized grains can form during solution treatment due to the particle stimulated nucleation (PSN) effect of coarse particles and pinning effect of finer particles. The main texture components include CubeND18, Goss{011}<100>, P{011}<122> and Cu{112}<111> for the alloy with fine-grained structure. At last, according to the relationship among alloy composition, thermomechanical processing and microstructure, the model of nucleation and growth of recrystallized grains affected by the particles with different sizes was also proposed.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-18 Cooperative journals: 《材料研究学报》
Abstract： The corrosion behavior of the newly developed Al-Mg alloys was investigated in terms of their alloying element and thermo-mechanical treatments by means of intergranular corrosion test, exfoliation corrosion test, scanning electron microscopy and transmission electron microscopy. The results show that, with the increase of Mg content the mass loss of the alloys in concentrated nitric acid increased, correspondingly their intergranular corrosion resistance decreases. Zn addition to the Al-Mg alloys led to the formation of Mg32(Al, Zn)49 phase at the grain boundary, which dramatically increased the intergranular corrosion resistance of the alloys. The corrosion resistance of the alloys was also modified by thermo- mechanical treatment. Both intergranular corrosion resistance and exfoliation corrosion resistance of the alloys were dramatically increased by a proper post stabilizing treatment after cold rolling reduction. The residual stress, higher dislocation density and morphology of elongated grains after cold rolling reduction can lead to more continuous precipitation at the grain boundary, thus decrease the corrosion resistance of the alloys.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-18 Cooperative journals: 《材料研究学报》
Abstract： The hot deformation behavior of as spray-formed Nb-containing AISI M3: 2 high speed steel has been investigated by compression tests at a temperature range of 950-1150℃ and a strain range of 0.001-10 s- 1 with 50% reduction. Processing maps were developed according to the principles of Dynamic Material Model. It was found that the flow curves assumed the classic shape of dynamic recrystallization (DRX)-rising to a peak, following a softening to a steady state. The hot working process of the steel can be carried out safely in the domain of (Td: 1050-1150℃, ε̇: 0.01-0.1 s-1). To obtain microstructures of the steel with fine grains and uniform distribution of fine granular carbides, the hot working process should be carried out at 1150℃ and strain rate of 0.1 s- 1. The flow instability took place when strain rates exceed 1 s-1. After a proper hot working and heat treatment, the hardness and bending strength of the spray-formed Nb-containing M3:2 high speed steel is 67 HRC and 3467 MPa, respectively.
Subjects: Materials Science >> Materials Science (General) submitted time 2016-11-15 Cooperative journals: 《金属学报》
Abstract：通过弯边变形、拉伸实验、OM、SEM、TEM观察等手段研究了不同浓度富铁相粒子对Al-Mg-Si-Cu系合金板材弯边性能的影响规律。结果表明，随着富铁相浓度增加，合金板材沿纵横向的弯边性能存在较大差异，不含富铁相粒子的合金板材沿两个方向弯边变形180°后外表面粗糙度增加，处于中等浓度（含0.2wt%Fe）时沿两个方向弯边变形后均具有优异的外表面质量，但是随着富铁相浓度的进一步增加（至0.5wt%Fe），合金板材沿两个方向弯边变形后均在外表面出现微裂纹。虽然富铁相粒子浓度增加对合金板材纵横向延伸率影响不大，但是拉伸断口、弯边和拉伸断裂后的侧表面组织均表明，不含富铁相的合金板材弯边变形后外表面粗糙度增加与剪切带组织密切相关，而含有较高浓度富铁相粒子的合金板材弯边变形后外表面出现的微裂纹与粗大富铁相粒子的尺寸、形态以及分布状态密切相关。此外，本文根据不同浓度富铁相粒子与合金弯边性能间的定量关系提出了合金弯边变形后外表面粗糙度增加和微裂纹形成的模型示意图。
Subjects: Materials Science >> Materials Science (General) submitted time 2016-11-15 Cooperative journals: 《金属学报》
Abstract：本文取初始织构为c轴与板面法向垂直的强织构AZ31镁合金板材为初始样品，经液氮温度深低温轧制多道次至不同变形量，研究所得的轧制板材的显微组织与织构演变，及其对轧制力学性能的影响。本研究利用SEM、EBSD和X射线衍射表征分析了轧制板材的显微组织和织构，重点研究了板材深低温轧制过程中的孪生变形，以及孪晶组织对板材微观组织与织构的影响，并应用准静态单轴拉伸实验分别测试了深低温轧制板材沿轧向（RD）和横向（TD）的室温力学性能，讨论研究了板材的力学性能与显微组织和织构之间的联系。研究表明，当板材织构有利于拉伸孪生时，{10-12}拉伸孪晶在深低温轧制过程中十分活跃，深低温轧制过程中活跃的孪生行为对于镁合金的显微组织、织构及力学性能都有着重要的影响。
Subjects: Materials Science >> Materials Science (General) submitted time 2016-11-04 Cooperative journals: 《金属学报》
Abstract：采用DSC、力学性能测试及TEM研究了预变形对Al-Mg-Si-Cu合金析出行为的影响。结果表明，预时效态合金在慢速率升温过程中，GP区溶解速率均随预变形量的增加而降低，利用Avrami-Johnson-Mehl方法求得经0%、5%和15%预变形后合金的GP区溶解激活能分别为137.1KJ/mol、189.5KJ/mol和141.3KJ/mol；若合金经不同预变形后直接进行185℃/20min烤漆硬化，预变形可有效促进沉淀相析出，提高烤漆硬化增量，最高达160MPa，不过预变形量大于10%时合金烤漆硬化增幅减缓；此外，经预变形处理后烤漆态合金的GP区溶解速率在一定温度下均较低，但高于某一温度后，相应的GP区溶解速率均高于未经预变形处理的，最终获得的ln[(dY/dT)φ/f(Y)]-1/T曲线甚至会出现高激活能向低激活能转化现象；不过随预变形量增加，β〃相析出激活能不断降低，析出速率不断增加，相应的TEM组织观察结果与此非常吻合。
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