AZ31镁合金板材超塑性气胀成形研究

研究了AZ31镁合金板材不同工艺条件下的气胀成形性能。实验表明,胀形高度随温度的升高而增大,且应变速率敏感指数值均大于0．3。在673 K,0．7 MPa下胀形25 min所得的胀形件胀形高度达23．34 mm,高径比为0．67。由金相及SEM电镜观察可知,在胀形件的顶端晶界处聚集了大量空洞。通过动态再结晶,晶粒得到了很大细化。并且随变形程度的增大,晶粒细化更明显。AZ31镁合金板材的超塑性胀形主要由晶界滑移控制,动态再结晶则为重要的辅助机制。     

AZ31镁合金变通道角挤压工艺

将Φ40mm×50mm的AZ31镁合金圆棒经变通道角挤压(Change channel angular extrusion,CCAE)成厚度约为5mm的板材。通过TEM观察表明,AZ31镁合金的形核机制主要是动态再结晶形核。结果表明,经CCAE变形后,由晶粒的剪切破碎和动态再结晶使得镁合金晶粒明显细化。内角、挤压比、挤压温度和挤压速率等对板材的显微组织结构有重要的影响。在100~450℃温度范围内进行CCAE变形,AZ31镁合金的晶粒尺寸随变形温度的升高而增大。AZ31镁合金经CCAE热变形后,合金的综合力学性能得到改善。     

基于FEM的战地车用AZ31镁合金板材ICAE工艺研究

细化晶粒、提高金属综合性能是当前战地车用AZ31镁合金板材塑性成形工艺迫切需要研究的内容。提出了AZ31镁合金板材的减径通道转角(Ironing channel angular extrusion,ICAE)挤压成形工艺。采用数值模拟结合理论分析的方法,对ICAE工艺下200 mm×2 mm的AZ31镁合金板材微观组织演化规律进行了研究。结果表明:ICAE通过晶粒破碎和动态再结晶可以显著细化AZ31镁合金晶粒,板材平均晶粒尺寸可达15μm。     

挤压态镁合金板材复合变形过程中的孪晶组织演变规律

采用扫描电镜(SEM)研究了挤压态镁合金压痕—压平复合变形过程中动态再结晶及孪晶组织演变规律。结果表明:在镁合金压痕—压平复合变形过程中,随着复合变形系数和变形温度的增加,AZ31镁合金的孪晶数量逐渐增多,动态再结晶程度增大,晶粒细化效果明显。动态再结晶的主要形核之处是原始晶粒的晶界处,动态再结晶新晶粒产生于原始晶粒的晶界处,且形状为典型的项链状结构,孪晶界也是动态再结晶的有利形核位置。在压痕-压平复合变形过程中,较低的变形温度和较大的变形程度有利于孪晶组织的形成,且孪晶组织容易出现在大晶粒内部。     

AZ31镁合金差温热轧过程的晶粒细化

实验研究了经不同道次差温热轧AZ31镁合金的金相组织,结合对轧制过程,尤其是轧件温度场的数值模拟结果,分析了AZ31镁合金差温热轧过程晶粒细化机制与主要影响因素,获得了通过轧制过程动态再结晶,使轧材晶粒尺寸随轧制道次增加,而持续细化的工艺参数,并制备出平均晶粒尺寸为5μm左右的细晶AZ31镁合金板材。     

非对称轧制AZ31镁合金板材组织与性能

采用商用连铸连轧AZ31镁合金板材,通过小辊径非对称轧制工艺,研究在150,200,250℃温度条件下多道次非对称轧制对镁合金板材组织、织构和力学性能的影响。结果表明,不同轧制温度下,镁合金板材的晶粒细化机理不同,150℃时以孪晶细化为主,部分晶粒发生动态再结晶,200和250℃时板材晶粒细化机理为动态再结晶。对比分析了对称轧制和非对称轧制板材织构演化规律,随着轧制温度的升高,非对称轧制板材基面织构依次增强,但明显低于对称轧制板材。     

AZ31镁合金挤压板材的显微组织与力学性能研究

为了获得高性能镁合金板材,采用正向热挤压将铸态AZ31镁合金坯料挤压成2 mm厚的板材,研究了其显微组织演变及力学性能等。结果表明:铸态AZ31镁合金坯料挤压成板材后可以获得均匀细小的再结晶晶粒组织,其力学性能(屈服强度、抗拉强度、伸长率)大幅度提升。铸态AZ31镁合金坯料在400、450℃挤压成板材后,平均晶粒尺寸可由390μm分别细化至3.9、5.6μm。挤压后的AZ31镁合金板材展现出典型的(0001)基面织构,大部分晶粒的c轴垂直于板材表面。铸态AZ31镁合金的力学性能较差,而AZ31镁合金挤压板材在三个拉伸方向上均展现出优越的力学性能。随挤压温度的升高,AZ31镁合金挤压板材晶粒长大且显微组织不均匀,综合力学性能也有所下降。     

汽车用AZ31镁合金板材轧制工艺研究

采用光学显微镜(OM)、硬度测试等手段研究了轧制温度和压下率对AZ31镁合金铸轧板材显微组织和硬度的影响。结果表明:轧制温度350℃和总压下率72%轧制的AZ31镁合金试样组织中有大量孪晶出现,细小的再结晶晶粒分布在孪晶内部和α相晶界处,将大尺寸晶粒分割成较小晶粒,未发生再结晶的晶粒明显发生扭曲变形,组织得到明显细化。在350～410℃,随着轧制温度的升高,AZ31镁合金试样平均晶粒尺寸逐渐增大,试样硬度逐渐降低。轧制温度350、380、410℃,总压下率72%时,试样的硬度分别为86.6、84.7、79.5HV。     

变形参数对AZ31镁合金组织性能的影响

对AZ31镁合金铸棒在不同变形温度和变形程度下的再结晶行为进行了观察，并测量了各变形条件下的拉伸性能。结果表明，挤压变形及动态再结晶，可以显著的细化铸造AZ31合金的晶粒(由铸态的约100μm减少到约5μm)。随变形温度的升高，AZ31合金的抗拉强度下降，到一定温度后，趋于稳定。     

挤压比对AZ31镁合金组织结构的影响

试验研究了挤压比对AZ31镁合金组织结构的影响.结果表明,258℃挤压变形镁合金在形变初期容易出现孪晶,再结晶晶粒一般出现在晶界和孪晶附近.挤压比小时动态再结晶晶粒平均尺寸为2 μm,动态结晶细化了晶粒.随着挤压比增大,晶粒尺寸减小,组织趋于均匀.挤压比达到16时,动态再结晶基本完成.挤压比为25时,能得到晶粒细小且均匀的组织,平均晶粒尺寸为7.3 μm.     

LZ91镁锂合金板材的超塑性气胀成形研究

本文利用热拉伸实验、气胀成形实验、金相分析和扫描电镜观察,研究LZ91镁锂合金板材的超塑性、气胀成形性能及其组织结构。结果表明:在热拉伸变形温度为573 K、应变速率为0.001 s-1时,其伸长率可达343.7 %,应变速率敏感指数为0.697,轧制态的LZ91合金板材表现出优良的超塑性;在胀形温度573 K,胀形气压0.06 MPa条件下,板材成形高度为51.14 mm,高径比达1.279,说明该镁锂合金板材具有良好的超塑性成形潜力;在热拉伸变形和超塑性气胀成形过程中,均有动态再结晶现象产生,可有效提高该合金的塑性成形能力;在拉伸断口和胀形件破裂处断口均存在典型的超塑性空洞形貌特征,说明两者的主要变形机制均为晶界滑移,且合金超塑性失效的主要原因是空洞的长大和连接。     

THE SUPERPLASTICITY AND DEFORMATION MECHANISM OF ULTRALIGHT BINARY Mg-8wt%Li ALLOY

1.IntroductionMagnesium--lithiumalloysarethelightestalloysamongthenonpoisonousmetalsandalloystll.Becauseoftheirlowdellsityandhighspecificstiffness,theyhavethepotentialforuseinaerospacesarmoredvehicle,automobileandelectricindustry.InordertoturnMg--Lialloysintospecificcomponentswithminimumamountofmachiningandjoining,itisdesirablethatMg-Lialloysexhibitsuperplasticity.UPtonow,superfinegraillscanbeobtainedinmagnesium-lithiumalloysbythefollowingways'(1)staticannealingafterthermomechanicaltreatment…     

EFFECT OF TENSION CONDITIONS ON SUPERPLASTIC BEHAVIOUR OF AN Al-Li-Cu-Mg-Zr ALLOY

A much higher elongation of a warm rolled superplastic Al-Li-Cu-Mg-Zr alloy was madeunder two-stage strain rate tests comparing with the single ones.During initial stage ofdeformation a deformation-induced continuous recrystallization which converted a subgrainstructure into a recrystallized grain structure by a continuous increase in boundarymisorientations had occurred.The higher the strain rate,the faster the continuousrecrystallization and the finer the recrystallized grains.The fine recrystallized grain structureformed during the first stage deformation is the essential condition for the material to havehigh strain rate hardening and strain hardening during the low second stage superplasticdeformation.The combination of strain rate hardening and strain hardening is the reason whythe higher elongation may be obtained during two-stage superplastic deformation of the alloy.     

拉伸条件对Al—Li—Cu—Mg—Zr合金超塑变形行为的影响

研究了拉伸条件对温轧态Al-Li-Cu-Mg-Zr合金超塑变形行为的影响.结果表明:该合金在变形初期发生了界面取向差逐渐增大,从亚晶组织向再结晶组织变化的形变促使连续再结晶过程;应变速率越高,再结晶速度越快,再结晶晶粒越细;经高应变速率的第一阶段拉伸变形后,形成的细晶组织具有高的应变速率敏感性,同时在低应变速率的第二阶段拉伸变形中发生晶粒长大而具有高的应变硬化效果,两者的综合作用是两段速率拉伸获得高延伸率的根本原因。     

Fe_3Al－Ti超塑性变形过程中晶粒形态演化规律

对Ｆｅ_３Ａｌ－Ｔｉ合金超塑性变形中不同应变量下的晶粒形态进行了研究。发现Ｆｅ_３Ａｌ－Ｔｉ超塑性行为与连续再结晶有关，随变形量增大，晶粒逐渐细化，但晶粒形状变化不大，并沿拉伸方向有所伸长。ＴＥＭ分析表明，晶粒细化过程与超塑性过程中亚晶界向大角晶界的演化有关。本文对Ｆｅ３Ａｌ－Ｔｉ合会超塑性变形机理进行了初步的探讨。     

Mechanism of dynamic continuous recrystallization during superplastic deformation in a microduplex stainless steel

Microstructural evolution during the cyclic cold-rolling and annealing process in an (α + γ) microduplex stainless steel, which consists of α subgrains and fine γ particles, has been studied in detail with the aim of clarifying the mechanism of dynamic continuous recrystallization. A continuous increase in α subgrain boundary misorientation is obtained by the present processing where grain boundary sliding does not occur and the effect of increasing boundary misorientation with cumulative strain is comparable to those observed in dynamic continuous recrystallization of superplastic aluminium alloys. The increase in boundary misorientation is accounted for by the absorption of dislocations into subgrain boundaries during annealing, dislocations which had operated to accommodate the plastic strain incompatibility of the neighboring phases undergoing slip deformation. The present results show that grain boundary sliding is not indispensable but the difference in accommodation deformation between adjacent subgrains is of great importance for the dynamic continuous recrystallization during superplastic deformation.     

DYNAMIC RECRYSTALLIZATION IN SUPERPLASTIC DEFORMATION OF TiAl BASED ALLOY

１ＩＮＴＲＯＤＵＣＴＩＯＮＳｕｐｅｒｐｌａｓｔｉｃｉｔｙｏｆＴｉＡｌｂａｓｅｄａｌｏｙｈａｓａｔｔｒａｃｔｅｄｍｏｒｅａｎｄｍｏｒｅａｔｅｎｔｉｏｎ［１－３］．Ｉｔｈａｓｂｅｅｎｆｏｕｎｄｔｈａｔ,ｄｙｎａｍｉｃｒｅｃｒｙｓｔａｌｉｚａｔｉｏｎｉｓ...     

DYNAMIC RECRYSTALLIZATION AND SUPERPLASTICITY IN Al-Li ALLOY

The behavior of dynamic recrystallization in the superplastic deformation of 8090 and 2091 aluminum-lithium alloys have been investigated.TEM observations indicated that dynamic recrystallization occurs at thetriple junction of grain boundaries.The measurement of grain boundary angle showed that recrystallization indynamic equilibrium exists in the process of superplastic deformation of 8090 Al-Li alloy.It is also indicatedthat,besides the role of refining grains and the grain boundary sliding,dynamic recrystallization playsconcurrently a role of stablizing microstructure.Thus dynamic rccrystallization can be used to induce metalssuperplasticity,which leads to a simplification of pretreatment for superplastic deformation.     

Electron backscattered diffraction study on superplastic coarse-grained Fe–27 at.%Al: processing effects

An electron backscattered diffraction technique has been used to investigate detailed crystallographic features of a superplastic coarse-grained Fe–27 at.%Al alloy. Alloy samples studied have been tensile tested to failure at 800°C in air under an initial strain rate of 1×10⁻⁴ s⁻¹. To examine processing effects, the hot isostatic pressing (HIP) has been applied prior to the superplastic deformation. The HIPed sample shows no observable pores in the fracture region while the sample without HIP reveals an elongated pore fracture structure. Nevertheless, HIP is shown to have no beneficial effects on the superplastic elongation, suggestive of the fact that the alleviation of cavity formation alone is insufficient in achieving better superplastic properties. After the superplastic deformation and the refined grains are formed, the presence of numerous small angle subboundaries in the large grain interior indicates the continuous event of recovery and recrystallization that occurs throughout the course of superplastic deformation. The post-deformation annealing yields a classic recrystallized large-grain structure, resulting from the surface-tension-induced boundary migration that reduces the grain surface-area. Conversely, the superplastic deformation of Fe–27 at.%Al involves a strain-induced boundary migration that causes the grain surface-area increase and results in a refined grain structure. The dynamic nature of recovery and recrystallization is therefore confirmed.