Effect of twins and non-basal planes activated by equal channel angular rolling process on properties of AZ31 magnesium alloy

Ultrafine-grained (UFG) metallic materials because of their superior properties have received considerable research interest. Recently, severe plastic deformation (SPD) processes are widely used for refining the grain size in magnesium alloys. Equal channel angular rolling (ECAR) is a SPD process based on equal channel angular pressing (ECAP) which is carried out on large, thin sheets. After doing this process, no significant change is occurred in cross-sectional area of specimen. In this research, an AZ31 magnesium alloy was subjected to ECAR. After completing eight passes of process, significant grain refinement was occurred, and the average grain size of about 3.9 μm was achieved. The distribution of grain size becomes more limited by increasing number of passes. Rotation of basal plane and activation of non-basal and twin planes were clearly observed in X-ray diffraction (XRD) pattern results. Mechanical properties were studied via tensile and hardness tests at room temperature. Tension tests indicated that better ductility due to the rotation of basal plane was achieved. Elongation-to-failure was increased from 8% of as-received material to 19% after two passes of process. Hardness values showed an increase of about 53% at eighth pass.     

循环道次对高温叠轧AZ31镁合金板材组织与性能的影响

目的通过高温累积叠轧工艺制备出高强度的镁合金,并研究该过程中循环道次对AZ31镁合金板材的微观组织与性能的具体影响。方法对累积叠轧1～5次板材进行微观组织观察,并进行显微硬度的测试,得到不同板材的硬度值,通过X射线衍射分析得到不同板材的取向结果,最终进行力学性能实验,并对比分析。结果随着循环道次的增加,板材抗拉强度有明显改变。从260 MPa先增加至310 MPa,最后稳定在350 MPa左右;非基面织构比重增加;断裂伸长率先降低后升高并稳定在10%左右。结论累积叠轧工艺使得AZ31镁合金板材产生了加工硬化,并显著细化了晶粒。循环道次的增加、孪晶产生和晶界数量显著增多导致强度进一步提高。     

Processing of AZ31 magnesium alloy by a new noble severe plastic deformation method

In the present investigation a wrought magnesium alloy (AZ31) has been processed applying the accumulative back extrusion (ABE) method. This was performed through different thermomechanical processing routes (different ABE deformation passes at temperatures of 80-380 °C). The results indicate that AZ31 alloy may successfully be deformed through ABE processing even at temperatures as low as 80 °C. Following the ABE processing a sophisticated microhardness testing was conducted and thorough microstructural observations were undertaken using optical microscopy. The results show that the equiaxed submicron size grains have been achieved. As the number of passes was increased, a more homogeneous microstructure with finer mean grain size was obtained. It was also found that increasing the temperature resulted in larger mean grain size and also higher microstructural homogeneity.     

Investigation on the effect of friction stir processing on the superplastic forming of AZ31B alloy

Superplastic forming has now become conventional for forming complex parts from sheet metals. In many superplastically formed aerospace components, only a selective region undergoes superplastic forming. In those cases, instead of selecting a material exhibiting superplastic properties, a light weight and low cost material can be chosen and its microstructure can be modified locally by the Friction Stir Processing (FSP) technique. In this work, AZ31B magnesium alloy is chosen, and friction stir processing is performed by varying the process parameters, such as tool axial force, tool traversing speed and tool rotational speed. The process parameter that produced equiaxed grains in the stirred zone with a grain size less than 10 μm is selected. With this parameter, single pass FSP, multiple pass FSP without overlapping and multiple pass FSP with overlapping are performed on the AZ31B magnesium alloy sheets and their superplastic behaviour was examined. Also the theoretical modelling was carried out to determine the strain rate sensitivity for the friction stir processed AZ31B magnesium alloy and for the nonprocessed AZ31B magnesium alloy. It is found that the strain rate sensitivity for the friction stir processed component has increased, when compared to the base metal.     

Microstructure and mechanical properties of Mg-Al-Zn alloy sheet fabricated by cold extrusion

Cold extrusion of AZ31 magnesium alloy sheets was studied in this paper. Microstructure and texture distributions of the as-extruded sheet were investigated by electron backscattered diffraction (EBSD) method. The grains were significantly refined and the average grain size was 1.6 μm. Dynamic recrystallization has taken place during the extrusion process, which resulted in the high frequency of high angle grain boundaries in the sheet. After the cold extrusion, a weak double-peak type basal texture was formed. The formation of the texture was ascribed to the non-basal <c + a> slips. Tensile tests revealed that mechanical properties were enhanced due to grain size refinement, but mechanical anisotropy was obvious. It is believed that mechanical anisotropy was related to the splitting of basal texture.     

Effect of rotational speed and probe profile on microstructure and hardness of AZ31/Al2O3 nanocomposites fabricated by friction stir processing

Friction stir processing (FSP) was used to fabricate AZ31/Al₂O₃ nanocomposites for surface applications. The effects of probe profile, rotational speed and the number of FSP passes on nanoparticle distribution and matrix microstructure were studied. The grain refinement of matrix and improved distribution of nanoparticles were obtained after each FSP pass. By increasing the rotational speed, as a result of greater heat input, grain size of the base alloy increased and simultaneously more shattering effect of rotation, cause a better nanoparticle distribution. The average grain size of matrix of the composites was in the range of 1–5 μm and the microhardness of them was 85–92 Hv.     

初始晶粒尺寸对高温交叉轧制镁合金板材组织和力学性能的影响

目的 揭示晶粒尺寸对多道次高温交叉轧制AZ31镁合金板材组织和力学性能的影响规律及机制.方法 通过对不同初始晶粒尺寸的镁合金板材进行高温交叉轧制变形及热处理,获得不同状态的镁合金板材,采用金相显微分析、X射线衍射(XRD)分析及室温拉伸实验等手段研究镁合金板材的晶粒组织(形态、尺寸、取向)及力学性能.结果 经过多道次交...     

Microstructure and properties of magnesium alloy processed by a new severe plastic deformation method

A new severe plastic deformation (SPD) method called C shape equal channel reciprocating extrusion (CECRE) was developed to fabricate fine grained AZ31 Mg alloys. The results show that homogeneous microstructure with mean grain size of 3.6 μm is obtained as the accumulated true strain is increased to 11. Strain localization leading to dynamic recrystallizaion (DRX) occurring is the main reason for grain refinement during CECRE process. At the same time, the hardness of AZ31 alloy increases from 62.6 of as-extruded to 74.6 of CECRE 4 passes.     

The microstructure,tensile, and shear deformation behavior of an AZ31 magnesium alloy after extrusion and equal channel angular pressing

The shear punch testing (SPT) technique and the uniaxial tension tests were employed to evaluate the mechanical properties of the equal channel angularly pressed (ECAPed) AZ31 magnesium alloy. After extruding, the material was ECAPed for 1, 2, and 4 passes using route B_C. The grain structure of the material was refined from 20.2 to 1.6 μm after 4 passes of ECAP at 200 °C. The 4 pass ECAPed alloy showed lower yield stress and higher ductility as compared to the as-extruded condition, indicating that texture softening has overcome the strengthening effects of grain refinement. The same trends in strength and ductility were also observed in shear punch testing. Similar shear strength and ductility values of the samples taken perpendicular to the extrusion direction (ED) and normal direction (ND) after 4 passes of ECAP indicated that {0 0 0 2} basal planes were inclined (∼45°) to the extrusion axis. The shear punch testing technique was found to be a useful method for verifying directional mechanical properties of the miniature samples of the ECAPed magnesium alloys.     

Multi-pass large strain rolling of as-cast AZ31 magnesium alloy

As-cast AZ31 magnesium alloy subjected to multi-pass large strain rolling was investigated. A successive rolling process up to three passes was carried out at 370°C with a pass reduction of 30%. Deformation microstructure characteristics prove that the dynamic recrystallisation (DRX) mode changed with the increase of rolling passes. In the first pass, DRX related to twinning played a dominant role. But in the third pass, DRX grains mainly appeared around the pre-existing grain boundaries. The ultimate strength and elongation of rolled sheets after three passes rolling are enhanced by 37 and 39%, respectively, compared to the as-cast alloy. Meanwhile, the tensile fracture mode was ductile fracture which was different from the ductile–brittle fracture of as-cast.     

AZ31镁合金薄板的无针搅拌摩擦加工

目的 研究有针、无针搅拌摩擦加工对AZ31镁合金薄板的微观组织和力学性能的影响。方法 通过搅拌摩擦加工技术（FSP）以不同的转速对AZ31镁合金薄板进行加工,采用拉伸试验机、金相显微镜、UMT摩擦磨损试验机、维氏硬度机对无针搅拌加工后的AZ31镁合金加工表面的晶粒形貌、拉伸性能、磨损性能和硬度进行研究分析,并与同转速有针搅拌进行比较。结果 无针条件下与同转速下的有针搅拌相比,焊缝表面更加细密、美观,无针加工下焊缝的抗拉强度最大为242MPa,测得的维氏硬度最大为97.6HV,且焊缝的平均摩擦因数最小,为0.31。结论 无针时FSP镁合金焊缝的抗拉强度随刀具转速的提高而增大;焊缝的硬度与镁合金母材相比有明显的提高,且随刀具转速的提高,维氏硬度值逐渐降低;无针条件下获得的AZ31镁合金焊缝的平均摩擦因数随刀具转速的增加而增大,与同转速时有针条件下获得的焊缝的平均摩擦因数相比,无针时获得的焊缝平均摩擦因数明显更高;无针时获得的焊缝表面的晶粒尺寸随转速的增加而增大。     

Effect of impurity reduction on rollability of AZ31 magnesium alloy

The effect of impurity reduction on the hot rolling behavior of AZ31 magnesium alloy was systematically investigated in this study. In the as-cast alloys, the total content of main impurity elements such as Fe, Si, Cu, and Ni was varied from 0.0462 to 0.0163 wt% by changing the purity of used raw magnesium metals. The alloys after homogenization were subjected to hot rolling at 300 °C with a reduction of 20% per pass. It was found that the initiation of edge cracks is postponed with reducing impurity level in the alloys. And the maximum rolling reduction prior to edge cracking increases from 34 to 58% as the impurity content drops from 0.0462 to 0.0163 wt%. Microstructural observations showed that smaller grains are present in the alloy with lower impurity content in the cast and homogenization states. Moreover, decreasing impurity content leads to a reduced number of deformation twins and an enhanced volume fraction of small recrystallized grains in the as-rolled microstructure, which indicates that impurity reduction is beneficial to the recrystallization process and subsequent plastic deformation. Based on the results, the enhancement in hot rollability of the AZ31 sheet by impurity reduction should be due to finer grain size, the reduced number of deformation twins and the enhanced extent of recrystallization.     

Improvement in rollability of AZ91 magnesium alloy by carbon addition

The present study aims to investigate the effect of carbon addition on the hot rolling behavior of as-cast AZ91 alloy. The AZ91 and C-added AZ91 alloys were subjected to hot rolling at 400 °C with a reduction of 30% per one pass. The as-cast C-added AZ91 alloy with very fine equi-axed grains of approximately 75 μm exhibited excellent hot rollability compared to as-cast AZ91 alloy with coarse dendrite structure, although the final grain size of the rolled C-added AZ91 alloy sheet was slightly larger than that of the rolled AZ91 alloy sheet. The side-crack occurrence on the surface during hot-rolling is mainly affected by the existence of twin boundary and the area fraction of grain boundaries. Based on the results, the improvement in rollability of the C-added AZ91 alloy is attributed to fine equi-axed grains and the polygonal Al₈Mn₅ phase located inside grains, which can homogeneously distribute and effectively absorb strain energy and prohibit crack growth.     

铸态AZ81镁合金ECAP态组织与性能研究

采用自制的90°模具,经Bc路径在温度为300℃下研究对比了铸态及不同道次的等通道挤压(ECAP)态AZ81镁合金微观组织和力学性能.结果表明ECAP随着挤压道次的增加,AZ81镁合金显微组织和力学性能发生显著变化.当挤压到4道次,平均晶粒尺寸由原来铸态的145um细化为9.6um,拉伸断口韧窝明显增多;抗拉强度从180 MPa提高到306 MPa,延伸率和硬度分别达到15.8%和142HL.分析表明,AZ81镁合金在高温挤压过程中Mg17Al12相粒子被破碎,并部分溶入基体,$-Mg基体与%-Mg17Al12相互相阻碍其晶粒长大,获得细小晶粒组织.     

循环扩挤变形AZ80镁合金的组织、织构与力学性能

采用循环扩挤(Cyclic expansion-extrusion,CEE)变形工艺对AZ80镁合金的块状材料进行热挤压加工,观察试样的微观组织与织构,并测试了力学性能。结果表明:AZ80镁合金经过CEE变形后,晶粒的尺寸明显细化,第4道次CEE变形之后,晶粒尺寸从150~230 μm细化至2 μm,整体分布均匀且呈等轴晶;2道次变形后,随着挤压道次的增加,晶粒的细化程度减慢;同时经过CEE变形的AZ80镁合金织构包括了(0001)基面平行于挤压方向与(1120)棱柱面垂直于挤压方向的两种不同纤维织构,随着挤压道次的增加,织构总体强度出现先减后增再减的变化;力学性能相对于均匀化态有着明显的变化,第1道次CEE变形之后,抗拉强度与屈服强度分别达到各自的最大值,为290 MPa和180 MPa,第2道次CEE变形之后,强度出现不随晶粒细化而增强的现象(反Hall-Petch理论),这是因为织构的软化作用强于晶粒的细化作用,而伸长率随着挤压道次的增加而提高。     

High shear dispersion technology prior to twin roll casting for high performance magnesium/SiCp metal matrix composite strip fabrication

SiC particulate (SiC_p) reinforced AZ31 magnesium alloy composite strips were produced by a novel process. In the process, a high shear technique was utilised to disperse the reinforcing particles uniformly into the matrix alloy, and AZ31/5 vol%SiC_p slurry was solidified into thin strip by a horizontal twin roll caster. The experimental results showed that the AZ31/5 vol%SiC_p strip obtained with high shear treatment exhibited a significantly refined microstructure and uniform distribution of reinforcing SiC particles. High cooling rate in the TRC process was also considered to contribute to the grain refinement of the matrix alloy, together with the possible heterogeneous nucleation effect of the reinforcing particles. The mechanical properties of the high shear treated composites strips showed enhanced modulus, yield strength and ductility by hardness and tensile tests. The experimental results were discussed in terms of the microstructural features and the macroscopic reliability, where necessary, analytical and statistical analyses were conducted.     

Processing of a magnesium alloy by equal-channel angular pressing using a back-pressure

Experiments were conducted on the magnesium AZ31 alloy to evaluate the significance of conducting equal-channel angular pressing (ECAP) with a back-pressure. Following processing by ECAP, the values of the Vickers microhardness were recorded on the cross-sectional planes and microstructural observations were undertaken using transmission electron microscopy. The results show an increase in the hardness in the first pass with significant microstructural inhomogeneity and a transition towards a more homogeneous structure with subsequent passes. The grain size was measured as 0.9 μm after 8 passes. A comparison with published data on the same alloy processed by ECAP without a back-pressure suggests several advantages in incorporating a back-pressure into ECAP. These advantages include the ability to achieve greater grain refinement, a potential for pressing at lower temperatures and the development of a more rapid evolution towards a homogeneous microstructure.     

Microstructural evolution and its effect on Hall-Petch relationship in friction stir welding of thixomolded Mg alloy AZ91D

Microstructural evolution of a thixomolded magnesium (Mg) alloy AZ91D during friction stir welding was investigated. Friction stir welding resulted in a homogeneous microstructure consisting of fine recrystallised -Mg grains in the thixomolded material. The microstructural homogenisation and refinement was attributed to dynamic recrystallisation accompanied by the dissolution of the eutectic structure during the welding. The grain refinement in the stir zone was effective in increasing the hardness, as predicted by the Hall-Petch equation. The effect of grain size on hardness was smaller than that in conventional and rapidly solidified AZ91. This phenomenon may be explained as being due to the microstructure of the stir zone which consisted of fine equiaxed grains with a high density of dislocations.     

Microstructure and mechanical properties of SPD-processed an as-cast AZ91D+Y magnesium alloy by equal channel angular extrusion and multi-axial forging

The aim of this work is to study the microstructure and mechanical properties of an as-cast AZ91D+Y magnesium alloy processed via two different severe plastic deformation techniques, equal channel angular extrusion (ECAE) and multi-axial forging (MAF). The grains were significantly refined after only one pass for both ECAE and MAF processed billets. However, the homogeneity of the SPD-processed microstructure increased with increasing number of passes. Micro-hardness and tensile tests showed that billets processed by ECAE and MAF techniques followed a same behaviour. With the increase of the number of processing passes (accumulated strain), the values of micro-hardness, yield strength, ultimate tensile strength and elongation were observed to increase. Grain refinement caused by dynamic recrystallization was introduced to explain the effects of the number of processing passes (accumulated strain) on the microstructure and mechanical properties of AZ91D+Y magnesium alloys processed by ECAE and MAF techniques.     

Texture and microstructure evolution in ultrafine-grained AZ31 processed by EX-ECAP

Extruded AZ31 alloy was processed by equal channel angular pressing (ECAP) up to 12 passes at 180 °C following route B_c, i.e. rotating the sample 90° between individual passes. Microstructure evolution was investigated using EBSD and TEM, as a function of strain imposed by ECAP. The first ECAP pass resulted in the formation of a new texture component which relates to the bimodal grain structure observed in this specimen. The grains larger than 10 μm show the orientation changes corresponding to the ECAP shear, which is characterised by the rotation of the basal poles by approximately 40° from the initial orientation. The fine grains with the average size of 1 μm maintain the initial orientation. The character of the bimodal grain structure and the distinct texture components between large and small grains remained unchanged up to 4 ECAP passes. Further ECAP pressing to 8 and 12 passes leads to a grain refinement through the whole sample volume and the orientation changes of all grains corresponding to the ECAP shear.