Grain refinement of DC cast AZ91D Mg alloy by intensive melt shearing

Abstract

Melt conditioning by advanced shear technology (MCAST) is a new process for microstructural refinement of both cast and wrought magnesium alloys. Melt conditioned direct chill (MCDC) casting combines the MCAST process with conventional direct chill (DC) casting. In the present work, melt conditioning has been combined with permanent mould casting to simulate the production of DC cast AZ91D billets and slabs. The results show that the MCDC process can achieve significantly finer grain size and more uniform microstructure than conventional DC process for both billets and slabs. Grain refinement in the MCDC process is due to the fine and well dispersed oxide particles produced after processing in the MCAST unit.     

Influence of undissolved second-phase particles on dynamic recrystallization behavior of Mg-7Sn-1Al-1Zn alloy during low-and high-temperature extrusions

This study investigates the effects of fine and coarse undissolved particles in a billet of the Mg-7Sn-1Al-1Zn (TAZ711) alloy on the dynamic recrystallization (DRX) behavior during hot extrusion at low and high temperatures and the resultant microstructure and mechanical properties of the alloy.To this end,partially homogenized (PH) and fully homogenized (FH) billets are extruded at temperatures of 250 and 450 ℃.The PH billet contains fine and coarse undissolved Mg2Sn particles in the interdendritic region and along the grain boundaries,respectively.The fine particles (＜1 μm in size) retard DRX during extrusion at 250 ℃ via the Zener pinning effect,and this retardation causes a decrease in the area fraction of dynamically recrystallized (DRXed) grains of the extruded alloy.In addition,the inhomogeneous distribution of fine particles in the PH billet leads to the formation of a bimodal DRXed grain structure with excessively grown grains in particle-scarce regions.In contrast,in the FH billet,numerous nanosized Mg2Sn precipitates are formed throughout the material during extrusion at 250 ℃,which,in turn,leads to the formation of small,uniform DRXed grains by the grain-boundary pinning effect of the precipitates.When the PH billet is extruded at the high temperature of 450 ℃,the retardation effect of the fine particles on DRX is weakened by their dissolution in the α-Mg matrix and the increased extent of thermally activated grain-boundary migration.In contrast,the coarse Mg2Sn particles in the billet promote DRX during extrusion through the particle-stimulated nucleation phenomenon,which results in an increase in the area fraction of DRXed grains.At both low and high extrusion temperatures,the extruded material fabricated using the PH billet,which contains both fine and coarse undissolved particles,has a lower tensile strength than that fabricated using the FH billet,which is virtually devoid of second-phase particles.This lower strength of the former is attributed mainly to the larger grains and/or absence of nanosized M2Sn precipitates in it.     

等径道角挤压制备高力学性能细晶Mg-6Al合金

为了制备高力学性能细晶Mg-6Al合金坯料,采用金相显微镜、材料拉伸实验机等手段对Mg-6Al合金铸坯进行等径道角挤压实验研究.并利用热处理工艺对挤压后材料进行处理,研究热处理工艺参数对材料力学性能的影响规律.结果表明,Mg-6Al合金的铸坯的抗拉强度为196.4MPa,延伸率为12.6%.经过等径道角挤压的Mg-6Al合金坯料的晶粒被大大细化,其晶粒尺寸由铸坯的140μm左右细化到8μm左右.其力学性能有很大提高,抗拉强度由196.4MPa提高到308.2MPa;延伸率由12.6%提高到30.6%.等径道角挤压工艺是一种非常好的制备高力学性能、细晶Mg-6Al合金的工艺方法.固溶和人工时效热处理工艺对等径道角挤压的Mg-6Al合金坯料的强度有较大影响,对延伸率影响较小.     

Microstructure and mechanical properties of hot extruded Mg–Al–Mn–Ca alloy produced by rapid solidification powder metallurgy

The microstructure and mechanical properties of hot extruded Mg–Al–Mn–Ca alloy was investigated. Both rapid solidified powders and cast billets were extruded at 573, 623 and 673 K to optimize the processing conditions for obtaining better mechanical response. Powder was consolidated to prepare the extrusion billets using both cold compaction and Spark Plasma Sintering at 473 K. The tensile properties of the extruded alloy were then evaluated and correlated to the observed microstructure. The results show that the use of rapid solidified powder could lead to effective grain refinement, which in turn resulted in the improved mechanical response, especially compared to the extruded conventional cast material.     

镁合金ZK60-RE半固态坯的微观组织演变

采用常规铸造法和等径道角挤压分别制备了镁合金ZK60-RE半固态坯;用金相显微镜研究了2种半固态坯料在等温热处理过程中的微观组织演变.结果表明:与传统铸造方法制备的半固态坯相比,采用等径道角挤压制备的半固态坯的晶粒细小、圆整,适合于半固态成形.在等温热处理过程中,2种坯料晶粒粗化的机制是合并长大和Ostwald长大.铸...     

挤压温度对AZT802镁合金组织和性能的影响

将铸态AZT802合金分别在350℃、380℃和400℃下挤压,随后进行T5时效处理,研究不同挤压温度对AZT802镁合金挤压态和时效态组织和性能的影响。结果表明,当挤压温度为350℃时,晶粒尺寸分布不均匀,同时观察到大块的条状第二相沿挤压方向析出。当挤压温度高于350℃时,挤压态合金获得均匀等轴晶粒,第二相以颗粒状形貌沿晶界均匀分布。经T5时效处理后,颗粒状Mg_2Sn相均匀分布于基体中,Mg_(17)Al_(12)相以连续相和非连续相析出,非连续析出相随时效前挤压温度的升高而逐渐增多。力学性能测试结果表明,AZT802合金在380℃下挤压,并进行175℃(3h)T5时效处理后,获得最佳综合力学性能。     

Microstructures and mechanical properties of Mg–Al–Zn–Ca alloys fabricated by high frequency electromagnetic casting method

The microstructures and mechanical properties of AZ31 and 1 wt%Ca-containing AZ31 billets fabricated using EMC (Electromagnetic Casting) and EMS (Electromagnetic Stirring) were examined. The results show a great potential of producing high-quality surface magnesium billets with fine-grained microstructure at a relatively high casting speed. Application of EMC + EMS for production of the 1 wt%Ca-AZ31 alloy billet with a diameter of 150 mm produced a reasonably homogeneous microstructure composed of fine grains with an average size of 45 μm. Attainment of the fine-grained and homogeneous microstructure by EMC + EMS was attributed to reduction of temperature gradient and fragmentation of dendrite structure under electromagnetic force. Strength of the EMC and EMC + EMS 1 wt%Ca-AZ31 billets was higher than that of the EMC AZ31 billet due to the grain size and particle strengthening effects.     

大塑性变形的AM60镁合金半固态等温处理研究

为了制备晶粒细小且球化程度高的的AM60镁合金半固态坯料,对铸态和等径道角挤压态的AM60镁合金半固态等温处理过程进行了研究.借助金相显微镜对AM60镁合金铸坯和等径道角挤压后的铸坯在半固态等温处理中的微观组织演变进行了观察.研究结果表明:对于AM60镁合金,直接等温处理获得的半固坯晶粒很粗大,其平均晶粒尺寸都在100μm以上,晶粒球化效果不理想,很难获得合格的半固态坯;新SIMA法是一种非常理想制备AM60镁合金半固态坯的方法,利用该方法制备的AM60半固态坯的微观组织晶粒十分细小,平均晶粒尺寸在8～22μm,晶粒球化程度高;随着保温时间的延长,新SIMA法制备的AM60半固态坯的微观组织出现长大现象;随着等温处理温度的升高,固相晶粒的平均尺寸先增加后减小,晶粒球化程度越来越高.     

Microstructure and Properties of As-Cast Mg–2.0Sn–1.0Zn–1.0Y–0.3Zr Alloys at Different Extrusion Ratios

Herein, the effect of hot extrusion with different extrusion ratios (λ = 6, 8, 10, and 12) on the microstructure evolution and properties of as-cast Mg–2.0Sn–1.0Zn–1.0Y–0.3Zr magnesium alloys, using optical microscopy (OM), scanning electron microscopy (SEM), immersion corrosion and electrochemical corrosion experiment, and tensile testing, is investigated. The results show that the Mg₁₄SnY and Mg₆SnY precipitated phases exist in the alloy before and after extrusion. After hot extrusion, the second phase of the alloy is broken into particles along the extrusion direction, whereas the grain size is significantly reduced, and dynamic recrystallization and deformed grains exist in the microstructure. The mechanical properties of the extruded alloy improve, but the corrosion resistance weakens. When the extrusion ratio is λ = 10, the extruded alloy exhibits relatively good mechanical properties and corrosion resistance. The corrosion behaviors of the extruded alloys are affected by both the grain size and galvanic corrosion. In the initial stage of corrosion, intergranular corrosion plays a major role in reducing the corrosion resistance of the extruded alloys. With prolonged corrosion time, galvanic corrosion has a more significant effect on weakening the corrosion resistance of the extruded alloys.     

Effect of extrusion temperature on mechanical properties of as-extruded Zn–22Al alloys

ABSTRACT

The effect of extrusion temperature on the microstructures and mechanical properties of as-extruded Zn–22Al alloys was investigated in this study. With decrease of extrusion temperature from 350 to 200°C, the elongation of as-extruded Zn–22Al alloys increases remarkably at low strain rate and has no change at high strain rate, which implies that the Zn–22Al alloys extruded at lower temperature exhibit high-ductility behaviour. X-ray diffraction and electron back-scattered diffraction analysis demonstrated that the maximum elongation of Zn–22Al alloys extruded at the extrusion temperature of 200°C can be attributed to the elimination of the lamellar structure and the refinement in grain size of the Zn-rich phase.     

Effect of consolidation temperature on mechanical properties of rapidly solidified Al–7Mg–1 Zr alloy

Abstract

The mechanical properties achieved via the extrusion of non-degassed billets prepared from an inert gas atomised powder of nominal composition Al–7Mg–lZr are reported. The alloy was extruded over the temperature range 350–550°C and the tensile mechanical properties and plane strain fracture toughness were evaluated. It was found that the yield strength remained fairly constant over the entire temperature range, with only a small decrease in strength observed at the highest extrusion temperature. The strength could be related to microstructure using standard models for solid solution, dispersoid, and substructural strengthening mechanisms, and the last was found to make the greatest contribution. The sensitivity of strength to subgrain size was found to be nearly three times higher than that for pure Al. The optimum combination of strength and fracture toughness was obtained for extrusion at 500°C (yield strength 400 MN m^?2; T–L K_Iv 21 MN m^?3; elongation 20%). The poor values of K_lv obtained at other temperatures were attributed to coarse dispersoids (highest extrusion temperature), undeformed powder particles (lowest extrusion temperature), and inhomogeneous dispersoid distributions (intermediate temperatures). It is concluded that extrusion process control plays an important role in determining the mechanical properties of consolidated rapidly solidified powders. Considering the excellent ductility and toughness obtained, vacuum degassing before extrusion may not be essential in the processing of inert gas atomised powders of a non heat treatable composition.

MST/1721     

High performance stainless steel via powder metallurgy hot isostatic pressing

Abstract

Argon atomised austenitic stainless steel (AISI 304) powder was characterized for its physical properties such as particle shape, microstructure, median particle size, particle size distribution, apparent density, tap density, and jlowrate. Subsequently, the tap density of the as received powder was improved to the desired level by adjusting the powder distribution followed by mixing and blending. This powder was subjected to hot isostatic pressing (hipping) at two different combinations of temperature and pressure to optimise the microstructure and mechanical properties. Mechanical properties of the stainless steel obtained by the powder metallurgy (PM) hipping route were found to be superior to those of conventionally processed wrought steel. The superior performance of PM hipped steel is attributed to its low oxygen content, fine grain size, and high degree of chemical homogeneity. Although the production of billets by the hipping route does not appear to be economical owing to the high capital cost of the hot isostatic press, the added advantage of obtaining a nearnet shape makes the process economically viable for production of intricate shapes.     

Fabrication Conditions, Microstructures and Mechanical Properties of P/M Processed 2XXX Al-SiCW Composites

The powder metallurgy fabrication of 2XXX Al composites reinforced with SiC whiskers was studied by investigating the evolution of microstructure and its relation to the mechanical properties. In this study, SiC whiskers and gas-atomized aluminum powders were mixed by fluid zone mixer, consolidated by vacuum hot press, and then extruded. The optimum condition for consolidation was 620°C and 50 MPa, at which fully densified pore-free billets were obtained. The composites with relatively homogeneous microstructures were produced by extrusion at 450–500°C under the extrusion pressure of 700–1000 MPa. The mechanical properties of the extruded bars were found to be comparable with those of the composites processed by Advanced Composite Materials Corp. The optimum fabrication conditions have been proposed for producing composites of improved mechanical properties through elimination of coarse intermetallic particles, uniform distribution of reinforcements, and minimization of whisker breakage. The possibility of using particulates rather than whiskers, and the modification of the alloy matrices for high temperature applications are also discussed in relation to the distribution of reinforcements and the optimization of the consolidation temperature.     

Microstructure development and tensile mechanical properties of Mg–Zn–RE–Zr magnesium alloy

The Mg–5.3 wt.%Zn–1.13 wt.%Nd–0.51 wt.%La–0.28 wt.%Pr–0.79 wt.%Zr alloy prepared by direct chill casting is subjected to hot extrusion. The effects of extrusion ratio and temperature on microstructure and tensile mechanical properties have been studied. The results indicate coarse grains of as-cast alloys are refined with extrusion ratio increasing from 0 to 9. The eutectic constituents are elongated along extrusion direction. However, further increase of extrusion ratio has a little influence on grain refinement and the improvement of mechanical properties of the alloy. Dynamic recrystallisation is the main mechanism of grain refinement during hot extrusion. Raising extrusion temperature results in grain coarsening. Grain shape becomes more equiaxed-like with raising extrusion temperature. At the same time, mechanical properties decrease with the increase of extrusion temperature.     

Direct chill casting of magnesium alloy under pulsed magnetic field

To refine the grain size and improve the hot workability, the billets of AZ80 magnesium alloy were cast by a new process of the pulsed magnetic field-direct chill casting. The effect of pulsed magnetic field on the grain size, segregation and mechanical properties of cast billet were investigated by experiments. The results indicate that the grain size of cast billet with pulsed magnetic field was greatly refined, and the homogeneity of microstructure was improved significantly. Meanwhile, the macrosegregation and microsegregation of main alloying elements in the cast billet with pulsed magnetic field were suppressed. Compared with the conventional cast billet, the yield strength, ultimate tensile strength and elongation of the cast billet with pulsed magnetic field were increased obviously.     

Dynamic recrystallization behavior of AZ31 magnesium alloy processed by alternate forward extrusion

One of the important factors that affect the microstructure and properties of extruded products is recrystallization behavior. Alternate forward extrusion (AFE) is a new type of metal extrusion process with strong potential. In this paper, we carried out the AFE process experiments of as-cast AZ31 magnesium alloy and obtained extrusion bar whose microstructure and deformation mechanism were analyzed by means of optical microscopy, electron backscattered diffraction and transmission electron microscopy. The experimental results indicated that homogeneous fine-grained structure with mean grain size of 3.91 μm was obtained after AFE at 573 K. The dominant reason of grain refinement was considered the dynamic recrystallization (DRX) induced by strain localization and shear plastic deformation. In the 573-673 K range, the yield strength, tensile strength and elongation of the composite mechanical properties are reduced accordingly with the increase of the forming temperature. Shown as in relevant statistics, the proportion of the large-angle grain boundaries decreased significantly. The above results provide an important scientific basis of the scheme formulation and active control on microstructure and property for AZ31 magnesium alloy AFE process.     

Design of powder metallurgy aluminium alloys for applications at elevated temperatures Part 2 Tensile properties of extruded and Conformed gas atomised powders

Abstract

The effects of aging treatments on the tensile properties and microstructure of Al–Cr–Zr–Mn powder metallurgy aluminium alloys prepared from high pressure gas atomised powders were investigated. The alloy compositions were designed to give powders with or without Al₁₃Cr₂ intermetallics in the <45 μm size fraction. The Al–5·2Cr–1·4Zr–1·3Mn alloy is typical of the former (concentrated alloy) and the Al–3·3Cr–0·7Zr–0·7Mn alloy of the latter (dilute alloy). The alloys were prepared using a canning/degassing/extrusion sequence or the Conform consolidation process. Measurements of micro hardness and electron microscopy were used to correlate the microstructure with the tensile properties. The extruded powders of both alloys exhibited better properties than those of the Conformed powders. A large contribution to the strength of the extruded materials is made by their stabilised fine grain size. The dilute alloys had consistently better ductility. Neither alloy retained its strength after prolonged aging at 400°C, but the results indicate that a service temperature of 300°C may be possible.

MST/1247b     

Characterization of hot extruded Mg/SiC nanocomposites fabricated by casting

Mg–1%SiC nanocomposites were fabricated using an ultrasonic cavitation based casting method, resulting in the dispersion of the reinforcing SiC nanoparticles to form Mg–metal matrix nanocomposite (Mg–MMNC) billets. The MMNC billets were then processed using hot extrusion at 350 °C. Micrographic observations illustrate a significant grain size reduction and the presence of microbands that align the SiC nanoparticles parallel to the direction of extrusion for Mg–MMNCs. Observations from the cross-section at 90° of the extrusion direction show uniform nanoparticles dispersion. Results from the extruded Mg–MMNCs tensile testing at different temperatures (25, 125 and 177 °C) reveal an increase of the yield strength, ultimate tensile strength, and ductility values as compared to the un-reinforced and extruded Mg-alloy; such increase was also observed from the microhardness testing results where an increase from 19 to 34% was measured.     

Zn添加对挤压态Mg-Al-Ca-Mn合金微观组织和力学性能的影响

目的 研究不同含量Zn元素对镁合金塑性、强度的改良效果.方法 以Mg-Al-Ca-Mn合金为基础,采用热挤压成形加工方法,分析不同Zn含量对其显微组织和力学性能的影响.结果 Zn元素可以改变挤压态镁合金的显微组织,对其主合金相影响不大,但可以改变衍射峰强度.Zn元素可以提高挤压态镁合金的屈服强度和伸长率,提高镁合金韧性;ACMZ2合金综合性能最佳.结论 热挤压成形能够细化镁合金晶粒,Zn元素含量可以改善镁合金微观组织和力学性能,可在实际生产中根据需求调整Zn元素含量以获得最佳性能.     

Influence of texture on the mechanical properties of commercially pure magnesium prepared by powder metallurgy

In the present work the influence of texture on the mechanical properties up to 500 °C of commercially pure magnesium prepared by PM was determined. Extrusion of magnesium powders was carried out between 250 and 450 °C. All extruded materials exhibited an intense fibre texture with the basal planes parallel to the extrusion direction whose intensity increased in line with the extrusion temperature. The microstructure consisted of highly elongated magnesium powder particles. All the materials presented a heterogeneous grain size resulting from the size distribution of the original magnesium powder particles. In addition, small MgO particles were found mainly decorating the original powder boundaries. The best mechanical properties corresponded to the materials extruded at 400 and 450 °C. This behaviour was associated particularly with the intense fibre texture of these materials.