Abnormal mechanical property evolution induced by heat treatment for a semi-solid forming hypereutectic Al-Fe base alloy

In the present study, Al-5.5Fe-4Cu-2Zn-0.4Mg-0.5Mn al oy samples were prepared by electromagnetic stirring and semi-solid forming processing, and then the effects of T6 and T1 heat treatments on the microstructures and mechanical properties of the semi-solid forming samples were investigated. The results indicate that after semi-solid forming, the mechanical properties of the sample improved significantly compared to that of the merely electromagnetically stirred sample. The grains of semi-solid forming alloy became almost fine equiaxed; big long strip-shaped Al3 Fe phases became short rod-like morphology and distributed uniformly in the matrix. However, the mechanical properties of the T6-treated semi-solid forming sample decreased significantly instead of increasing and, with solution temperature rising, the tensile strength of the al oy decreased further. The results of EDS show that after high temperature solid-solution treatment, the Cu element in the semi-solid forming alloy sample is mainly concentrated at the boundaries of the Al3 Fe phases instead of being dissolved in the matrix. At the same time, the grains of the semi-solid forming sample grew slightly after solid-solution treatment. Therefore, the growth of the grains and the accumulation of Cu element at Al3 Fe phase boundaries during solution treatment of the semi-solid forming alloy were the main reasons for the mechanical properties decreasing after T6 treatment. The mechanical properties of the alloy were improved after T1 heat treatment due to aging strengthening phase being precipitated in the matrix.

Preparation of AZ91D Slurries for Semi-Solid Forming Using Al8 ( Mn, Fe) 5 Precipitates

During the solidification of the AZ91D-alloys, the Al8(Mn, Fe)5 phase is generally precipitated in the melt in advance of the precipitation of the primary α-Mg. The basic principle for manufacturing AZ91D-alloy slurries for semi solid forming is to use the Al8(Mn, Fe)5 precipitates as the heterogeneous nucleation sites for primary α-Mg phases. Microscopic analysis for the location of the Al8(Mn, Fe)5 precipitate explains that the Al8(Mn, Fe)5 precipitate is the effective heterogeneous nucleation site for the primary α-Mg phase. It was also observed that increase of the Mn content in the melt and the cooling rate below to the solid/liquid two-phase region resulted in smaller and more globular primary α-Mg due to the increase of heterogeneous nucleation sites. It was found that the average α-Mg diameter grew as a function of t0.278, where t is the holding time at the solid/liquid two-phase region. This would be attributed to the Ostwald type ripening and coalescence between primary α-Mg phases. The cooling rate below to the solid/liquid two-phase region, Mn content in AZ91D alloy, and the holding time and temperature affected on the quality of slurry.

The Formablity of AZ31B Magnesium Alloy Sheet

The forming limit diagrams （ FLD ） of AZ31B magnesium alloy sheet were tested by means of the electro etching grid method based on the forming experiment of magnesium alloy sheet carried out with a BCS- 30D sheet forming testing machine and the strain testing analysis made with an advanced ASAME automatic strain measuring system. Experiments show that, at room temperature, the mechanical properties and deep drawing peorformance of AZ31B cold-rolled magnesium alloy sheet were so poor that it failed to test the forming limit diagrams without an ideal forming and processing capacity, while the hot-rolled magnesium alloy sheet was of a little better plasticity and forming peorformance after testing its forming limit diagrams. It can be concluded that the testing of the forming limit curves （ FLC ） offers the theoretical foundation for the drawing of the deep drawing and forming process of magnesium alloy sheet.

Hot deformation behavior of a spray-deposited AZ31 magnesium alloy

The flow stress behavior of an as-spray-deposited AZ31 magnesium alloy with fine grains was investigated by means of compression tests with a Gleeble 1500 thermal mechanical simulator at isothermal constant strain rates of 0.01, 0.1, 1.0, and 10 s^-1; the testing temperatures ranged from 623 to 723 K. It is demonstrated that a linear equation can be fitted between the Zemer-Hollomon parameter Z and stress in a double-log scale. The effect of deformation parameters on the behavior of recrystallization was analyzed. Dynamic recrystallization （DRX） generally occurs at a higher temperature and at a lower strain rate. The constitutive equation of the spray-deposited AZ31 magnesium alloy is presented by calculating the deformation activation energy （199.8 kJ·mol^-1）. The as-spray-formed AZ31 alloy is easier for DRX nucleation at elevated temperatures due to the fine grain, which provides a large amount of nucleation sites and a high-diffusivity path for the atom.

Hot rolling characteristics of spray-formed AZ91 magnesium alloy

AZ91 magnesium alloy was prepared by spray forming. The spray-deposited alloy was subsequently hot-rolled with a 80% reduction at 350 ℃. The microstructural features of the as-spray-deposited and hot-rolled alloy were examined by optical microscopy, scanning electron microscopy and X-ray diffractometry. The results show that the spray-formed AZ91 magnesium alloy has, compared with the as-cast ingot, a finer microstructure with less intermetallic phase Mg17Al12 dispersed in the matrix due to fast cooling and solidification rates of spray forming process, and, therefore showing excellent workability. It can be hot-rolled with nearly 20% reduction for one pass at lower temperatures (330-360℃), and the total reduction can reach 50% prior to annealing. After proper thermo-mechanical treatment, the spray-formed AZ91 magnesium alloy exhibits outstanding mechanical properties.

Precision forging technologies for magnesium alloy bracket and wheel

Fundamental investigations on precision forging technology of magnesium alloys were studied.As-cast billet prestraining and a new concept of hollow billet were proposed in order to reduce the maximum forming load.A schcme of isothermal forming and the use of combined female dies were adopted,which can improve the die filling capacity and ensure the manufacture of high quality forgings.By means of the developed technique,AZ80 alloy wheel and AZ31 alloy bracket were produced successfully at suitable process parameters and applied in the automotive industries.The results show that the hot compression of AZ80 magnesium alloy has the peak flow stresses of pre-strained alloy with finer grain,which are lower by 20%than those of as-cast alloy under the same deformation conditions.The forming load is related to contact area and average positive stress on interface during forging process.

Consideration of Castability and Formability for New Magnesium Alloys

A comprehensive consideration based on castability or plastic formability, as well as mechanical properties for development of either cast magnesium alloys or wrought magnesium alloys is a very important issue. To develop new magnesium alloy sheets with high formability at room temperature, the microstructure, texture, ductility and anisotropy of rolled Mg-Zn-Gd alloy sheets were investigated. The sheets exhibit an excellent ultimate elongation of nearly 50% and an uniform elongation greater than 30% with a very low planar anisotropy. The new sheet has a random basal texture and the basal pole is tilted by maximum 40° from the normal direction towards the transverse direction. The majority of grains in the tilted texture have an orientation favorable for both basal slip and tensile twining because of their high Schmid factor. The low planar anisotropy, the large uniform elongations and the high strain hardening rate observed in the Mg-Zn-Gd sheets result in excellent room temperature formability, the Erichsen values reach ~8, well comparable with the conventional aluminum alloys sheets at room temperature. The solidification pathways and phase equilibria of Mg-Al-Ca alloys have been profoundly investigated by using thermal analysis and thermodynamic calculations. The relationship between hot tearing tendency and alloy compositions were discussed in terms of strength of the mushy zone, solidification pathways and feeding mechanisms, et al. Thixoforming refers to as that metal components are formed in their semi-solid state. Criteria for thixoforming are summarized and then the thixoformability of Mg-Al-Ca based alloys (AC alloys) are evaluated using the thermodynamic calculations based on the consideration of metallurgical parameters.

Microstructural evolution of equal channel angular pressed AZ91D magnesium alloy during semi-solid isothermal heat treatment

The microstructural evolution of AZ91D magnesium alloy processed by equal channel angular pressing during isothermal heat treatment at 570℃was investigated.The results indicated that the equal channel angular pressing followed by semi-solid isothermal heat treatment was an effective method to prepare semi-solid nondendritic slurry of AZ91D magnesium alloy.During this process,its microstructure change underwent four stages,the initial coarsening stage,the structure separation stage,the spheroidization stage and the final coarsening stage.The microstructural spheroidization effect was the best after being heated for 15 min for the alloy pressed for four passes,and the grain size was the smallest.With the further increase of heating time,the grain size and shape factor increased.When the heating time was kept constant,the grain size and shape factor decreased with the increase of pressing passes.

Partial melting behavior and thixoforming properties of extruded magnesium alloy AZ91 with and without addition of SiC particles with a volume fraction of 15%

A series of reheating-isothermal holding experiments and compression tests were conducted on pristine magnesium alloy AZ91 extruded by equal channel angular extrusion （ECAE） and SiC particles （a volume fraction of 15%） reinforced AZ91 composite （AZ91-SiCp） by regular extrusion. Dissolution of eutectic com- pounds and partial melting of the ct-Mg matrix occurred during the reheating of these materials. Spherical semisolid slurries of these materials were obtained when the reheating temperature and isothermal hold- ing time were 550℃ and 20 s, respectively. The presence of SiCp in AZ91-SiCp not only caused lower liquid fractions of semisolid slurries but also resulted in higher values of flow stress during semisolid compression tests. Both AZgl alloy and AZ91-SiCp composite exhibited better thixoforming properties at high temperatures. Segregation of SiCp did not occur during thixoforming of AZ91-SiCp composite after an isothermal holding at semisolid temperatures for 20 s.2018 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science ＆ Technology.

Plastic deformation of magnesium alloy with different forming parameters during ultrasonic vibration-assisted single-point incremental forming

The research of forming parameters on the ultrasonic vibration single-point incremental forming of magnesium alloy plastic deformation can provide a theoretical basis for the establishment of the forming parameters.According to the forming characteristics of magnesium alloy sheet,a new method of ultrasonic vibration-as sis ted single-point incremental forming was proposed.The influence of forming parameters on the plastic deformation of magnesium alloy was studied by finite element simulation and experimentation.The influence of vibration frequency,amplitude,friction coefficient,and tool head size on stress and thinning rate of magnesium alloy during ultrasonic vibration-as sis ted single-point asymptotic forming was studied.The results show that the vibration frequency of 20 kHz and forming tool radius of about 5 mm are beneficial for plastic deformation magnesium alloy in ultrasonic vibration-assisted single-point incremental forming.With vibration amplitude increasing,the maximum shear stress tends to decrease as a whole,but at the amplitude of 0.16 mm,the thinning rate is large and fracture occurs easily.With friction coefficient increasing,the maximum shear stress tends to increase,and there is a good linear relationship between the maximum thinning rate and the friction coefficient.

Additive manufacturing of magnesium and its alloys: process-formabilitymicrostructure-performance relationship and underlying mechanism

Magnesium and its alloys,as a promising class of materials,is popular in lightweight application and biomedical implants due to their low density and good biocompatibility.Additive manufacturing(AM)of Mg and its alloys is of growing interest in academia and industry.The domain-by-domain localized forming characteristics of AM leads to unique microstructures and performances of AM-process Mg and its alloys,which are different from those of traditionally manufactured counterparts.However,the intrinsic mechanisms still remain unclear and need to be in-depth explored.Therefore,this work aims to discuss and analyze the possible underlying mechanisms regarding defect appearance and elimination,microstructure formation and evolution,and performance improvement,based on presenting a comprehensive and systematic review on the relationship between process parameters,forming quality,microstructure characteristics and resultant performances.Lastly,some key perspectives requiring focus for further progression are highlighted to promote development of AM-processed Mg and its alloys and accelerate their industrialization.

Effect of microstructure on outer surface roughening of magnesium alloy tubes in die-less mandrel drawing

The crystal orientation and outer surface roughening of magnesium alloy tubes were evaluated to clarify the effect of the mandrel on the microstructure and outer surface roughness in die-less mandrel drawing. Locally heated ZM21 tubes with an outer diameter of 6.0 mm and an inner diameter of 3.8 mm were drawn with and without a mandrel. The outer surface roughness and crystal orientation were evaluated in the same measurement area. The results indicated that the outer surface becomes rougher in the die-less mandrel drawing than in die-less drawing for a given outer circumferential strain. The outer surface roughness developed when there was large difference in the pyramidal slip system Schmid factor. Therefore, the slip deformation of the pyramidal slip system seems to be mainly responsible for the outer surface roughening in the die-less mandrel drawing. Furthermore, the crystal grain with the {2110} crystal plane vertical to the normal direction of outer surface had a larger Schmid factor than the other crystal grains. The large number of crystal grains with the {2110} crystal plane in the die-less mandrel drawing is one of the reasons that the outer surface roughness develops more in the die-less mandrel drawing than in die-less drawing for a given outer circumferential strain. These results will contribute significantly to the development of fabrication process of the microtube with high surface quality, which prevents rapid corrosion of biomedical applications.

Preparation and thixoforging of semisolid billet of AZ80 magnesium alloy

Semisolid billet of AZ80 magnesium alloy was prepared by new strain induced melt activated (new SIMA) process and thixoforging experiment was performed.The results show that after as-cast AZ80 magnesium alloy is processed by equal channel angular extrusion, microstructure is refined well due to heavy dynamic recrystallization occurring in severe plastic deformation.Compared with semisolid isothermal treatment and conventional SIMA, semisolid billet with fine and spheroidal grains are achieved in new SIMA.Thixoforging process of semisolid billet prepared by new SIMA has many advantages such as good surface quality of final component, high ability to fill cavity and net-shape.The fine and spheroidal grains and high mechanical properties such as tensile strength of 298 MPa and elongation of 28% can be developed in final part thixoforged.

Semi-solid Forming of a Damper Housing with Dendritic and Non-dendritic Al-Si-Mg Alloy

A motorcycle component of damper housing was made by semi-solid forming process. This was used to investigate the effect of microstructures of feedstock on the formability of semisolid process. The soundness and microstructures of casting parts made by dendritic and non-dendritic feedstock were investigated. Separating of liquid phase was found in the casting produced by dendritic feedstock, which might result in defects of porosity, while uniform microstructures were found in the casting produced by no...

Effect of spray forming on evolution of microstructure and mechanical behavior of Mg-9Al-1Zn alloy

The cylindrical billet of Mg-9Al-1Zn alloy was produced by spray forming and hot extruded at 698-723 K with an extrusion ratio of 22-1. The microstructural evolution was investigated systematically. The mechanisms of grain refinement,solubility extension of Al and Zn elements in Mg matrix,and the precipitation of the second phase β-Mg17Al12 as well as their effect on the mechanical properties of the Mg-9Al-1Zn alloy were analyzed and discussed. The results show that the spray formed Mg-9Al-1Zn alloy has homogeneous equiaxed fine grains with the average grain size of 17 μm,of which the average grain size is refined further to 5 μm due to dynamic recrystallization during hot extrusion process. The evolution of size,proportion and distribution of β-Mg17Al12 phase was also observed and analyzed. The mechanical properties for the extruded rods are improved remarkably at ambient temperature.

Promotion mechanism of the pulse current on the superplastic deformation of AZ31 magnesium alloy

The effects of pulse current on the superplastic deformation of AZ31 magnesium alloy with different microstructures were examined. The results of TEM analysis showed that the dislocation movement was mainly glide, and the dislocation lines were approximate parallel with few dislocation tangles observed, which indicated that the dislocation movementwas promoted during the deformation, and therefore the formability of the coarse-grained AZ31 magnesium alloy was enhanced by the pulse current. This effect was also indicated by the asymmetrical contour of the free bulging sample, which was observed in the unidirectional pulses auxiliary equi-biaxial tensile test of coarse-grained alloy. In addition, the phenomenon of the restrained cavity growth caused by the thermoelectrical effect of the pulse current was discovered and studied.

Structural evolution of spray-formed Al-70wt.%Si alloys during iso-thermal holding in the semi-solid state

The grain growth behavior of spray-formed Al-70wt.%Si alloys was studied in the semi-solid state. The specimens were isothermally heat-treated at various temperatures between the solidus and liquidus of Al-Si alloys and then quenched in water. The microstructure of reheated specimens was characterized using optical and scanning electron microscopies. The isothermal holding experiment was carried out to investigate grain growth behavior as a function of holding time and temperature in the semi-solid state. The coarsening mechanism and the effect of porosity on microstructure were also studied.

基于DEFORM-3D的镁合金齿轮热锻成形工艺优化

以经均匀化处理后的铸态AZ80A镁合金的本构模型和动态再结晶(DRX)模型为基础,基于DEFORM-3D软件进行二次开发,通过正交试验设计,研究了铸态AZ80A镁合金直齿锥齿轮的最优热锻成形工艺参数,并对其进行验证和分析。结果表明,最优的工艺参数组合为:模具温度为623 K,坯料温度为693 K,摩擦因子为0.2,模具速度为0.5 mm/s.在该工艺下进行热锻成形,锻件大部分区域发生完全动态再结晶(DRX),齿形处平均晶粒尺寸小于10μm,锻件成形效果良好。

Forming conditions of blisters during solution heat treatment of Al–Si alloy semi-solid die castings

In the present study,numerical simulation method was used to analyze the conditions,resulting in the formation of blisters during solution heat treatment.Blister formation is considered to occur as the height of blister reaches 5μm.The effects of process parameters on the magnitude of the forming temperatures of blister(Tb)were discussed.Two kinds of Al-Si alloys were used to analyze the effect of mechanical properties of the alloys on blister forming conditions.Simulation results show that decreasing the initial pressure in gas hole,the long-short axial ratio of gas hole and the size of gas hole,as well as increasing the depth of gas hole and the strength of alloy are helpful to increase the critical temperature of forming blister.These conclusions are helpful for casters to understand the conditions controlling blister formation during solution heat treatment and take actions to avoid the blister defects.

Microstructural Evolution and Phase Transformations of Mechanically Stirred Non-dendritic ZA27 Alloy during Partial Remelting

The microstructural evolution and phase transformations of mechanically stirred non-dendritic ZA27 alloy during partial remelting were studied by using scanning electron microscopy and X-ray diffraction technique.The partial remelting temperature was 460℃ and lower than the stirring temperature of 465℃.So the microstructure with globular grains needed for semi-solid forming can not be obtained and the starting primary non-dendritic grains change in turn to connect non-dendritic grains, long chain-like structures and finally to coarsen connect grains.However,the small near-equiaxed grains between the primary non-dendritic grains are evolved into small globular grains gradually,some of which are also attached to the primary non-dendritic grains during the subsequent heating.The X-ray diffraction results show that a series of phase transformations, α+η+ε→β,η+β→L,β→α′+L,α+η+ε→α′ and α′→L, occur successively during this process.The main reason why the starting primary non-dendritic grains do not separate into the needed independent globular grains is that the reactions of η+β→L and α′→L do not occur or occurr incompletely in the layers used to connect the primary non-dendritic grains.