A new die-cast magnesium alloy for applications at higher elevated temperatures of 200-300℃

The development of lightweight magnesium(Mg)alloys capable of operating at elevated temperatures of 200-300℃and the ability of using high pressure die casting for high-volume manufacturing are the most advanced developments in manufacturing critical parts for internal combustion engines used in power tools.Here we report the microstructure and mechanical properties of a newly developed die-cast Mg-RE(La,Ce,Nd,Gd)-Al alloy capable of working at higher elevated temperatures of 200-300℃.The new alloy delivers the yield strength of 94 MPa at 300℃,which demonstrates a 42%increase over the benchmark AE44 high temperature die-cast Mg alloy.The new alloy also has good stiffness at elevated temperatures with its modulus only decreasing linearly by 13%from room temperature up to 300℃.Thermal analysis shows a minor peak at 364.7℃in the specific heat curve of the new alloy,indicating a good phase stability of the alloy up to 300℃.Nd and Gd have more affinity to Al for the formation of the minority of divorced Al-RE(Nd,Gd)based compounds,and the stable Al-poor Mg_(12)RE(La_(0.22)Ce_(0.13)Nd_(0.31)Gd_(0.31))Zn_(0.39)Al_(0.13)compound acts as the continuous inter-dendritic network,which contribute to the high mechanical performance and stability of the new die-cast Mg alloy at 200-300℃.

Section thickness-dependent tensile properties of squeeze cast magnesium alloy AM60

The development of alternative casting processes is essential for the high demand of light weight magnesium components to be used in the automotive industry, which often contain different section thicknesses. Squeeze casting with its inherent advantages has been approved for the capability of minimizing the gas porosity in magnesium alloys. For advanced engineering design of light magnesium automotive applications, it is critical to understand the effect of section thickness on mechanical properties of squeeze cast magnesium alloys. In this study, magnesium alloy AM60 with different section thicknesses of 6, 10 and 20 mm squeeze cast under an applied pressure of 30 MPa was investigated. The prepared squeeze cast AM60 specimens were tensile tested at room termperature. The results indicate that the mechanical properties including yield strength （YS）, ultimate tensile strength （UTS） and elongation （A） decrease with an increase in section thickness of squeeze cast AM6O. The microstructure analysis shows that the improvement in the tensile behavior of squeeze cast AM60 is primarily attributed to the low-gas porosity level and fine grain strucuture which result from the variation of cooling rate of different section thickness. The numerical simulation （Magmasoft） was employed to determine the solidification rates of each step, and the simulated results show that the solidification rate of the alloy decreases with an increase in the section thickness. The computed solidification rates support the experimental observation on grain structural development.

Characterization of mechanical properties of aluminum cast alloy at elevated temperature

The tensile response, the low cycle fatigue （LCF） resistance, and the creep behavior of an aluminum （A1） cast alloy are studied at ambient and elevated temperatures. A non-contact real-time optical extensometer based on the digital image correlation （DIC） is developed to achieve strain measurements without damage to the specimen. The optical extensometer is validated and used to monitor dynamic strains during the mechanical experiments. Results show that Young＇s modulus of the cast alloy decreases with the increasing temperature, and the percentage elongation to fracture at 100 ℃ is the lowest over the temperature range evaluated from 25 ℃ to 300 ℃. In the LCF test, the fatigue strength coefficient decreases, whereas the fatigue strength exponent increases with the rising temperature. The fatigue ductility at 100 ℃. As expected, the resistance to and changes from 200 ℃ to 300 ℃. coefficient and exponent reach maximum values creep decreases with the increasing temperature

Microstructure and elevated-temperature tensile properties of differential pressure sand cast Mg-4Y-3Nd-0.5Zr alloy

The microstructures of an Mg-4Y-3Nd-0.5Zr alloy by differential pressure casting were investigated using scanning electron microscopy （SEM） and transmission electron microscopy （TEM）, and its tensile deformation behavior was measured using a Gleeble1500D themo-simulation machine in the temperature range of 200 to 400 ℃ at initial strain rates of 5×10^-4 to 10^-1 s^-1. Results show that the as-cast microstructure consists of primary α-Mg phase and bone-shaped Mg5RE eutectic phase distributed along the grain boundary. The eutectic phase is dissolved into the matrix after solution treatment and subsequently precipitates during peak aging. Tensile deformation tests show that the strain rate has little effect on stress under 300 ℃. Tensile stress decreases with an increase in temperature and the higher strain rate leads to an increase in stress above 300 ℃. The fracture mechanism exhibits a mixed quasi-cleavage fracture at 200 ℃, while the fracture above 300 ℃ is a ductile fracture. The dimples are melted at 400 ℃ with the lowest strain rate of 10^-4 s^-1.

Microstructure evolution and tensile mechanical properties of thixoformed AZ61 magnesium alloy prepared by squeeze casting

A novel process that combines squeeze casting with partial remelting to obtain AZ61 magnesium alloy with semi-solid microstructures was proposed. In this route, the squeeze casting was used to predeform the magnesium alloy billets to obtain small dendritic structures. During subsequent partial remelting, small dendritic structures transform into globular grains surrounded by liquid films. The results show that the squeeze casting AZ61 alloy after partial remelting produces more ideal, finer semi-solid microstructure compared with as-cast AZ61 alloy treated by the same isothermal holding conditions. Moreover, the mechanical properties of the thixoformed AZ61 alloy prepared by squeeze casting plus partial remelting are better than those of the thixoformed alloy prepared by conventional casting plus partial remelting.

Microstructure and mechanical properties of AZ91D magnesium alloy by expendable pattern shell casting with different mechanical vibration amplitudes and pouring temperatures

To refine the microstructure and improve the mechanical properties of AZ91 D alloy by expendable pattern shell casting(EPSC),the mechanical vibration method was applied in the solidification process of the alloy.The effects of amplitude and pouring temperature on microstructure and mechanical properties of AZ91 D magnesium alloy were studied.The results indicated that the mechanical vibration remarkably improved the sizes,morphologies and distributions of the primaryα-Mg phase andβ-Mg17 Al12 phase,and the densification and tensile properties of the AZ91 D alloy.With an increase in amplitude,the microstructures were gradually refined,resulting in a continuous increase in mechanical properties of the AZ91 D alloy.While,with the increase of pouring temperature,the microstructures were continuously coarsened,leading to an obvious decrease of the mechanical properties.The tensile strength and yield strength of the AZ91 D alloy with a vibration amplitude of 1.0 mm and a pouring temperature of 730℃were 60%and 38%higher than those of the alloy without vibration,respectively.

Effect of mischmetal on mechanical properties and microstructure of die-cast magnesium alloy AZ91D

Effects of the mischmetal addition in range of 0.4 wt.% to 1.7 wt.% on the microstructure and mechanical properties of die-cast magnesium AZ91D were investigated to improve the elevated temperature mechanical properties of the alloy by scanning electron microscopy, transmission electron microscopy, X-ray diffraction and tensile tests. The results revealed that mechanical prop- erties of die-cast magnesium alloy AZ91D-0.4%MM at 100 ℃ were near to those of die-cast magnesium alloy AZ9 ID. The ultimate tensile strength, 0.2% proof yield strength and elongation to failure of die-cast magnesium alloy AZ91D at 170 ℃ were 178, 129 MPa and 20%, respectively. In comparison, the ultimate tensile strength, 0.2% proof yield strength and elongation to faihtre of die-cast magnesium alloy AZ91D-0.4%MM at 170℃reached to 206, 142 MPa and 26%, respectively increased by 15.7%, 10% and 30%. Proper addition of mischmetal could enhance the mechanical properties at an elevated temperature, which was attributed to the formation of A1-RE phases with high thermal stability. Hence sliding of grain boundaries and cracks could be effectively hindered by A1-RE phases.

Low temperature mechanical properties of as-extruded Mg-10Gd-3Y-0.5Zr magnesium alloy

Influence of multi-cycle cryogenic treatment and tensile temperature on microstructure, mechanical properties and fracture mechanism of as-extruded Mg-10Gd-3Y-0.5Zr magnesium alloy was investigated. The results show that there have no significant changes in tensile properties of the tested alloy after 10 d in liquid nitrogen immersion or 10 cycles of high-low temperature treatment at all test temperatures. The room temperature ultimate tensile strength increases from 398 MPa to 417 MPa after 20 cycles of high-low temperature treatments. Compared with the room temperature, the tested alloys exhibit higher tensile properties at low temperatures. At -196 °C, the yield strength and ultimate tensile strength of the as-extruded-T5 Mg-10Gd-3Y-0.5Zr alloy are 349 MPa and 506 MPa, respectively, increasing by about 18% and 27%, respectively. The transgranular cleavage fracture mechanism is observed at room temperature, while at low temperatures both ductile fracture and cleavage fracture behaviors coexist.

Processing of AM60 magnesium alloy by hydrostatic cyclic expansion extrusion at elevated temperature as a new severe plastic deformation method

Hydrostatic cyclic expansion extrusion(HCEE) process at elevated temperatures is proposed as a method for processing less deformable materials such as magnesium and for producing long ultrafine-grained rods. In the HCEE process at elevated temperatures, high-pressure molten linear low-density polyethylene(LLDPE) was used as a fluid to eliminate frictional forces. To study the capability of the process,AM60 magnesium rods were processed and the properties were investigated. The mechanical properties were found to improve significantly after the HCEE process. The yield and ultimate strengths increased from initial values of 138 and 221 MPa to 212 and 317 MPa, respectively.Moreover, the elongation was enhanced due to the refined grains and the existence of high hydrostatic pressure. Furthermore, the microhardness was increased from HV 55.0 to HV 72.5. The microstructural analysis revealed that ultrafine-grained structure could be produced by the HCEE process. Moreover, the size of the particles decreased, and these particles thoroughly scattered between the grains. Finite element analysis showed that the HCEE was independent of the length of the sample, which makes the process suitable for industrial applications.

Influence of temperature and strain rate on flow stress behavior of twin-roll cast, rolled and heat-treated AZ31 magnesium alloys

The effects of temperature and strain rate on the flow stress behavior of twin-roll cast, rolled and heat-treated AZ31 magnesium alloys were investigated under uniaxial tension. At high temperatures, dynamic recovery, continuous dynamic recrystallization, grain boundary sliding and the activation of additional slip systems lead to an improvement of the ductility of the alloys. The elongation to failure is nearly independent of the strain rate between 473 and 523 K at 10-2 s-1 and 10-1 s-1, which is related to the strain rate dependence of the critical resolved shear stress（CRSS） for nonbasal slip. Despite the high temperature, twins are even observed at 573 K and 10-3 s-1 because they have a low CRSS.

Influence of Ce addition on microstructure and mechanical properties of high pressure die cast AM50 magnesium alloy

The influence of Ce addition on the microstructure and mechanical properties of AM50 magnesium alloy was investigated to improve its mechanical properties.The results show that the addition of Ce to AM50 alloy results in the grain refinement and the mechanical properties of the Ce-modified AM50 at room and elevated temperatures are remarkably improved.AM50 magnesium alloy containing 1% Ce（mass fraction） shows better refinement and mechanical properties compared with the AM50 magnesium alloy with 0.5% Ce and even AM50 alloy without any Ce.

Re-evaluation of the mechanical properties and creep resistance of commercial magnesium die-casting alloy AE44

This paper presents a re-evaluation of the room temperature mechanical properties and high temperature creep resistance of magnesium die-casting alloy AE44(Mg-4Al-4RE)in light of the influence of minor Mn addition.It is shown that the Mn-containing AE44 exhibits distinct age hardening response upon direct ageing(T5)due to the precipitation of nanoscale Al-Mn particles,as reported previously in a similar alloy.The T5 ageing leads to a significant improvement in strength with similar ductility.Consequently,the T5-aged AE44 has a remarkably better strength-ductility combination than most Mg die-casting alloys and even the Al die-casting alloy A380.Minor Mn addition is also shown to be critical for the creep resistance of AE44 whereas the influence of the RE constituent is not as significant as previously thought,which reaffirms that precipitation hardening of theα-Mg matrix is more important than grain boundary reinforcement by intermetallic phases for the creep resistance of die-cast Mg alloys.The findings in this work could provide new application perspectives for AE44,particularly in the automotive industry.

Influence of TiB_2 nanoparticles on elevated-temperature properties of Al-Mn-Mg 3004 alloy

Two contents(1.5%and3%)of TiB2nanoparticles were introduced in Al?Mn?Mg3004alloy to study their effects on theelevated-temperature properties.Results show that TiB2nanoparticles were mainly distributed at the interdendritic grain boundarieswith a size range of20?80nm,which is confirmed by transmission electron microscopy(TEM)and X-ray diffraction(XRD).Therefore,the volume fraction of the dispersoid free zones is greatly reduced and the motion of grain boundaries and dislocations isinhibited more effectively at elevated temperature.After peak precipitation heat treatment,the yield strengths in the alloy with3%TiB2addition at room temperature and300°C were increased by20%and13%respectively,while the minimum creep rate at300°Cwas reduced to only1/5of the base alloy free of TiB2,exhibiting a considerable improvement of elevated-temperature properties inAl?Mn?Mg alloys.

Microstructure and mechanical properties of AZ91-Ca magnesium alloy cast by different processes

The microstructure and mechanical properties of magnesium （Mg） alloys are significantly influenced by the casting process. In this paper, a comparative study on microstructure and mechanical properties at ambient and elevated temperatures of AZ91-2wt.% Ca （AZX912） Mg alloy samples prepared by gravity casting （GC）, squeeze casting （SC） and rheo-squeeze casting （RSC）, respectively, was carried out. The results show that mMg grains in SC and RSC samples are significantly refined compared to the GC sample. The average secondary dendritic arm spacing of AZX912 alloy samples decreases in the order of GC, SC and RSC. As testing temperature increases from 25 ~C to 200 ~C, strength of AZX912 alloy samples is reduced, while their elongation is increased continuously. Compared to GC and SC processes, RSC process can improve the mechanical properties of AZX912 alloy at both ambient and elevated temperatures. The enhancement of mechanical properties of RSC sample over GC and SC samples mainly results from grain refinement in the as-cast microstructure of AZX912 alloy.

Structure and mechanical properties of AZ31 magnesium alloy billets by different hot-top semi-continuous casting technology

AZ31 alloy billets of 200 mm in diameter were produced by three different processes of conventional direct chill （DC） casting, low-frequency electromagnetic casting （LFEC） and low-frequency electromagnetic vibration casting （LFEVC）, respectively. The effect of LFEC and LFEVC on the microstructures, macmsegregation and mechanical properties of AZ31 alloy billets was investigated. In conventional DC casting, the AZ31 alloy billets exhibited coarse grains （about 370 μa） and severe segregation of A1 and Zn. In the presence of a solo low-frequency alternating magnetic field or a low-frequency electromagnetic vibration field applied during DC casting of Ф200 mm AZ31 billets, grains in the AZ31 alloy billets were effectively reffmed （about 210 μa） and the macrosegregation of A1 and Zn in the billets was greatly decreased. Furthermore, the tensile strength, fracture elongation and hardness of the as-cast AZ31 alloy billets were improved by the processes of LFEC and LFEVC relative to that cast by the process of conventional DC casting.

Effect of Adding Extruded Mg-Mischmetal Intermediate Alloy on Microstructure and Properties of Die-Casting Magnesium Alloy AZ91D

The preparation techniques of Mg-mischmetal intermediate alloy and the effects of the mischmetal addition ranging from 0.45% to 1.04% on the microstructure and properties of AZ91D alloy prepared by die casting were investigated. The Mg-MM intermediate alloy was prepared by permanent mold casting and then was extruded into the bars. The microstructure and analytical studies were carried out using optical microscopy and differential scanning calorimetry (DSC). Testing results shows the Mg-MM intermediate alloy could melt easily down at die casting temperature of 680 ℃ that was lower than the melting point of lanthanum (918 ℃) and that of cerium (798 ℃). This was propitious to protection the alloy from the oxidation at high temperatures. Then magnesium alloy test bars were produced under conventional cold chamber die casting condition with addition of different weight of the Mg-MM intermediate alloy. Observation and analysis indicated that the microstructures of the alloy were refined and RE containing Al phase was formed with increasing RE addition. The data obtained by tensile tests showed that alloying with mischmetal improved the tensile property of the AZ91D magnesium die casting alloy at ambient temperature.

On the deformation behavior of heterogeneous microstructure and its effect on the mechanical properties of die cast AZ91D magnesium alloy

Both a conventional flow distributer and an improved one with a flow buffer were applied respectively during the high pressure die casting(HPDC)process,and samples of AZ91D magnesium alloy with different microstructure mainly consisting ofα-Mg grains,β-phase and porosities were obtained.According to the grain orientation analysis,the predominant deformation behavior inα-Mg grains was dislocation slip,supplemented by deformation twinning.Dislocation slip was more difficult to occur in the samples with the improved flow distributer on account of the fact that the size ofα-Mg grains in the microstructure was finer and more uniform.During the in situ tensile deformation test,cracks were observed to initiate from gas-shrinkage pore and island-shrinkage,and two main crack propagation mechanisms,porosity growth and coalescence were found accordingly.When the crack was in contact with theβ-phase,it would pass through and fracture the networkβ-phase,whereas bypass the islandβ-phase by detaching it from the surroundingα-Mg grains.Mechanical property tests showed that the samples with relatively more homogeneous microstructure would perform higher mechanical properties,which was the combined effect of matrixα-Mg grains,β-phase,and porosities.

Microstructure and mechanical properties of magnesium alloy prepared by lost foam casting

The microstructure and mechanical properties of AZ91 alloy prepared by lost foam casting（LFC） and various heat treatments have been investigated. The microstructure of the AZ91 alloy via LFC consists of dominant α-Mg and β-Mg17Al12 as well as a new phase Al32Mn25 with size of about 550 μm, which has not been detected in AZ91 alloy prepared by other casting processes. The tests demonstrate that the as-cast mechanical properties are higher than those of sand gravity casting because of chilling and cushioning effect of foam pattern during the mould filling. The solution kinetics and the aging processes at different temperatures were also investigated by hardness and electrical resistivity measurements. The kinetics of aging are faster at the high temperature due to enhanced diffusion of atoms in the matrix, so the hardness peak at 380 ℃ occurs after 10 h; while at the lower aging temperature（150 ℃）, the peak is not reached in the time（24 h） considered.

As-cast Microstructure,Mechanical Properties and Casting Fluidity of ZA84 Magnesium Alloy Containing TiC

The as-cast microstructure, mechanical properties and casting fluidity of ZA84 alloy containing TiC were investigated. The experimental results indicate that adding 0.5wt%TiC to ZA84 alloy can refine the as-cast microstructure, and do not cause the formation of any new phase. After 0.5wt%TiC was added to the ZA84 alloy, the morphology of ternary phases on the grain boundaries changed from coarse quasi-continuous net to fine disconnected net, and the distribution of ternary phases became dispersive and homogeneous. At the same time, the tensile properties of ZA84＋0.5TiC alloy at room temperature were comparable to those of AZ91D alloy, and were higher than those of ZA84 alloy. At 150 ℃, the tensile and creep properties of ZA84＋0.5TiC alloy were also higher than those of ZA84 and AZ91D alloys. In addition, compared with the AZ91D alloy, the casting fluidity of ZA84＋0.5TiC alloy was slightly poor, but better than that of ZA84 alloy. The reason could be related to the effect of TiC on the solidification temperature range of ZA84 alloy.

Inhomogeneity of microstructure and mechanical properties of a 500 mm diameter heavy section semi-continuous cast AZ61 magnesium alloy ingot

For the large magnesium alloy ingot, there is a considerable difference in cooling rate of different parts in the ingot, which leads to non-uniform distribution of the secondary phases, solute segregation and tensile properties. In the present research, an heavy AZ61 alloy ingot with a diameter of 500 mm was made by semi-continuous casting. The microstructure and mechanical properties at different positions along the radial direction of the large ingot were investigated by using an optical microscope(OM), a scanning electron microscope(SEM), an energy dispersive spectroscope(EDS), and a micro-hardness tester. The results indicate that the microstructure of the AZ61 ingot is non-uniform in different locations. It changes from equiaxed to columnar grains from the center to the edge; the average grain size gradually reduces from 1,005 μm to 763 μm, the secondary dendrite arm spacing reduces from 78 μm to 50 μm, and the Mg17(Al,Zn)12 phase is also refined. The micro-hardness value increases from 55.4 HV at the center to 72.5 HV at the edge of the ingot due to the microstructure differences, and the distribution of micro-hardness at the edge of the ingot is more uniform than that in the center. The tensile properties at room temperature show little difference from the center to the edge of the ingot except that the elongation at the edge is only 3.5%, much lower than that at other areas. The fracture mechanism is ductile fracture at the center and cleavage fracture at the edge of the ingot, and at the 1/2 radius of the ingot, a mixture of ductile and cleavage fracture is present.