Effect of slow shot speed on externally solidified crystal,porosity and tensile property in a newly developed high-pressure die-cast Al-Si alloy

The effect of slow shot speed on externally solidified crystal(ESC),porosity and tensile property in a newly developed high-pressure die-cast Al-Si alloy was investigated by optical microscopy(OM),scanning electron microscopy(SEM)and laboratory computed tomography(CT).Results showed that the newly developed AlSi9MnMoV alloy exhibited improved mechanical properties when compared to the AlSi10MnMg alloy.The AlSi9MnMoV alloy,which was designed with trace multicomponent additions,displays a notable grain refining effect in comparison to the AlSi10MnMg alloy.Refining elements Ti,Zr,V,Nb,B promote heterogeneous nucleation and reduce the grain size of primaryα-Al.At a lower slow shot speed,the large ESCs are easier to form and gather,developing into the dendrite net and net-shrinkage.With an increase in slow shot speed,the size and number of ESCs and porosities significantly reduce.In addition,the distribution of ESCs is more dispersed and the net-shrinkage disappears.The tensile property is greatly improved by adopting a higher slow shot speed.The ultimate tensile strength is enhanced from 260.31 MPa to 290.31 MPa(increased by 11.52%),and the elongation is enhanced from 3.72%to 6.34%(increased by 70.52%).

Thin-walled and large-sized magnesium alloy die castings for passenger car cockpit:Application,materials,and manufacture

In order to effectively reduce energy consumption and increase range mile,new energy vehicles represented by Tesla have greatly aroused the application of integrated magnesium(Mg)alloy die casting technology in automobiles.Previously,the application of Mg alloys in automobiles,especially in automotive cockpit components,is quite extensive,while it has almost disappeared for a period of time due to its relatively high cost,causing a certain degree of information loss in the application technology of Mg alloy parts in automobiles.The rapid development of automotive technology has led to a higher requirement for the automotive components compared with those traditional one.Therefore,whatever the components themselves,or the Mg alloy materials and die casting process have to face an increasing challenge,needing to be upgraded.In addition,owing to its high integration characteristics,the application of Mg alloy die casting technology in large-sized and thin-walled automotive parts has inherent advantages and needs to be expanded urgently.Indeed,it necessitates exploring advance Mg alloys and new product structures and optimizing die casting processes.This article summarizes and analyzes the development status of thin-walled and large-sized die casting Mg alloy parts in passenger car cockpit and corresponding material selection methods,die casting processes as well as mold design techniques.Furthermore,this work will aid researchers in establishing a comprehensive understanding of the manufacture of thin-walled and large-sized die casting Mg alloy parts in automobile cockpit.It will also assist them in developing new Mg alloys with improved comprehensive performance and new processes to meet the high requirements for die casting automotive components.

Effects of magnesium and copper additions on tensile properties of Al-Si-Cr die casting alloy under as-cast and T5 conditions

Aluminum high pressure die casting(HPDC)technology has evolved in the past decades,enabling stronger and larger one-piece casting with significant part consolidation.It also offers a higher design freedom for more mass-efficient thin-walled body structures.For body structures that require excellent ductility and fracture toughness to be joined with steel sheet via self-piercing riveting(for instance,shock towers and hinge pillars,etc.),a costly T7 heat treatment comprising a solution heat treatment at elevated temperatures(450℃-500℃)followed by an over-ageing heat treatment is needed to optimize microstructure for meeting product requirement.To enable cost-efficient mass production of HPDC body structures,it is important to eliminate the expensive T7 heat treatment without sacrificing mechanical properties.Optimizing die cast alloy chemistry is a potential solution to improve fracture toughness and ductility of the HPDC components.The present study intends to tailor the Mg and Cu additions for a new Al-Si-Cr type die casting alloy(registered as A379 with The Aluminum Association,USA)to achieve the desired tensile properties without using T7 heat treatment.It was found that Cu addition should be avoided,as it is not effective in enhancing strength while degrades tensile ductility.Mg addition is very effective in improving strength and has minor impact on tensile ductility.The investigated Al-Si-Cr alloy with a nominal composition of Al-8.5wt.%Si-0.3wt.%Cr-0.2wt.%Fe shows comparable tensile properties with the T7 treated AlSi10MnMg alloy which is currently used for manufacturing shock towers and hinge pillars.

Characteristics and distribution of microstructures in high pressure die cast alloys with X-ray microtomography:A review

Al and Mg alloy high pressure die castings(HPDC)are increasingly used in automotive industries.The microstructures in the castings have decisive effect on the casting mechanical properties,in which the microstructure characteristics are fundamental for the investigation of the microstructure-property relation.During the past decade,the microstructure characteristics of HPDC Al and Mg alloys,especially micro-pores andα-Fe,have been investigated from two-dimensional(2D)to threedimensional with X-ray micro-computed tomography(μ-CT).This paper provides an overview of the current understanding regarding the 3D characteristics and formation mechanisms of microstructures in HPDC alloys,their spatial distributions,and the impact on mechanical properties.Additionally,it outlines future research directions for the formation and control of heterogeneous microstructures in HPDC alloys.

Low cycle fatigue behavior of T4-treated Al-Zn-Mg-Cu alloys prepared by squeeze casting and gravity die casting

Gravity die casting(GC) and squeeze casting(SC) T4-treated Al-7.0Zn-2.5Mg-2.1Cu alloys were employed to investigate the microstructures,mechanical properties and low cycle fatigue(LCF) behavior.The results show that mechanical properties of SC specimens are significantly better than those of GC specimens due to less cast defects and smaller secondary dendrite arm spacing(SDAS).Excellent fatigue properties are obtained for the SC alloy compared with the GC alloy.GC and SC alloys both exhibit cyclic stabilization at low total strain amplitudes(less than 0.4%) and cyclic hardening at higher total strain amplitudes.The degree of cyclic hardening of SC samples is greater than that of GC samples.Fatigue cracks of GC samples dominantly initiate from shrinkage porosities and are easy to propagate along them,while the crack initiation sites for SC samples are slip bands,eutectic phases and inclusions at or near the free surface.

Microstructure,mechanical properties and corrosion behavior of Al-Si-Cu-Zn-X(X=Bi,Sb,Sr) die cast alloy

The microstructure evolution, mechanical and corrosion properties of Al-11Si-2Cu-0.8Zn die cast alloy treated with Bi, Sb and Sr additions were investigated. The results of mechanical testing showed that all additions increased impact toughness, ultimate tensile strength, and elongation of the alloy as a result of change in eutectic Si morphology. The analysis of fracture surfaces revealed that with addition of Sr and to lesser extent Bi and Sb, the alloy exhibited a predominantly ductile fracture rather than quasi-cleavage brittle fracture. Moreover, with the additions of Sr, Bi and Sb, the quality index increased to 164.7 MPa, 156.3 MPa and 152.6 MPa respectively from 102 MPa for the base alloy. Polarization corrosion tests conducted in sodium chloride solution showed that the corrosion potential shifted to more negative values with additions of Sb, Bi and Sr, respectively. Corrosion immersion tests also revealed that the element additions have a detrimental effect on the corrosion rate of alloys, due to the increase of boundaries between the Al and eutectic Si phases.

Influence of high pressure and manganese addition on Fe-rich phases and mechanical properties of hypereutectic Al-Si alloy with rheo-squeeze casting

The influence of high pressure and manganese addition on Fe-rich phases(FRPs)and mechanical properties of Al-14Si-2Fe alloy with rheo-squeeze casting(RSC)was investigated.The semi-solid alloy melt was treated by ultrasonic vibration(UV)firstly,and then formed by squeeze casting(SC).Results show that the FRPs in as-cast SC alloys are composed of coarseβ-Al5(Fe,Mn)Si,δ-Al4(Fe,Mn)Si2 and bone-shapedα-Al15(Fe,Mn)3Si2 phases when the pressure is 0 MPa.With RSC process,the FRPs are first refined by UV,and then the solidification under pressure further causes the grains to become smaller.The peritectic transformation occurs during the formation ofαphase.For the alloy with the same composition,the ultimate tensile strength(UTS)of RSC sample is higher than that of the SC sample.With the same forming process,the UTS of Al-14Si-2Fe-0.8Mn alloy is higher than that of Al-14Si-2Fe-0.4Mn alloy.

Microstructural characteristics of near-liquidus cast AZ91D alloy during semi-solid die casting

Near-liquidus cast ingot was reheated to semi-solid firstly, and then a bracket of motor was prepared by die casting the semi-solid ingot into mould. The microstructural characteristics of AZ91D alloy in these processes were investigated. In the process of near-liquidus casting, primary α-Mg grains tend to be rosette-like because of the increase of plentiful quasi-solid atom clusters in molten alloy with the decrease of pouring temperature. These rosette-like a-Mg grains in ingots fabricated by near-liquidus casting are fused off and refined into near-globular structure owing to the solute diffusion mechanism and the minimum surface energy mechanism during reheating. After semi-solid die-casting, a-Mg grains, located in biscuit, impact and connect with each other; α-Mg grains, located in inner gate, congregate together; while α-Mg grains, located in component, distribute uniformly and become into globularity or strip. Because the inner gate limits the flowing of semi-solid slurry, and the pressure acted on the semi-solid slurry decreases gradually along the filling direction of semi-solid slurry in Cavity, microstructural segregation of unmelted a-Mg grains appears along this direction. Shrinkage holes in casting are caused by two different reasons. For biscuit, the shrinkage holes are caused by the blocked access of feeding liquid to the shrinkage zone for the agglomerated unmelted α-Mg grains. For component, the shrinkage holes are caused by the lack of feeding of liquid alloy.

Feasibility of semi-solid die casting of ADC12 aluminum alloy

The feasibility of semi-solid die casting of ADC12 aluminum alloy was studied. The effects of plunger speed, gate thickness, and solid fraction of the slurry on the defects were determined. The defects investigated are gas and shrinkage porosity. In the experiments, semi-solid slurry was prepared by the gas-induced semi-solid （GISS） technique. Then, the slurry was transferred to the shot sleeve and injected into the die. The die and shot sleeve temperatures were kept at 180 ℃ and 250 ℃, respectively. The results show that the samples produced by the GISS die casting give little porosity, no blister and uniform microstructure. From all the results, it can be concluded that the GISS process is feasible to apply in the ADC12 aluminum die casting process. In addition, the GISS process can give improved properties such as decreased porosity and increased microstructure uniformity.

Characterization on the formation of porosity and tensile properties prediction in die casting Mg alloys

The 3D visualization of the porosity in high-pressure die casting(HPDC)Mg alloys AZ91D and Mg4Ce2Al0.5Mn(EA42)was investigated by X-ray computed tomography.It was demonstrated that the volumetric porosity at the near-gate location for alloy EA42 was significantly higher than that far from the gate location.This difference resulted from the low valid time during intensified casting pressure conditions.Specimens of alloy EA42 exhibited a narrow pore distribution in the side view(~0.5 mm)compared to the wide distribution(~1.8 mm)of alloy AZ91D,which was mainly attributed to the formation mechanism of the defect band.The formation of microporosity in the defect band of alloy EA42 was inhibited because of the significant latent heat released by a large amount of the Al11Ce3phase segregated in the defect band during solidification.Additionally,an effective estimator(Z-Propagation)was introduced,which is proposed to predict the projected area fraction of the porosity(f)involved during tensile failure with better effectiveness compared with traditional methods based on the actual fractured surface.By coupling the Z-Propagation method with the critical local strain model,the logarithmic fracture strain and true fracture stress of the specimens were predicted within 3.03%and 1.65%of the absolute value of the average relative error(AARE),respectively.

Performance of Semi-solid Slurry Produced by Twin-screw Stirring Mixer and Rheo-diecasting Process of AZ91D Alloy

The microstructure of semi-solid slurry of AZ91D alloy, which was produced by twin-screw stirring mixer under the different parameters, was investigated.Rheoforming by cold chamber die casting process was performed thereafter. The results indicate that with decreasing of the barrel temperature of the mixer and the pouring temperature of molten Mg alloy, the solid fraction of semi-solid slurry increases and the size of non-dendritic grains becomes smaller. While the shear rate increases, the solid fraction of semi-solid slurry decreases. The tensile strength and elongation of metal rheoformed by die casting are higher by about 37% and 44% respectively than those produced by conventional liquid die casting.

Effect of holding pressure on density and cooling rate of cast Al-Si alloy during additive pressure casting

The cast Al-Si alloy was fabricated using the Additive Pressure Casting(APC)method.The effects of holding pressure from 50 to 400 k Pa on the density,cooling rate,and mechanical properties of the alloy,and the corresponding mechanism were discussed.The results indicate that the application of high holding pressure(300 k Pa)enhances the feeding ability of the alloy,leading to an increase of the density.Meanwhile,the cooling rate of the alloy is increased by 100%.In addition,the tensile testing results show that the increase of holding pressure from 50 to 300 k Pa improves the tensile strength and elongation of the alloy by 6.2%and 81.3%,respectively.However,excessive holding pressure(400 k Pa)might lower the density and cooling rate of the alloy due to the feeding channels being blocked.

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.

Effects of combined addition of Y and Ca on microstructure and mechanical properties of die casting AZ91 alloy

A series of die casting heat-resistant magnesium alloys based on Mg-Al system were developed for automotive application by adding Y and various amounts of Ca. The mechanical properties and microstructures of die casting AZ91 alloy with combined addition of Y and Ca were investigated by optical microscopy, scanning electronic microscopy, X-ray diffractometry and mechanical property test. The results show that the combined addition of Y and Ca can refine the as-die-cast microstructure, result in the formation of Al2Ca phase and Al2Y phase, and inhibit the precipitation of Mg17Al12 phase. The combined addition of Y and small amount of Ca has little influence on the ambient temperature tensile properties, but increasing the content of Ca can improve significantly the tensile strength at both ambient and elevated temperatures. It is found that for AZ91-1Y-xCa alloy, the hardness and the elevated temperature tensile strength increase, while the elongation decreases with increasing the addition of Ca. The mechanism of mechanical properties improvement caused by the combined addition of Y and Ca was also discussed.

Rheo-diecasting of AZ91D magnesium alloy

A rheo-diecasting process （RDC） was investigated for semisolid processing of an AZ91D magnesium alloy. The results of the RDC samples in as-cast state indicate that the microstructure of primary α-Mg particles has a fine size, nearly spherical morphology, and uniform distribution throughout the components. Due to the advanced microstructure and reduced level of defects, the RDC AZ91D Mg alloy exhibits an apparent improvement in mechanical properties. The quantitative metallographic investigations reveal that increasing the intensity of forced convection during the slurry preparation results in a promoted nucleation and reduced volume fi-acfion of the primary phase solidified in the slurry maker.

High thermal conductivity and high strength magnesium alloy for high pressure die casting ultrathin-walled components

With the rapid development of 3C industries,the demand for high-thermal-conductivity magnesium alloys with high mechanical performance is increasing quickly.However,the thermal conductivities of most common Mg foundry alloys(such as Mg-9wt%-1wt%Zn)are still relatively low.In this study,we developed a high-thermal-conductivity Mg-4Al-4Zn-4RE-1Ca(wt%,AZEX4441)alloy with good mechanical properties for ultrathin-walled cellphone components via high-pressure die casting(HPDC).The HPDC AZEX4441 alloy exhibited a fine homogeneous microstructure(average grain size of 2.8μm)with granular Al_(11)RE_(3),fibrous Al_(2)REZn_(2),and networked Ca_(6)Mg_(2)Zn_(3) phases distributed at the grain boundaries.The room-temperature thermal conductivity of the HPDC AZEX4441 alloy was 94.4 W·m^(-1)·K^(-1),which was much higher than 53.7 W·m^(-1)·K^(-1) of the HPDC AZ91D alloy.Al and Zn in the AZEX4441 alloy were largely consumed by the formation of Al_(11)RE_(3),Al_(2)REZn_(2),and Ca_(2)Mg_(6)Zn_(3) phases because of the addition of RE and Ca.Therefore,the lattice distortion induced by solute atoms of the AZEX4441 alloy(0.171%)was much lower than that of the AZ91D alloy(0.441%),which was responsible for the high thermal conductivity of the AZEX4441 alloy.The AZEX4441 alloy exhibited a high yield strength of~185 MPa,an ultimate tensile strength of~233 MPa,and an elongation of~4.2%.This result indicated that the tensile properties were comparable with those of the AZ91D alloy.Therefore,this study contributed to the development of high-performance Mg alloys with a combination of high thermal conductivity,high strength,and good castability.

Characteristics and formation mechanisms of defect bands in vacuum-assisted high-pressure die casting AE44 alloy

The microstructure in vacuum-assisted high-pressure die casting(HPDC) Mg-4Al-4RE(AE44) alloy was studied. Special attention was paid to the characteristics of defect bands and their formation mechanisms. Since double defect bands are commonly observed, the cross section of die cast samples is divided into five parts with different grain morphologies and size distributions. The inner defect band is much wider than the outer one. Both the defect bands are solute segregation bands, resulting in a higher area fraction of Al;RE;phase than that in the adjacent regions. No obvious aggregation of porosities is observed in the defect bands of AE44 alloy. This may be due to a narrow solidification temperature range of AE44 alloy and a large amount of latent heat released during the precipitation of intermetallic phases. The formation of the defect bands is related to the shear stress acting upon the partially solidified alloy, which can lead to collapse of the grain network. However, the generation mechanisms of shear stress in the outer and inner defect bands are quite different.

In-situ observation of porosity formation during directional solidification of Al-Si casting alloys

In-situ observation of porosity formation during directional solidification of two Al-Si alloys (7%Si and 13%Si) was made by using of micro-focus X-ray imaging.In both alloys,small spherical pores initially form in the melt far away from the eutectic solid-liquid (S/L) interface and then grow and coagulate during solidification.Some pores can float and escape from the solidifying melt front at a relatively high velocity.At the end of solidification,the remaining pores maintain spherical morphology in the near eutectic alloy but become irregular in the hypoeutectic alloy.This is attributed to different solidification modes and aluminum dendrite interactions between the two alloys.The mechanism of the porosity formation is briefly discussed in this paper.

Study on interfacial heat transfer coefficient at metal/die interface during high pressure die casting process of AZ91D alloy

The high pressure die casting (HPDC) process is one of the fastest growing and most efficient methods for the production of complex shape castings of magnesium and aluminum alloys in today's manufacturing industry. In this study, a high pressure die casting experiment using AZ91D magnesium alloy was conducted, and the temperature profiles inside the die were measured. By using a computer program based on solving the inverse heat problem, the metal/die interfacial heat transfer coefficient (IHTC) was calculated and studied. The results show that the IHTC between the metal and die increases right after the liquid metal is brought into the cavity by the plunger, and decreases as the solidification process of the liquid metal proceeds until the liquid metal is completely solidified, when the IHTC tends to be stable. The interfacial heat transfer coefficient shows different characteristics under different casting wall thicknesses and varies with the change of solidification behavior.

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.