Effect of heat treatment on microstructure and properties of semi-solid squeeze casting AZ91D

Semi-solid AZ91D magnesium alloy billets were prepared by near-liquidus heat holding. Semi-solid squeeze casting was conducted at 575, 585 and 595 ℃, respectively, with 1 mm·s~(-1) squeeze speed. The semisolid squeeze casting AZ91D samples were heat treated by T4(solution at 415 ℃ for 24 h) and T6(solution at 415 ℃ for 24 h + 220 ℃ for 8 h) processes, respectively. The microstructure and mechanical properties of the alloy in different states were investigated by means of OM, SEM and tensile testing machine. The results show that compared to as-cast alloy, the grain size of the semi-solid squeezed AZ91D decreased significantly, and with the increase of semi-solid squeeze temperature, the grain size of AZ91D increased. The grains of the alloy were refined by T4 treatment, and further refined by T6 treatment. T6 treatment greatly improved the tensile strength, elongation, and hardness, but did not significantly improve yield strength. After 575 ℃ squeeze casting and T6 treatment, the ultimate tensile strength(UTS) reached 285 MPa, the elongation reached 13.36%, and the hardness also reached the maximum(106.8 HV), but the yield strength(YS) was only 180 MPa. During the process of semi-solid squeeze casting and heat treatment, the matrix grain was refined and a large number of precipitated and secondary precipitated phases of Mg_(17)Al_(12) appeared. Both the average size of matrix grain and secondary precipitated phase decreased, while the volume fraction of secondary precipitated phase increased. All these resulted in high tensile strength, elongation and hardness.     

Microstructure evolution and mechanical properties of Mg-12Zn-2Y alloy containing quasicrystal phase fabricated by different casting processes

Although icosahedral quasicrystal phase (denoted as I-phase) has been verified as an outstanding reinforcing phase, the mechanical properties of quasicrystal-reinforced Mg-Zn-Y alloys fabricated by traditional casting processes are still unsatisfactory due to the serious segregation of intermetallic compounds. In this study, the microstructure and mechanical properties of Mg-12Zn-2Y alloy fabricated by different casting processes, including permanent mold casting, squeeze casting and rheo-squeeze casting with ultrasonic vibration, were systematically investigated and compared. The results show that massive, large-sized I-phase and Mg₇Zn₃ phase gather together in the permanent mold cast sample, while the squeeze casting process leads to the transformation of I-phase into fine lamellar morphology and the amount of Mg₇Zn₃ decreases. As to the rheo-squeeze casting process, when the ultrasonic vibration is exerted with power from 800 W to 1,600 W, the α-Mg grains are refined and spheroidized to a large extent, and the lamellar spacing of the eutectic structure is significantly reduced, accompanied by some tiny granular I-phase scattering in the α-Mg matrix. However, when the ultrasonic power continuously increases to 2,400 W, the eutectic structure becomes coarse. The best mechanical properties of the rheo-squeeze cast alloy are obtained when the ultrasonic power is 1,600 W. The microhardness, yield strength, ultimate tensile strength and elongation are 79.9 HV, 140 MPa, 236 MPa, and 3.25%, which are 44.1%, 26.1%, 25.5%, 132.1% respectively higher than the corresponding values of the squeeze casting sample, and are 47.6%, 44.3%, 69.8%, and 253.3% respectively higher than the corresponding values of the permanent mold casting sample.

     

Ultrasonic Vibration and Rheocasting for Refinement of Mg–Zn–Y Alloy Reinforced with LPSO Structure

In this work, ultrasonic vibration (UV) and rheo-squeeze casting was first applied on the Mg alloy reinforced with long period stacking ordered (LPSO) structure. The semisolid slurry of Mg–Zn–Y alloy was prepared by UV and processed by rheo-squeeze casting in succession. The effects of UV, Zr addition and squeeze pressure on microstructure of semisolid Mg–Zn–Y alloy were studied. The results revealed that the synergic effect of UV and Zr addition generated a finer microstructure than either one alone when preparing the slurries. Rheo-squeeze casting could significantly refine the LPSO structure and α-Mg matrix in Mg_96.9Zn₁Y₂Zr_0.1 alloy without changing the phase compositions or the type of LPSO structure. When the squeeze pressure increased from 0 to 400 MPa, the block LPSO structure was completely eliminated and the average thickness of LPSO structure decreased from 9.8 to 4.3 μm. Under 400 MPa squeeze pressure, the tensile strength and elongation of the rheocast Mg_96.9Zn₁Y₂Zr_0.1 alloy reached the maximum values, which were 234 MPa and 17.6%, respectively, due to its fine α-Mg matrix (α1-Mg and α2-Mg grains) and LPSO structure.     

Effects of Sr on microstructure and corrosion resistance in simulated body fluid of as cast Mg–Nd–Zr magnesium alloys

Mg–2·2Nd–xSr–0·3Zr alloys (wt-%, x?=?0, 0·4, 0·7 and 2·0) were prepared by gravity casting to study the effects of Sr addition on the microstructure and corrosion resistance of Mg–Nd–Zr alloys in simulated body fluid (SBF). Phases were identified by X-ray diffraction, and microstructure was observed with optical microscopy and scanning electron microscopy. Corrosion resistance of the alloys was determined by evaluating mass loss and hydrogen evolution during immersion in SBF. Mg₁₇Sr₂ phase was formed, and the grain size decreased with additional Sr addition. For the grain refinement and more continuous second phase, which could improve the corrosion resistance, the alloy with 0·7 wt-%Sr showed the slowest corrosion rate, whereas the alloy with 2·0 wt-% showed the fastest corrosion rate due to the increased volume fraction of Mg₁₇Sr₂, which led to severe local microgalvanic corrosion.     

Passivation behaviors of 6082 aluminium alloy under stress corrosion process

We systematically studied the passivation process of 6082 aluminium alloy under the bending stress situation by combining electrochemical measurement techniques with three-point bending stress fixture designed by our lab, and then examined the microstructures of corroded specimens to analyze electrochemical corrosion mechanism in 1.5% NaCl solution. The results show that secondary Mg₂Si phase acts as the anodic electrode, leading to the self-corrosion of Mg₂Si phase. As a result, the spots of self-corroded Mg₂Si phase within grains act as initial pitting corrosion site, combined with tiny, massive precipitated Mg₂Si particles at the grain boundaries and bending stress, leading to the failure of surface of the 6082 aluminium alloy. The corrosion current density increases from 3.422 × 10⁻⁷ to 13.77 × 10⁻⁷ A/cm² when bending stress level was increased from 0% up to 100% of yield stress. Passive film formation process occurred between polarization potential area of −1.05 and −0.65 V. Sectional microstructural investigations show that the corrosion starts penetrating vertically into the material before it develops corrosion paths extending parallel to the surface, leading to massive stress-induced corrosion cracks. The maximum corrosion depth increases from ∼24 μm on specimen without any stress applied to 85 μm when bending stress of 100% yield strength is applied.     

Corrosion and chemical behavior of Mg97Zn1Y2-1wt.%SiC under different corrosion solutions

To explore the corrosion properties of magnesium alloys, the chemical behavior of a high strength Mg₉₇Zn₁Y₂-1 wt.%Si C alloy in different corrosion environments was studied. Three solutions of 0.2 mol·L^-1 NaCl, Na₂SO₄ and NaNO₃ were selected as corrosion solutions. The microstructures, corrosion rate, corrosion potential, and mechanism were investigated qualitatively and quantitatively by optical microscopy(OM), scanning electron microscopy(SEM), immersion testing experiment, and electrochemical test. Microstructure observation shows that the Mg₉₇ Zn₁Y₂-1 wt.%Si C alloy is composed of α-Mg matrix, LPSO(Mg₁₂ ZnY) phase and Si C phase. The hydrogen evolution and electrochemical test results reflect that the Mg₉₇Zn₁Y₂-1 wt.%SiC in 0.2 mol·L^-1 Na Cl solution has the fastest corrosion rate, followed by Na₂SO₄ and NaNO₃ solutions, and that the charge-transfer resistance presents the contrary trend and decreases in turn.     

The effect of casting temperature on the properties of squeeze cast aluminium and zinc alloys

Gravity casting and squeeze casting were carried out on an aluminium alloy with 13.5% silicon and a zinc alloy with 4.6% aluminium with different temperatures, 660, 690 and 720 °C for the former and 440, 460 and 480 °C for the latter. A top-loading crucible furnace was used to melt the alloys. The die-preheat temperatures used were 200–220 °C for the aluminium alloy and 150–165 °C for the zinc alloy. A K-type thermocouples with digital indicator were used to measure the die surface temperature and the molten metal temperature; while a 25 t hydraulic press with a die-set containing a steel mould was used to perform the squeeze casting with a pressure of 62 MPa. Tensile, impact and density tests were carried out on the specimens. It was found that casting temperature had an effect on the mechanical properties of both gravity cast and squeeze cast aluminium and zinc alloys. The best temperatures to gravity cast the aluminium alloy and the zinc alloy were 720 and 460 °C, respectively. For the squeeze casting of the aluminium alloy, the best temperature to use was either 690 or 660 °C; the former would give a better property at the top of the casting while the latter, at the bottom of the casting. However, for the squeeze casting of the zinc alloy, the best temperature was again 460 °C.     

Pitting corrosion of thixoformed,rheocast and gravity cast A356-T6 alloy in chloride media

Abstract

In the research work presented here, the pitting behaviour of thixoformed A356-T6 alloy, with different reheating temperatures, was evaluated and compared with the pitting behaviour of rheocast and gravity cast A356-T6 alloys with the same composition. To study the pitting behaviour, linear sweep voltammetric tests were performed on thixoformed, rheocast and gravity cast A356-T6 alloys in a 3·5%NaCl solution. A simulation method was also used to identify local galvanic corrosion current density between local galvanic couples. Results obtained show that the resistance to pitting corrosion of thixoformed samples formed at 590°C was higher than that of the samples formed at 600°C as well as rheocast and gravity cast samples. The improvement in the pitting corrosion resistance due to the thixoforming process is attributed to morphological aspects of the silicon phase as well as the area effect as related to galvanic corrosion between the silicon particles and the eutectic aluminium phase.     

Mn含量对挤压铸造Al-5.0Cu-0.5Fe合金显微组织和力学性能的影响(英文)

采用拉伸性能测试、光学显微镜、扫描电镜和定量金相测试手段研究Mn含量对不同压力下挤压铸造Al-5.0Cu-0.5Fe合金显微组织和力学性能的影响。结果表明:当挤压压力为0MPa,Mn/Fe质量比达到1.6时才能将针状β-Fe相(Al7Cu2Fe)完全转变成汉字状α-Fe相(Al15(FeMn)3(CuSi)2)。而对于挤压铸造,当挤压压力为75MPa时,在Mn/Fe质量比为0.8时就可以将β-Fe相完全转变成α-Fe相。挤压铸造合金中需要的Mn含量较低,即Mn/Fe质量比较小,这主要是由于在挤压压力下富Fe相的细化以及相比例的减少。然而,加入过量的Mn将导致合金力学性能的下降,这是因为过量的Mn将导致α-Fe相的增多及这些多余的硬脆相导致的孔洞增多。     

Microstructure,Tensile and Fatigue Properties of Al–5 wt.%Mg Alloy Manufactured by Twin Roll Strip Casting

This study investigated the microstructure, tensile and fatigue properties of Al–5 wt.%Mg alloy manufactured by twin roll strip casting. Strips cast as a fabricated (F) specimen and a specimen heat treated (O) at 400 °C/5 h were produced and compared. In the F specimen, microstructural observation discovered clustered precipitates in the center area, while in the O specimen precipitates were relatively more evenly distributed. Al, Al₆(Mn, Fe), Mg₂Al₃ and Mg₂Si phases were observed. However, most of the Mg₂Al₃ phase in the heat-treated O specimen was dissolved. A room temperature tensile test measured yield strength of 177.7 MPa, ultimate tensile strength of 286.1 MPa and elongation of 11.1% in the F specimen and 167.7 MPa (YS), 301.5 MPa (UTS) and 24.6% (EL) in the O specimen. A high cycle fatigue test measured a fatigue limit of 145 MPa in the F specimen and 165 MPa in the O specimen, and the O specimen achieved greater fatigue properties in all fatigue stress conditions. The tensile and fatigue fracture surfaces of the above-mentioned specimens were observed, and this study attempted to investigate the tensile and fatigue deformation behavior of strip cast Al–5 wt.%Mg based on the findings.     

Influence of Applied Pressure on Tensile Behaviour and Microstructure of Squeeze Cast Mg Alloy AM50 with Ca Addition

The development of alternative manufacturing processes is essential for the success in applying Ca-containing magnesium alloys for automotive applications due to their relatively poor die castability. Squeeze casting with its inherent advantages has been demonstrated capable of minimizing the formation of casting defects in Mg-Al-Ca alloys. In this study, the effect of applied pressures on tensile behavior and microstructure of squeeze cast Mg-5wt.%Al-1%wt.%Ca alloy (AMX501) was investigated with the applied pressure varying from 3 to 90 MPa. The results of tensile testing indicate that the tensile properties of AMX501 alloy including ultimate tensile strength, yield strength, and elongation (E _f) increase from 153.7, 80 MPa and 3.26% to 183.7, 90.5, and 5.42% with increasing applied pressure levels from 3 to 90 MPa, respectively. The analysis of true stress versus strain curves shows that an increase in applied pressure levels result in high straining hardening rates during the plastic deformation of the alloy. Microstructural analysis and density measurements indicate that, as the applied pressure increases, the porosity levels of the alloy decrease considerably, despite of almost no significant reduction in grain sizes of the squeeze cast alloys due to their high aspect ratio of cylindrical castings. Hence, the improvement in tensile properties should be primarily attributed to casting densification resulting from applied pressures. The scanning electron microscopy observation on fractured surfaces reveals that the fracture modes of the squeeze cast alloys transit to ductile from brittle with increasing applied pressures.     

Mg-Zn-Y-Nd-Zr合金的显微组织、织构、力学性能

对Mg-Zn-Y-Nd-Zr合金的显微组织和力学性能进行了研究。结果表明,Nd元素的加入部分取代了W相（Mg₃Zn₃Y₂）中的Y元素,形成了新的第二相Mg₃Zn₃(Y, Nd)₂。热挤压后观察到由细小的等轴再结晶晶粒和粗大的细长未再结晶晶粒组成的典型双峰结构。Nd元素的加入促进了热挤压过程中的动态再结晶,随着Nd含量的增加,动态再结晶率增加,挤压态合金的整体织构强度减弱。Nd的加入细化了晶粒并改善了合金的力学性能。添加0.5%（质量分数）Nd时,挤压态合金表现出高强度和高塑性的良好结合：屈服强度为362 MPa,极限抗拉伸强度为404 MPa,延伸率为10.2%。时效处理后合金的抗拉伸强度进一步提高,峰值时效极限抗拉伸强度可达421 MPa。合金的高强度主要归功于超细再结晶晶粒和析出强化。     

Corrosion resistance of WE43 and AZ91D magnesium alloys with phosphate PEO coatings

The corrosion performance of WE43-T6 and AZ91D magnesium alloys with and without treatment by plasma electrolytic oxidation (PEO) was investigated by electrochemical measurements in 3.5 wt.% NaCl solution. For untreated WE43-T6 alloy, formation of a uniform corrosion layer (Mg(OH)₂) was accompanied by initial pits around magnesium-rare earth intermetallic compounds. The AZ91D alloy disclosed increased corrosion susceptibility, with localized corrosion around the β-phase, though the β-phase network phase acted as a barrier for corrosion progression. PEO treatment in alkaline phosphate electrolyte improved the corrosion resistance of WE43-T6 alloy only at the initial stages of immersion in the test solution. However, PEO-treated AZ91D alloy revealed a relatively high corrosion resistance for much increased immersion times, contrary to the relative corrosion resistances of the untreated alloys. The improved performance of the PEO-treated AZ91D alloy appears to be related to the formation of a more compact coating.     

Grain size refinement through gas bubble stirring in AC4CH cast aluminium alloy

A comparison has been made of the mechanical properties of gravity and squeeze cast aluminium alloys that have been grain refined using gas bubbling and those that have not. To find the optimum gas bubbling conditions, the alloy melt temperature, the gas flow rate and the gas bubbling times were varied over wide ranges. The microstructure of the gas bubbled gravity and squeeze cast materials is fine, equiaxed and non-dendritic with an average primary α size of 52 μm and 163 μm respectively. However, gas bubbling has no effect on the morphology of the eutectic Si. There seems to be no noticeable difference between the measured mechanical properties of the gravity and squeeze cast materials with or without the gas bubbling. The lack of improvement in the mechanical properties of the gravity cast alloy is due to casting defects and porosity, which offset the effects of the grain refinement. The crystal separation and showering mechanisms are operative for the formation of equiaxed grains.     

Mechanical and corrosion properties of newly developed Mg–Mn–Ca alloys as potential biodegradable implant materials

Calcium and manganese were selected as alloying elements to develop Mg–2Mn–xCa (x?=?0·8, 1·0, 1·2 wt-%) alloys as potential biodegradable implant materials. The mechanical properties and corrosion mechanism of both as cast and solution naturally age (T4) treated Mg–2Mn–xCa alloys were investigated. The results indicated that the distribution of the second phase dominated the corrosion process. T4 treatment could transfer coarse Mg₂Ca and α-Mn phases into dispersed fine precipitated phases, which improve mechanical and corrosion properties. Mg–2Mn–1·0Ca alloy has the best integrated performance among the developed alloys. This research indicated that T4 treated Mg–2Mn–xCa alloys are a promising candidate used as biodegradable implant material.     

Ca Addition Effects on the Microstructure,Tensile and Corrosion Properties of Mg Matrix Alloy Containing 8 wt.% Mg<Subscript>2</Subscript>Si

The microstructure, tensile properties and corrosion behavior of the Mg-8 wt.% Mg₂Si-x%Ca alloy have been studied by the use of optical microscopy, scanning electron microscopy equipped with energy-dispersive spectroscopy, x-ray diffraction, standard tensile testing, polarization test and electrochemical impedance spectroscopy (EIS) measurements. Microstructural studies indicated that Ca modifies both primary and eutectic Mg₂Si phase. It was found that the average size of primary Mg₂Si particles is about 60 μm, which is dropped by about 82% in the alloy containing 0.05 wt.% Ca. By the addition of different Ca contents, Ca-rich intermetallics (i.e., CaSi₂ and CaMgSi) were formed. The modification mechanism of adding Ca during solidification was found to be due to the strong effect of CaMgSi phase as a heterogonous nucleation site, apart from CaSi₂ which was reported before, for Mg₂Si intermetallics. Tensile testing results ascertained that Ca addition enhances both ultimate tensile strength (UTS) and elongation values. The optimum amount of Ca was found to be 0.1 wt.%, which improved UTS and elongation values from about 130 MPa and 2% to 165 MPa and 5.5%, whereas more Ca addition (i.e., 3 wt.%) reduced the tensile properties of the alloy to about 105 MPa and 1.8%, which can be due to the formation of CaMgSi intermetallics with deteriorating needle-like morphology. Polarization and EIS tests also showed that the Mg-3%Si-0.5%Ca alloy pronounces as the best anti-corrosion alloy. Nevertheless, further added Ca (up to 3 wt.%) deteriorated the corrosion resistance due to predominance of worse galvanic coupling effect stemmed from the presence of stronger CaMgSi cathode in comparison with Mg₂Si. With higher Ca additions, an adverse effect was seen on corrosion resistance of the Mg-3%Si alloy, as a result of forming a weak film on the alloy specimen surface.     

Effects of TiC additions on as cast and heat treated microstructures and mechanical properties of ZA84 magnesium alloy

Abstract

The effects of TiC additions on the as cast and heat treated microstructures and mechanical properties of ZA84 Mg alloy are investigated. The results indicate that, after adding 0·5 wt-%TiC to ZA84 alloy, the as cast microstructure of the alloy is refined. At the same time, the distribution of second phases in the alloy becomes relatively uniform and the quasi-continuous networked second phases in the alloy become discontinuous. As a result, the as cast tensile properties of the alloy are improved. In addition, after solutionisation at 345°C, parts of the Mg₃₂(Al,Zn)₄₉ phases in the ZA84 alloy without TiC modification still exhibit quasi-continuous distribution. The Mg₃₂(Al,Zn)₄₉ phases in the ZA84 alloy treated with 0·5 wt-%TiC change to disconnected particles with an angular morphology and gradually spheroidise. Under the optimum heat treated conditions, the ZA84 alloy treated with 0·5 wt-%TiC exhibits higher heat treated tensile properties than the ZA84 alloy without TiC modification.     

Influence of multipass high rotating speed friction stir processing on microstructure evolution,corrosion behavior and mechanical properties of stirred zone on AZ31 alloy

The influence of multipass high rotating speed friction stir processing (FSP) on the microstructure evolution, corrosion behavior, and tensile properties of the stirred zone (SZ) was investigated by EBSD, TEM, SEM, electrochemical workstation and electronic universal testing machine. The mean grain size of the SZ is significantly refined, and it increases with the increase of the processing pass. In addition to an obvious increase in the number, the distribution of β-Al₁₂Mg₁₇ precipitates also becomes more uniform and dispersed with increasing the processing pass. Compared with the as-received AZ31 alloy, the tensile properties of the SZ are hardly improved, but the corrosion resistances are significantly enhanced. The corrosion potential of the SZ prepared by 4-pass FSP is increased from −1.56 V for the unprocessed AZ31 alloy to −1.19 V, while the corrosion current is decreased from 1.55×10⁻⁴ to 5.47×10⁻⁵ A.     

Microstructures and properties in surface layers of Mg-6Zn-1Ca magnesium alloy laser-clad with Al-Si powders

Laser surface cladding with Al-Si powders was applied to a Mg-6Zn-1Ca magnesium alloy to improve its surface properties. The microstructure, phase components and chemical compositions of the laser-clad layer were analyzed by using X-ray diffractometry (XRD), scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS). The results show that the clad layer mainly consists of α-Mg, Mg₂Si dendrites, Mg₁₇Al₁₂ and Al₃Mg₂ phases. Owing to the formation of Mg₂Si, Mg₁₇Al₁₂ and Al₃Mg₂ intermetallic compounds in the melted region and grain refinement, the microhardness of the clad layer (HV_0.025 310) is about 5 times higher than that of the substrate (HV_0.025 54). Besides, corrosion tests in the NaCl (3.5%, mass fraction) water solution show that the corrosion potential is increased from –1574.6 mV for the untreated sample to –128.7 mV for the laser-clad sample, while the corrosion current density is reduced from 170.1 to 6.7 µA/cm². These results reveal that improved corrosion resistance and increased hardness of the Mg-6Zn-1Ca alloy can be both achieved after laser cladding with Al-Si powders.     

Effect of Sn addition on the corrosion behavior of Ti‐Ta alloy

In the present paper, the effect of Sn addition on phase constitution and corrosion behavior of Ti₆₅Ta_(35?x)Sn_x (x = 0, 1, 3, 5 at%) alloys has been investigated using X‐ray diffraction and electrochemical measurements. The results show that the β phase is stabilized when quenching after the addition of Sn into Ti₆₅Ta₃₅ alloy, with Ti₆₅Ta₃₀Sn₅ alloy consisting of single β phase at room temperature. The corrosion resistance is enhanced with increase in the Sn content. When the Sn content increases up to 5%, the addition of Sn results in a decrease of corrosion current density from 0.690 µA/cm² of the Ti₆₅Ta₃₅ alloy to 0.098 µA/cm², and corresponding passive current density decreased from 37.7 µA/cm² of the Ti₆₅Ta₃₅ alloy to 9.30 µA/cm².