The effect of Sr and Fe additions on the microstructure and mechanical properties of a direct squeeze cast Al-7Si-0.3Mg alloy

This article describes the results of an investigation into the microstructure and mechanical properties of a gravity die cast and direct squeeze cast LM25 alloy (Al-7Si-0.3Mg-0.3Fe). The direct squeeze cast LM25 alloy has superior mechanical properties compared to the gravity die cast LM25 alloy, especially with regard to ductility, which is increased from ∼1.7 pct for the gravity die cast LM25 alloy to ∼8.0 pct for the direct squeeze cast LM25 alloy in the T6 heat-treated condition. This increase in ductility is due to (1) the removal of porosity, (2) a decrease in Si particle size, and (3) a refinement of the Fe-Si-aluminide particles. High cooling rates in direct squeeze casting result in quench modification of the Si particles, such that chemical modification with Sr or Na may not be required. In addition, direct squeeze casting is more tolerant of Fe impurities in the alloy, due to the formation of smaller Fe-Si-aluminide particles than those in gravity die cast material. The direct squeeze cast LM25+Fe alloy (Al-7Si-0.3Mg-1.0Fe) has a ductility of ∼6.5 pct, compared to that of ∼0.5 pct for the gravity die cast LM25 + Fe alloy in the T6 heat-treated condition. This increase in tolerance to Fe impurities can lead to a substantial reduction in manufacturing costs due to (1) reduced raw-material costs, (2) reduced die sticking, and (3) improved die life.     

Quench Sensitivity of an Al-7 Pct Si-0.6 Pct Mg Alloy: Characterization and Modeling

The quench sensitivity of a cast Al-7 wt pct Si-0.6 wt pct Mg alloy was characterized by tensile tests and scanning electron microscopy. Specimens were cooled from the solution treatment temperature following 58 different cooling paths including interrupted and delayed quenches. Analysis of the microstructure showed that quench precipitates were Mg₂Si (β), which nucleated heterogeneously on Si eutectic particles as well as in the aluminum matrix, presumably on dislocations. The quench sensitivity of the alloy’s yield strength was modeled by multiple C-curves, using an improved methodology for quench factor analysis. The three C-curves used in the model represented loss of solute by (1) diffusion of Si to eutectic particles, (2) precipitation of β on Si eutectic particles, and (3) precipitation of β in the matrix. The model yielded a R ² of 0.994 and a root-mean-square error (RMSE) of 7.4 MPa. The model and the implications of the results are discussed in the article. This article is based on a presentation made in the symposium entitled “Simulation of Aluminum Shape Casting Processing: From Design to Mechanical Properties,” which occurred March 12–16, 2006 during the TMS Spring Meeting in San Antonio, Texas, under the auspices of the Computational Materials Science and Engineering Committee, the Process Modeling, Analysis and Control Committee, the Solidification Committee, the Mechanical Behavior of Materials Committee, and the Light Metal Division/Aluminum Committee.     

Wear mechanisms in hybrid composites of Graphite-20 Pct SiC in A356 Aluminum Alloy (Al-7 Pct Si-0.3 Pct Mg)

The wear behavior of A356 aluminum alloy (Al-7 Pct Si-0.3 Pct Mg) matrix composites reinforced with 20 vol Pct SiC particles and 3 or 10 vol Pct graphite was investigated. These hybrid composites represent the merging of two philosophies in tribological material design: soft-particle lubrication by graphite and hard-particle reinforcement by carbide particles. The wear tests were performed using a block-on-ring (SAE 52100 steel) wear machine under dry sliding conditions within a load range of 1 to 441 N. The microstructural and compositional changes that took place during wear were characterized using scanning electron microscopy (SEM), Auger electron spectroscopy (AES), energy-dispersive X-ray spectroscopy (EDXA), and X-ray diffractometry (XRD). The wear resistance of 3 Pct graphite-20 Pct SiC-A356 hybrid composite was comparable to 20 Pct SiC-A356 without graphite at low and medium loads. At loads below 20N, both hybrid and 20 Pct SiC-A356 composites without graphite demonstrated wear rates up to 10 times lower than the unreinforced A356 alloy due to the load-carrying capacity of SiC particles. The wear resistance of 3 Pct graphite 20 Pct SiC-A356 was 1 to 2 times higher than 10 Pct graphite-containing hybrid composites at high loads. However, graphite addition reduced the counterface wear. The unreinforced A356 and 20 Pct SiC-A356 showed a transition from mild to severe wear at 95 N and 225 N, respectively. Hybrid composites with 3 Pct and 10 Pct graphite did not show such a transition over the entire load range, indicating that graphite improved the seizure resistance of the composites. Tribolayers, mainly consisting of a compacted mixture of graphite, iron oxides, and aluminum, were generated on the surfaces of the hybrid composites. In the hybrid composites, the elimination of the severe wear (and hence the improvement in seizure resistance) was attributed to the reduction in friction-induced surface heating due to the presence of graphite- and iron-oxide-containing tribolayers.     

Microstructure and mechanical properties of spray-deposited Al-17Si-4.5Cu-0.6Mg wrought alloy

High Si content in Al-Si alloys usually leads to the formation of coarse, brittle Si phase under slow solidification conditions. In the present study, an Al-17Si-4.5Cu-0.6Mg (referred to hereafter as AS17) was synthesized using spray deposition to modify the Si phase. In the spray deposition process, the master alloy of AS17 was atomized using N₂ gas, and was deposited on a collecting substrate directly into a three-dimensional material. The microstructure and mechanical behavior of the spray-deposited AS17 were studied using optical microscopy (OM) scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction, and tensile tests. The present results indicate that in the spray-deposited AS17, the eutectic Si phase was modified from a “flakelike” morphology, characteristic of ingot metallurgy (IM) materials, into a “particulate” morphology. The formation of the coarse primary Si blocks was suppressed. Moreover, the size and morphology of Si particulates were found to have significant influences on the deformation behavior. During plastic deformation, extensive fracture of Si occurred. The percentage of fractured Si increased with the increasing amount of plastic deformation and the size of Si particulates. Finally, the room-temperature mechanical properties of the spray-deposited AS17 were compared with its IM counterpart A390 (an IM alloy with identical composition as AS17). The strength and ductility of AS17 were improved over those of A390. In the T6 condition, the yield strength and tensile elongation of AS17 were 503 MPa and 3.0 pct, respectively, whereas those of A390 were 374 MPa and 1.3 pet, respectively.     

Microstructure and mechanical properties of Al-7Si-0.7Mg alloy formed with an addition of(Pr+Ce)

Effects of(Pr+Ce) addition on the Al-7Si-0.7Mg alloy were investigated by optical microscope(OM), energy diffraction spectrum(EDS), X-ray diffraction(XRD) and tensile tests. The results showed that the Al-7Si-0.7Mg alloy was modified with(Pr+Ce) addition. The needle-like eutectic silicon phase developed into rose form and the crystalline grains decreased in size and showed a high degree of spheroidization. When the amount of the(Pr+Ce) addition reached 0.6 wt.%, the mean diameter was 31.8 μm(refined by 50%). The aspect ratio decreased to 1.35, and the tensile strength and ductility reached 192.4 MPa and 2.18%, respectively. At higher levels of addition, over-modification occurred, as indicated by increased grain size and reduced mechanical properties. The poisoning effect of the(Pr+Ce) addition on eutectic silicon and the constitutional supercooling caused by the(Pr+Ce) addition were the major causes of alloy modification, grain refinement, and the improvement of mechanical properties.     

喷射沉积Al-12Si-0.6Mg合金的微观组织与性能

共晶合金具有良好的激光焊接性能,为提高电子封装盖板用Al-12Si合金的强度并保持良好的热物理性能,采用喷射沉积与热压烧结技术制备Al-12Si合金,研究添加0.6％Mg对合金微观组织、力学性能和热物理性能的影响.结果表明,喷射沉积/热压烧结Al-12Si合金中Si相呈近球形颗粒,平均直径为(4.5±0.2)μm,均匀...     

The quench sensitivity of cast Al-7 wt pct Si-0.4 wt pct Mg alloy

The effect of quenching condition on the mechanical properties of an A356 (Al-7 wt pct Si-0.4 wt pct Mg) casting alloy has been studied using a combination of mechanical testing, differential scanning calorimetry (DSC), and transmission electron microscopy (TEM). As the quench rate decreases from 250 °C/s to 0.5 °C/s, the ultimate tensile strength (UTS) and yield strength decrease by approximately 27 and 33 pct, respectively. The ductility also decreases with decreasing quench rate. It appears that with the peak-aged condition, both the UTS and yield strength are a logarithmic function of the quench rate,i.e., UTS orσ _y =A logR +B. The termA is a measure of quench sensitivity. For both UTS and yield strength of the peak-aged A356 alloy,A is approximately 32 to 33 MPa/log (°C/s). The peak-aged A356 alloy is more quench sensitive than the aluminum alloy 6063. For 6063,A is approximately 10 MPa/log (°C/s). The higher quench sensitivity of A356 is probably due to the high level of excess Si. A lower quench rate results in a lower level of solute supersaturation in the α-Al matrix and a decreased amount of excess Si in the matrix after quenching. Both of these mechanisms play important roles in causing the decrease in the strength of the peak-aged A356 with decreasing the quench rate.     

Modeling of feeding behavior of solidifying Al-7Si-0.3Mg alloy plate casting

The systematic change of riser size, together with the variation of geometries of solidifying Al-7Si-0.3Mg plate castings, was tested by thermal analysis to model the interdendritic feeding behavior based on Darcy’s law. This law, however, is found to be only applicable to certain thermal conditions in the solidifying casting. The applicability of Darcy’s law depends on the regime of solidification time. A new feeding efficiency parameter integrating all individual ther-mal variables, denoted as(G · t ^2/3)V_s (whereG is the thermal gradient,t is local solidification time, and V_{s i} is solidus velocity), is found satisfactory to predict the formation of porosity. The combined geometries of a casting and its riser size exert a great influence on the thermal vari-ables of Al-7Si-0.3Mg alloy in a complicated way. Together, these thermal variables synergize to govern the feeding behavior of the casting.     

An induction heating process with coil design and solutions avoiding coarsening phenomena of Al-6 Pct Si-3 Pct Cu-0.3 Pct Mg alloy for thixoforming

During the thixoforming process, it is very important to obtain a uniform temperature, which affects the uniformity of the solid fraction, throughout the billet. Consequently, a heating method that can provide a suitable temperature profile throughout the billet must be chosen. Other parameters that must be considered include heating time (in order to minimize the total processing time), the level of control, and temperature consistency. An optimal design of the induction coil has been identified that best meets these criteria. In a previous study, the theoretical optimal coil design was verified through the FEM simulation of the induction heating process by using a general purpose finite element analysis code, ANSYS. So, in this study, the suitability of the coil design was also demonstrated by conducting induction-heating experiments. The optimal reheating conditions to apply the thixoforming (thixoforging and semisolid die casting) process were investigated by varying the reheating time, the holding time, the reheating temperatures, the capacity of the induction heating system, and the size of the adiabatic material. The final holding time was observed to be the most important factor in obtaining a fine globular microstructure and to prevent coarsening in the three-step reheating process.     

球磨时间对纳米晶Al-7Si-0.3Mg合金粉末微观组织及硬度的影响

以机械破碎Al-7Si-0.3Mg合金粉末为原料进行高能球磨, 对不同球磨时间的合金粉末进行金相观察、X射线衍射分析、透射电镜表征及显微硬度测试, 研究球磨时间对纳米晶Al-7Si-0.3Mg合金粉末的影响。结果发现, 高能球磨导致共晶硅颗粒从微米尺度细化到亚微米尺度, 颗粒形状从多面体转变成圆形, 颗粒内部有层错生成。随着球磨时间逐渐增加到60 h, 合金粉末平均颗粒尺寸从134μm逐渐下降到22μm, Al(Si, Mg)基体晶粒尺寸从438 nm降低到23 nm, 粉末显微硬度从HV 93增加到HV 289。粉末硬度的增加主要归功于球磨导致的晶粒细化(细晶强化作用), 此外, 球磨过程中硅颗粒的细化以及球磨引起的Mg、Si原子在基体内固溶度的增加也有利于粉末硬度的提高。     

微量元素对Al-1.2Mg-1.3Si-0.6Cu-0.3Mn铝合金组织性能的影响

The effects of impurity element of Fe and microelements of Zn, Cr and Ti on the constituents,dispersoids, grain size, mechanical properties and formability of Al-1.3Mg-1.2Si-0. 6Cu-0. 3Mn alloy were investigated in the present work by analyzing microstructures, phase composition, mechanical tests. The resuits indicated that with the increase of Fe content, the insoluble constituents increased, and the mechanical properties and r15 of alloy sheets debased, which can be recovered by the addition of microelements Zn, Cr and Ti. Al-1.3Mg-1.2Si-0. 6Cu-0. 3Mn alloy sheets in the T4 temper are difficult to strengthen with paint -bake treatment in automobile factory.     

固溶处理对Al-1.5Si-1.2Mg-0.6Cu-0.3Mn铝合金组织性能的影响

对汽车车身板用Al-1.5Si-1.2Mg-0.6Cu-0.3Mn铝合金冷轧薄板进行了固溶处理,研究了固溶温度、时间对第二相、晶粒及成形性能的影响规律.结果表明:在500～555℃之间进行固溶处理时,固溶温度升高,基体中残留的第二相数量逐渐减少,而再结晶晶粒尺寸形态变化不大;合金板材的强度和延伸率单调增大,,IE单调减小,n,r15变化不大.1.2 mm厚的冷轧合金薄板在540℃固溶处理时,保温时间需接近30 min才可使其具有良好的成形性,继续延长保温时间至180 min其成形性能变化不大.1.2 mm厚的A1-1.5Si-1.2Mg-0.6Cu-0.3Mn铝合金冷轧薄板合适的固溶处理温度为540℃,保温时间应接近30 min.常规T4状态的6xxx系铝合金薄板直接在汽车厂冲压成形后的烤漆涂装处理并不能起到提高车身构件强度的作用.     

Influence of calcium on the microstructure and properties of an Al-7Si-0.3Mg-xFe alloy

The effect of Ca addition on the microstructure, physical characteristics (density/porosity), and mechanical properties (tensile and impact strength) has been investigated in an Al-7Si-0.3Mg-xFe (x=0.2, 0.4, and 0.7) alloy. The size of Al-Fe intermetallic platelets (β-Al₅FeSi) increased with increasing Fe content. The addition of Ca modified the eutectic microstructure and also reduced the size of intermetallic Fe-platelets, causing improved elongation and impact strengths. A low level of Ca addition (39 ppm) reduced the proosity of the alloys. The tensile strength was decreased marginally with Ca addition. However, Ca addition improved the ductility of the alloy by 18.3, 16.7, and 44 pct and the impact strength by 44, 48, and 15.8 pct for Fe contents of 0.2, 0.4, and 0.7 pct, respectively.     

Al-12.7Si-0.7Mg合金脉冲MIG焊接头组织与性能

采用进口ER4047作为填充焊丝对Al - 12.7Si -0.7Mg合金热挤压板材实施脉冲MIG直缝对焊.利用金相观察、显微硬度测定及拉伸性能测试等方法研究了焊接接头的显微组织与力学性能.结果表明:利用优化的焊接工艺参数,获得了外观和内部质量良好的焊缝,焊接接头平均抗拉强度为180 MPa,达到基材的98％.     

The effect of predeformation and heat treatment on microstructure of Al-7.0% Si-0.45% Mg alloy cast through SIMA route

The effect of warm working, hot working and heat treatment conditions on microstructure of Al-7.0%Si-0.45%Mg alloys were investigated in strain induced melt activation (SIMA) process. Predeformation of 30%, 40% and 50% was done by hot working. The hot working has been carried out at 380°C. The samples of various deformations were kept at 580°C, 590°C, 600°C with varied soaking time for 10, 20, 30 min respectively. It was found that increased predeformation reduced the soaking time to obtain globular α Al grains. The shape factor and metallography were done on SIMA processed Al-7.0%Si-0.45%Mg alloys. Those were compared with as cast Al-7.0%Si-0.45%Mg alloy and it was observed that strain induced predeformation and subsequently melt activation has caused the globular microstructure of the alloy.     

The effect of two-step aging on the quench sensitivity of an Al-5 Pct Zn-2 Pct Mg alloy with and without 0.1 Pct Cr

The effect of two-step aging on the quench sensitivity of an Al-5 pct Zn-2 pct Mg alloy with and without 0.1 pct Cr has been studied. Results show that the quench sensitivity effect can be eliminated in thin samples of these alloys by two-step aging if the slow cooling during quenching does not allow the precipitation process to proceed too far. Lack of achievement of full strength in the aged condition due to a slow quench rate can be attributed to 1) loss of vacancies during quenching and 2) formation of incoherent-type precipitates during quenching. The trend to lower strength due to the loss of vacancies can be reversed by two-step aging; however, if incoherent-type precipitates form, some strength potential of the alloy is permanently lost. A 0.1 pct Cr addition increases the quench sensitivity effect by accelerating incoherent-type precipitation during quenching. These incoherent precipitates, which appear in the form of bands within the grains and in the grain boundaries, lead to an increase in ductility. Formerly Research Assistant, M.I.T., Cambridge, Mass.     

The role of oxides in the formation of primary iron intermetallics in an Al-11.6Si-0.37Mg alloy

Recent research suggest that the iron-rich intermetallic phases, such as α-Fe Al₁₅(Fe,Mn)₃Si₂ and β-Fe Al₅FeSi, nucleate on oxide films entrained in aluminum casting alloys. This is evidenced by the presence of crack-like defects within these iron-rich intermetallics. In an attempt to verify the role of oxides in nucleating iron-rich intermetallics, experiments have been conducted under conditions where in-situ entrained oxide films and deliberately added oxide particles were present. Iron-rich intermetallics are observed to be associated with the oxides in the final microstructure, and crack-like defects are often observed in the β-Fe plates. The physical association of the Fe-rich intermetallic phases with these solid oxides, either formed in situ or added, is in accordance with the mechanism suggesting that iron-rich intermetallics nucleate upon the wetted sides of double oxide films. This article is based on a presentation made in the John Campbell Symposium on Shape Casting, held during the TMS Annual Meeting, February 13–17, 2005, in San Francisco, CA.     

Superheat treatment of Al-7Si-0.55Mg melt and its influences on the solidification structures and the mechanical properties

Superheat treatment on Al-7Si-0.55Mg melt and its influences on the solidification structure both in the micro-and macroscales, as well as mechanical properties, are studied experimentally. The alloys were first melted and superheated for ∼30 minutes at temperatures between 720 °C and 950°C. The melts were then cooled slowly to 720 °C before casting into the specimens for microstructural analyses and mechanical property tests. The results show the following. (1) A superheating process above 850 °C for 30 minutes modifies the interdendritic eutectic and is comparable to that achieved by a Sr addition. A high cooling rate is necessary to ensure the modification efficiency. (2) Through superheat treatment, the morphology of Fe-rich phases can range from needlelike to skeleton. This effect is not observed in the case of Sr additions. (3) Superheat treatment reduces slightly the secondary dendrite arm spacing and coarsens the primary grain size. (4) The strength of specimens that underwent a superheat treatment during the melting process can be even 10 MPa higher than those modified by Sr addition.