The solidification characteristics of Fe-rich intermetallics in Al-11.5Si-0.4Mg cast alloys

After the nucleation and sedimentation of primary Fe-rich phases, the microstructures of Al-11.5Si-0.4Mg cast alloys with 0.35–1.03Fe and 0.18–0.59Mn have been studied to investigate the solidification characteristics of Fe-rich phases. Depending on the iron and manganese contents as well as cooling rates, Fe-rich phases may solidify as predendritic (primary), pre-eutectic, coeutectic, and posteutectic intermetallics at the different stages of solidification through three types of reactions: (1) predendritic (primary), (2) eutectic, and (3) peritectic reactions. It seems that Fe-rich phases may nucleate on the wetted sides of double oxide films, while the gap of the dry sides of oxide films constitutes the cracks commonly observed in the Fe-rich phases and aluminum matrix. Conventional metallurgical observations also suggest that the Fe-rich phases nucleated early during the solidification might act as nuclei for those formed subsequently, although it has not been ruled out that these phases may share the same oxide substrates. It is probable that these nucleation events may all work as suggested in the possible nucleation hierarchy for Al-11.5Si-0.4Mg cast alloys.     

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.     

Optimization of Solution Treatment of Cast Al-7Si-0.3Mg and Al-8Si-3Cu-0.5Mg Alloys

The influence of solidification rate on the solution-treatment response has been investigated for an Al-7Si-0.3Mg alloy and an Al-8Si-3Cu-0.5Mg alloy. The concentrations of Mg, Cu, and Si in the matrix after different solution-treatment times were measured using a wavelength dispersive spectrometer. All Mg dissolves into the matrix for the Al-Si-Mg alloy when solution treated at 803 K (530 °C) because the π-Fe phase is unstable and transforms into short β-Fe plates which release Mg. The Q-Al₅Mg₈Cu₂Si₆ phase do not dissolve completely at 768 K (495 °C) in the Al-Si-Cu-Mg alloy and the concentration in the matrix reached 0.22 to 0.25 wt pct Mg. The distance between π-Fe phases and Al₂Cu phases was found to determine the solution-treatment time needed for dissolution and homogenization for the Al-Si-Mg alloy and Al-Si-Cu-Mg alloy, respectively. From the distance between the phases, a dimensionless diffusion time was calculated which can be used to estimate the solution-treatment times needed for different coarsenesses of the microstructure. A model was developed to describe the dissolution and homogenization processes.     

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.     

离心转速对过共晶铝硅合金Al-19Si-5Mg组织的影响

通过改变离心铸造的转速,获得了内层聚集大量Mg2Si和初晶Si,中层不含Mg2Si和初晶Si,外层含有少量Mg2Si和初晶Si的Al-19％Si-5％Mg合金自生梯度功能复合材料圆筒形零件,分析了该复合材料零件组织分布特征和成型工艺过程及影响因素。实验结果表明：离心转速将明显影响第二相颗粒的分布;离心转速越高,颗粒的偏聚越明显。     

Eutectic Morphology of Al-7Si-0.3Mg Alloys with Scandium Additions

The mechanisms of Al-Si eutectic refinement due to scandium (Sc) additions have been studied in an Al-7Si-0.3Mg foundry alloy. The evolution of eutectic microstructure is studied by thermal analysis and interrupted solidification, and the distribution of Sc is studied by synchrotron micro-XRF mapping. Sc is shown to cause significant refinement of the eutectic silicon. The results show that Sc additions strongly suppress the nucleation of eutectic silicon due to the formation of ScP instead of AlP. Sc additions change the macroscopic eutectic growth mode to the propagation of a defined eutectic front from the mold walls opposite to the heat flux direction similar to past work with Na, Ca, and Y additions. It is found that Sc segregates to the eutectic aluminum and AlSi₂Sc₂ phases and not to eutectic silicon, suggesting that impurity-induced twinning does not operate. The results suggest that Sc refinement is mostly caused by the significantly reduced silicon nucleation frequency and the resulting increase in mean interface growth rate.     

The Structure and Kinetics of the Nanoscale Precipitation Processes in Al-1.0 wt pct Mg2Si-0.4 wt pct Mg-0.5 wt pct Ag Alloy

The influence of addition of 0.4 wt pct Mg on the precipitation sequence in the balanced Al-1.0 wt pct Mg₂Si bearing 0.5 wt pct Ag has been investigated during the continuous heating of the quenched alloy from the solid solution state. Differential scanning calorimetry (DSC) and high-resolution transmission electron microscopy techniques have been used. The DSC experiments showed that all processes occurred are thermally activated. The activation energies of the precipitation processes have been determined and hence the kinetics of these precipitates have been determined. The obtained results have shown that the existence of excess Mg inhibits the formation of the early stage clusters of solute-vacancy clusters. These clusters can be assisted by the binding energies between solute Si, Mg, and Ag atoms and the excess vacancies. On the other hand, excess Mg accelerates the precipitation of random, β′-phase and β-phase precipitates.     

Martensitic phases in splat quenchedβ Ag Mg alloys

Retainedβ-phase as well as martensitic phases have been obtained by splat quenching Ag-46.6 at. pct Mg and Ag-43.1 at. pct Mg alloys. The retainedβ-phase exhibitsB ₂ orDO ₃ type ordering. Two martensitic phases have been identified in these alloys. These phases are3R and2H stacking modulations derived by stacking (110) type planes of the (β-phases. A close examination of electron diffraction patterns of the retained (β-phase reveals extra intensity maxima. These extra maxima are explained in terms of fine particles of2H and3R type surface martensites in retainedβ-phase.     

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.     

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.     

The effect of mischmetal addition on the structure and mechanical properties of a cast Al-7Si-0.3Mg alloy containing excess iron (up to 0.6 Pct)

The effect of Fe content (0.2 to 0.6 pct) on the microstructure and mechanical properties of a cast Al-7Si-0.3Mg (LM 25/356) alloy has been investigated. Further, 1 pct mischmetal (MM) additions (a mixture of rare-earth (RE) elements) were made to these alloys, and their mechanical properties at room and at elevated temperatures (up to 200 °C) were evaluated. A structure-property correlation on this alloy was attempted using optical microstructure analysis, fractographs, X-ray diffraction, energy-dispersive analysis of X-rays (EDX), and quantitative metallography by image analysis. An increase in Fe content increased the volume percentage of Fe-bearing intermetallic compounds (β and π phases), contributing to the loweryield strength (YS), ultimate tensile strength (UTS), percentage elongation, and higher hardness. An addition of 1 pct MM to the alloys containing 0.2 and 0.6 pct Fe was found to refine the microstructure; modify the eutectic silicon and La, Ce, and Nd present in the MM; form different intermetallic compounds with Al, Si, Fe, and Mg; and improve the mechanical properties of the alloys both at room and elevated temperatures.     

固溶温度对新型Al-12.7Si-0.7Mg合金挤压材组织性能的影响

针对我国自主研制的新型Al-12.7Si-0.7Mg合金挤压型材,采用金相显微镜、扫描电镜及其附带的能谱分析仪以及拉伸实验研究了固溶温度对铝基体晶粒、合金相粒子尺寸、形状及数量以及挤压材力学性能的影响规律,结果表明:Al-12.7Si-0.7Mg合金挤压材基体中存在大量尺寸极其细小的点状Mg2Si相及大量微米级共晶Si粒子和少量微米级AlFeSi过剩结晶相;固溶温度从440℃升高至540℃,尺寸极其细小的点状Mg2Si相逐渐回溶入基体消失,微米级共晶Si颗粒及含AlFeSi的过剩结晶相粒子形状趋于球化,而铝基体晶粒呈现出略有长大的趋势,且共晶硅颗粒具有较明显的细化铝基体晶粒的作用;合金挤压材的强度及延伸率随固溶温度升高分别呈现出单调增大及总体下降的趋势;新型Al-12.7Si-0.7Mg合金挤压材比较适宜的固溶温度为520℃,合金挤压材经最佳固溶温度固溶水淬再经170℃×2.5 h时效处理后的Rp0.2≥279 MPa,Rm≥330 MPa,A≥9.9%。     

Influence of Grain Refinement on Slurry Formation and Surface Segregation in Semi-solid Al-7Si-0.3Mg Castings

This study aims to evaluate the effect of grain refinement on slurry formation and surface segregation in semi-solid castings produced by the Rheometal™ process. The effect of two grain refiners, Al-8B and Al-5Ti-1B, on the slurry α-Al grain size, shape factor and solid fraction was evaluated. The results suggest that the addition of a grain refiner can affect the solid fraction obtained in the Rheometal^TM process and, consequently, reduce the solute content near the casting surface. Grain refiner addition resulted in a larger fraction of α-Al grains ≤ 60 µm for the refined alloys compared with the unrefined alloy. Additionally, the growth of α-Al slurry globules was greater for the unrefined alloy compared with the refined alloy during solidification in the die-cavity. A more homogeneous and finer microstructure was observed near the surface in the grain-refined castings compared with the unrefined castings. Evidence of significant liquid penetration was identified in some α-Al globules, indicating that disintegration of α-Al globules may occur during the Rheometal™ casting process.

     

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.     

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.     

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％.