Grain Refiner Effect on the Microstructure and Mechanical Properties of the A356 Automotive Wheels

A356 aluminum alloy automotive wheels, 17 inch in diameter, were produced by low-pressure die casting. Contents of Al-5Ti-B (ATB) master alloy were added from 0 to 0.79 wt.%. Microstructural and mechanical properties were evaluated under industrial casting process conditions. The obtained results from mechanical testing provide evidence that additions of 0.13 and 0.27 wt.% of ATB have an improvement on the mechanical performance of the automotive wheels. This can be compared with the use of a grain refiner’s higher concentrations, leading to a significant reduction in the cost-benefit ratio for the manufacturing of A356 automotive wheels.     

Influence of cooling rate after homogenization on microstructure and mechanical properties of aluminum alloy 7050

The influence of cooling rate (0.009?C220 °C/s) after homogenization on the microstructure and mechanical properties of high strength aluminum alloy 7050 was investigated by tensile testing, optical microscope, X-ray diffraction, scanning electron microscope, and transmission electron microscope. A lower cooling rate after homogenization resulted in lower mechanical properties after aging. The drop in strength was significant when the cooling rate was decreased from 0.5 °C/s to 0.1 °C/s. A lower cooling rate gave rise to a larger amount of remnant S(Al₂CuMg) phase and a higher fraction of recrystallization after solution heat treatment. Consequently, the increase in strength after aging due to precipitation hardening and substructure hardening was less significant in the case of slow cooling. This was supposed to be responsible for the lower mechanical properties due to a lower cooling rate after homogenization.     

Microstructures and Properties of W-Ti Alloys Prepared Under Different Cooling Conditions

W-(10 to 15) wt.% Ti alloys were sintered at 1400 or 1500 °C and cooled under different cooling conditions. The microstructures and properties of W-Ti alloys were affected by the cooling conditions. XRD, SEM, EBSD, and TEM were carried out to investigate the effects of cooling conditions and sintering temperature on the microstructures of W-Ti alloys. The nanohardness and elastic modulus of the alloys were also investigated. The results showed that when the temperature was 1500 °C, the content of Ti-rich phase in W-(10 to 15) wt.% Ti alloys decreased obviously with the increase of cooling rate (the average cooling rate of furnace cooling, air cooling and water cooling was 0.2, 10, and 280 °C/s, respectively). For the W-10 wt.% Ti alloy, the content decreased from 20.5 to 9.7%, and the grain size decreased from 2.33 to 0.67 μm. When the temperature decreased to 1400 °C, the grain size was also decreased sharply with the increase of cooling rate, but there was a little change in the microstructure. Meanwhile, the grain sizes were smaller than those of the alloys sintered at 1500 °C. The nanohardness and elastic modulus increased with the increase of cooling rate, and the alloys sintered at different temperatures had different nanohardness and elastic modulus which depended on the cooling conditions. Sintering at a proper temperature and then cooling at a certain cooling condition was a useful method to fabricate alloy with less Ti-rich phase and high properties.     

The Mechanical and Microstructural Changes of Sn-Ag-Bi Solders with Cooling Rate and Bi Content Variations

The purpose of this study is to investigate the influence of cooling rate and Bi addition on the microstructure evolution and mechanical properties of Sn-3.5Ag alloy. A series of Sn-3.5Ag-xBi solders has been fabricated with Bi content in the range of 0.5-3.5 wt.%. After solution heat treatment at 170 °C for 24 h and subsequent aging heat treatment at 100 °C for 2 h, samples were divided into two groups. One group was rapidly quenched into iced water (water quenching) for the fast cooling rate (20 °C/s), while the second group was slowly cooled (furnace cooling) in the furnace for the slow cooling rate (0.2 °C/s) after the furnace reflow. The microstructural evolutions of the present solders have been investigated using x-ray diffraction and scanning electron microscopy. The microhardness was measured to correlate the mechanical properties to alloy compositions and cooling rate. It was found that the microhardness of Sn-3.5Ag-xBi solders increased with increasing cooling rate. The indentation creep curves have been evaluated from the obtained microhardness values. Results revealed the steady-state creep rate decreased with increasing Bi content exhibiting an anomalous behavior at 2.5Bi. The reason for improved creep resistance of Sn-3.5Ag-xBi solders is the result of the combination of the solid solution strengthening and precipitation strengthening of Bi. The mean values of stress exponent indicated that the operative creep mechanism is dislocation climb.     

低过热度铸造和触变锻造结合制备A356铝合金车轮的组织与力学性能

采用低过热度铸造和触变锻造相结合的方法制备A356铝合金车轮,研究低过热度铸造A356铝合金坯料的组织、坯料二次加热组织演变规律和触变锻造车轮的组织与力学性能。结果表明：熔体在635℃浇注,可获得具有细小、均匀的非枝晶晶粒的A356铝合金坯料。坯料在600℃等温加热60min后,非枝晶晶粒可转变成球形晶粒,在750kN锻压力下半固态坯料可触变锻造成铝合金车轮。经T6热处理,A356铝合金车轮的抗拉强度和伸长率分别为327．6MPa和7．8％,高于铸造铝合金车轮的拉伸力学性能。将低过热度铸造与触变锻造工艺相结合,可以制备具有较高力学性能的铝合金车轮。     

6351铝合金的淬火敏感性

采用分级淬火的实验方法,结合合金时效态硬度和淬火态电导率的测试拟合得到6351合金的TTP和TTT曲线,并采用透射电镜对6351合金的淬火敏感性进行研究.结果表明,当6351合金在相同温度下等温时,随着保温时间延长,淬火态电导率呈上升趋势,时效态硬度呈下降趋势.透射电镜分析发现,在等温初期,过饱和固溶体分解形成针状的β”相;随着保温时间延长,逐渐形成棒状β'相和片状β相.TTT和TTP曲线的鼻温为360℃,淬火敏感温度区间为230～430℃.在鼻温附近等温相转变最快,低温区相转变次之,高温区最慢.淬火因子分析结果表明,要获得最佳的力学性能,淬火敏感温度区间的冷却速率需大于15℃/s.     

Optimization of the Mechanical Properties and Residual Stresses in 2024 Aluminum Alloy Through Heat Treatment

Residual stresses induced during quenching of aluminum alloys cause dimensional instability and distortion. In this study, the effects of different concentrations of polyalkylene glycol (PAG) quenchants on residual stresses and mechanical properties of 2024 aluminum alloy were investigated. Surface residual stresses were measured by using hole-drilling strain-gauge method. Also, mechanical properties and microstructure of the heat-treated samples were analyzed using hardness measurements, tensile tests, and transmission electron microscopy. Results showed that quenching into a 15% polymeric solution and aging at 190 °C for 12 h cause 50% reduction in residual stress as compared with quenching in water at 20 °C and naturally aging. Moreover, tensile strength decreased by 104 MPa (~?20%) in compared with the T6 sample.     

Effect of heat treatment on microstructures and mechanical properties of A356 alloy cast through rapid slurry formation (RSF) process

The effect of T5 heat treatment on microstructure and mechanical properties of A356 alloy was observed. The as-cast A356 alloy exhibited coarse dendrites and long Si needles. RSF process changed the dendritic α-Al phase to globular morphology which helps in improving the mechanical properties of the alloy. The addition of 0.6wt-% Al–5Ti–1B grain refiner refined the average grain size of primary α-Al phase. T5 heat treatment at 170 °C for 20 h in different processing conditions was given to A356 alloy. T5 heat treatment led to further refinement of α-Al phase and Si needles, precipitation hardening due to Mg₂Si phase and reduction in the porosity level (%). The Quality Index for A356 alloy in different processing conditions was also measured. Results showed that RSF process with the use of baffles, addition of grain refiner and T5 heat treatment had improved the mechanical properties over other processing conditions.     

挤压铸造半固态A356铝合金的组织与力学性能

采用低温铸造方法制备A356铝合金半固态坯料.在200 t立式油压机上用挤压铸造方法将A356铝合金半固态浆料挤压成件.研究挤压铸造件的微观组织、力学性能,并与液态挤压铸造件进行比较.结果表明,A356铝合金半固态挤压铸造件组织由球形及椭圆形α-Al晶粒和α+Si共晶成分组成,且制件充型完整、无宏观缩孔、组织致密.在比压48.7 MPa,浇注温度575℃,保压时间3s条件下成形的半固态挤压铸造件的抗拉强度、屈服强度、伸长率分别达到278 MPa、225 MPa、13.2％,相比于在比压48.7 MPa,保压时间3s,710℃液态挤压铸造件性能分别提高了8.6％、8.2％、24.5％.A356铝合金半固态挤压铸造成形件具有较高的综合力学性能.     

Effect of foaming temperature on the mechanical properties of produced closed-cell A356Aluminum foams with melting method

In this study an attempt was carried out to determine the effect of production temperature on the mechanical properties and energy absorption behavior of closed-cell A356 alloy foams under uniaxial compression test. For this purpose, three different A356 alloy closed-cell foams were synthesized at three different casting temperatures, 650 °C, 675 °C and 700 °C by adding the same amounts of granulated calcium as thickening and TiH₂ as blowing agent. The samples were characterized by SEM to study the pore morphology at different foaming temperatures. Compression tests of the A356 foams were carried out to assess their mechanical properties and energy absorption behavior. The results indicated that increasing the foaming temperature from 650 °C to 675 °C and 700 °C reduces the relative density of closed cell A356 alloys by 18.3% and 38% respectively and consequently affects the compressive strength and energy absorption of cellular structures by changing them from equiaxed polyhedral closed cells to distorted cells. Also at 700 °C foaming temperature, growth of micro-pores and coalescence with other surrounding pores leads to several big voids.     

Microstructure and Mechanical Properties of Austempered Medium-Carbon Spring Steel

Changes in microstructure and mechanical properties of medium-carbon spring steel during austempering were investigated. After austempering for 1 h at 290 °C or 330 °C, the bainite transformation stabilized austenite, and microstructure consisting of bainitic ferrite and austenite could be obtained after final cooling; the retained austenite fraction was smaller in the alloy austempered at 290 °C because carbon redistribution between bainitic ferrite and austenite slowed as the temperature decreased, and thereby gave persistent driving force for the bainite transformation. The products of tensile strength and reduction of area in the austempered alloy were much larger in the austempered steel than in quenched and tempered alloy, mainly because of significant increase in reduction of area in austempered alloy.     

Fabrication and Characterization of Functionally Graded Al/SiCp Composites Produced by Remelting and Sedimentation Process

A new process termed here as remelting and sedimentation (RAS) was developed to produce functionally graded Al/SiC composites with a smooth concentration gradient of SiC particles along the height of samples, as opposed to a step change. For this purpose, first settling velocities of different-sized SiC particles in aluminum A356 melt were measured, and the results exhibited a reasonably good agreement with those predicted via the modified Stokes law. Then slices of particulate Al/SiC composites with different SiC contents of 5, 10, 15, and 20 vol.% were stacked in a cast iron mold and heated at 650 °C resulting in remelting and unification of the different composite parts. Considering the preliminary settling experiments, the composite slurry was held at this temperature for three different times to investigate the optimum holding time for obtaining a smooth gradient of SiC concentration along the height of the sample. After quenching, the samples were sectioned and subjected to metallographic studies and hardness measurements. The results confirmed that holding the melt for 60 s provides sufficient settling and redistribution of SiC particles and results in successful production of a functionally graded material.     

Rheo-squeeze casting of semi-solid A356 aluminum alloy slurry

The effect of pouring temperature, electromagnetic stirring power and holding process on semi-solid A356 aluminum alloy slurry was investigated, then the slurry was squeeze-cast. The results show that when the pouring temperatures are properly above the liquidus line, for example 630-650 °C, the slurry with spherical primary α(Al) grains can be prepared under the stirring power of 1.27 kW. The slurry is then homogeneously held for a short time, and the primary α(Al) grains are further ripened and distributed evenly in the slurry. The results of the rheo-squeezed casting experiments show that the injection specific pressure has a great effect on the filling ability of the semi-solid A356 aluminum alloy slurry, and the higher the injection specific pressure is, the better the ability for the slurry to fill the mould cavity is. When the injection specific pressure is equal to or above 34 MPa, the whole and compact rheo-squeezed castings can be obtained. The microstructure of the castings indicates that the shape, size and numbers of the primary α(Al) grains in different parts of the castings are highly consistent. After being held at 535 °C for 5 h and then aged at 155 °C for 12 h, the ultimate strength of the rheo-squeezed castings can reach 300-320 MPa, the yield strength 230-255 MPa, and the elongation 11%-15%.     

Study on the Mechanical Properties of Cast 6063 Al Alloy Using a Mixture of Aluminum Dross and Green Sand as Mold

The mechanical characteristics of 6063 aluminum alloy cast in a mixture of aluminum dross and silica sand as mold have been examined. The amount of dross in the green silica sand was varied in the range of 0?C80% with bentonite as binder. In all, 40 samples were cast, and 8 of these were left in the as-cast condition for control while 32 were first homogenized at 470°C for 6?h and then rolled in a two-high mill at ambient temperature to 10% reduction in one pass. The rolled samples were solution heat treated at 515°C for 8?h followed by normalizing, annealing, and quench tempering, respectively. The samples were then simulated and tensile behavior coupled with the evaluation of microhardness and microstructures developed. The results obtained demonstrate significant improvement in mechanical properties from 50% to 80% dross in the mold. Tensile strength increased to 177?MPa and 15% elongation compared with conventional 6063-T5 aluminum alloy with 145?MPa tensile strength and 8% elongation. The improvement in mechanical properties by the quench-tempered samples can be attributed to the inducement of fine and coherent Mg₂Si crystals within the matrix. Furthermore, the overall analysis of the proportion of dross to the size of cast show that about 64% of dross generated can be utilized as mold material.     

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.     

Optimization of heat treatment of gravity cast Sr-modified B356 aluminum alloy

In recent years, certain foundry processes have made it possible to obtain products with very thin parts, below the 4 mm threshold of the permanent mold casting technology. The safety margins of these castings have been reduced, so the T6 heat treatment conditions adopted for the Al–7Si–Mg alloys need to be investigated to identify the best combination of strength and ductility. Furthermore, the cost and the production time associated with T6 heat treatment have to be optimized. In the present work, an experimental study was carried out to optimize the solution treatment and artificial aging conditions in gravity cast thin bars of B356 aluminum alloy modified with Sr. Two solution temperatures were selected, 530 °C and 550 °C, respectively, with solution time ranging from 2 to 8 h, followed by water quenching and artificial aging at 165 °C with aging time from 2 to 32 h. The results of hardness and tensile tests were correlated with differential scanning calorimetry (DSC) analysis. The best combination of mechanical properties and heat treatment duration was obtained with 2 h solutionizing at 550 °C and 8 h aging at 165 °C. DSC analysis showed that the alloy's mechanical properties reach the maximum value when the β″ phase is completely developed during the artificial aging.     

Effects of Heat Treatment on Microstructural Modification of As-Cast Gamma-TiAl Alloy

Effects of normalizing and annealing treatments on the microstructure of Ti-48Al-2Cr-2Nb (at.%) were investigated. Normalizing processes were done at 1385 ± 5 °C in α-phase domain with the heating rate of 10 °C/min, the average cooling rate of 30 °C/min, and the holding times of 5, 10, 15, 20, and 25 min. The annealing process was done at the same temperature and heating rate, the holding time of 15 min, and the average cooling rate of 2 °C/min. Microstructures, phases, and hardness levels were studied by optical and field emission electron microscopic observations, x-ray diffractometry (XRD), and microhardness testing, respectively. Also, crystallographic texture variations were analyzed by means of texture coefficient and XRD results. Experimental results showed a linear direct relationship between treatment time and grain size, up to 15 min. A linear reversed behavior was observed for longer times. The untreated alloy consisted of γ and α₂ phases with a columnar morphology with the length of about 300 μm. A near-lamellar microstructure with equiaxed gamma grains, Widmansttäten, and laminar γ + α₂ colonies was obtained by the normalizing process. The maximum reduction of the grain size was about 70%, as achieved by normalizing with the 15 min holding time. A texture-free microstructure was acquired by normalizing treatment in comparison with strong texture of the as-cast and annealed alloys.     

Casting of Iron Aluminides

Iron aluminides form an interesting class of materials which combine excellent corrosion and oxidation resistance with good mechanical properties at moderate to high temperature (up to 500 °C). These materials, however, suffer from low room-temperature ductility (under 5% elongation in tension), which is mostly due to environmental effects. Casting is a processing route traditionally applied to brittle alloys (e.g., gray cast irons), but to cast a part without defects, several thermochemical properties are needed, as well as information on the tendency of the alloy to form foundry defects (e.g., shrinkage voids, pores). The present work aims to provide this information using parts produced on laboratory scale. In particular, the solidification contraction and the efficiency of TiB₂ as inoculant are investigated. Three alloys with nominal composition (in at.%) Fe28Al, Fe28Al6Cr, and Fe28Al6Cr1Ti (about 1.5 kg for each melt) were melted in an induction furnace under argon flux protection using conventional raw materials (carbon steel, commercial aluminum, metallic chromium, and commercial ferrotitanium). The resulting melts were treated by adding Al-TiB₂ master alloy used in the aluminum industry and poured into “staircase” molds, designed to investigate feeding distance effects in complex parts. Characterization of the microstructure of the alloys revealed that alloys Fe28Al and Fe28Al6Cr showed κ-carbide precipitation, while alloy Fe28Al6Cr additionally showed chromium carbides at dendritic boundaries. Addition of 1 wt.% Ti in alloy Fe28Al6Cr1Ti changed the solidification microstructure, refining the dendrite morphology and forming TiC-containing eutectic in interdendritic spaces.     

超快速加热退火下保温时间对5083铝合金组织及力学性能的影响

利用Gleeble-3500热模拟系统和电子背散射衍射(EBSD)技术对5083铝合金超快速退火组织的演变规律进行了研究,探讨了5083铝合金经过80%冷轧变形后以500 ℃/s加热至450 ℃时,不同保温时间(1~60 s,冷却速度40 ℃/s)对退火组织及力学性能的影响。结果表明,随退火保温时间从1 s延长到60 s,5083铝合金的平均晶粒尺寸由4.94 μm增大到6.44 μm,合金中主要产生了再结晶立方退火织构{001}<100>、旋转立方织构{001}<110>,以及少量的高斯织构{011}<100>和黄铜型织构{011}<211>。当退火保温时间从1 s增加到60 s,整体上合金中的再结晶退火织构先增强再减弱。退火保温时间对5083铝合金的强度影响较小,5083铝合金的屈服强度、抗拉强度没有明显的变化,分别约为170 MPa、326 MPa,而其伸长率由25.63%逐渐增大至30.06%,最后又降低至25.20%。     

Multiphase Microstructure in a Metastability-Assisted Medium Carbon Alloy Steel

A medium carbon alloy steel is processed by austenizing at 900 °C for 30 min, then rapid quenching into a patented quenching liquid and holding at 170 °C for 5 min, finally isothermally holding at 250 °C for different times. The morphology and mechanical properties are performed by using optical microscopy and scanning electron microscopy. A multiphase microstructure characterized by a mixture of lenticular prior martensite (PM), fine needle bainitic ferrite and filmy retained austenite (RA) is obtained. It is found that the PM formed firstly upon quenching can accelerate the subsequent bainitic transformation and promote refinement of multiphase colonies. The results show that an optimum mechanical property of a 4000.9 MPa bending strength and a 2030 MPa tensile strength is achieved at 250 °C for 120 min, which is attributed to the multiphase microstructural characteristics and a high product of the volume fraction of RA and the carbon content of austenite.