The effect of die geometry on the microstructure of indirect squeeze cast and gravity die cast 7050 (Al-6.2Zn-2.3Cu-2.3Mg) wrought Al alloy

The indirect squeeze casting process has been used to cast a 7050 (Al-6.2Zn-2.3Cu-2.3Mg) wrought Al alloy to near-net shape with excellent die replication. Defects which occur with gravity casting, in particular (1) shrinkage pipe, (2) macro-porosity and (3) hot-tearing, are largely removed by squeeze casting, although regions of macro-porosity can re-appear when thick sections are fed through substantially thinner sections. Squeeze casting results in a considerable refinement of microstructure compared to gravity casting due to a marked decrease in solidification time. The decrease in solidification time is caused by intimate contact between the pressurised melt and the die, which leads to an increase in the heat transfer coefficient. Decreasing the section thickness also results in a refinement of the microstructure due to a reduction in solidification time. This revised version was published online in September 2006 with corrections to the Cover Date.     

Predictive equations of the tensile properties based on alloy hardness and microstructure for an A356 gravity die cast cylinder head

One of the main problems in the design of complex Al–Si cast components is the wide variety of mechanical properties in different regions of the castings which is due to the wide range of solidification microstructures, related to the local solidification conditions. There are many papers available on the widely used A356/A357 Al–Si–Mg alloys, however, most experimental data on their tensile or fatigue properties are generally obtained from specimens cast separately or produced under controlled laboratory conditions, that are extremely different from those of industrially cast components. Moreover, most of these data often relate the mechanical properties to only one microstructural parameter, such as solidification defects or secondary dendrite arm spacing, and do not take their simultaneous effect into consideration. For all these reasons, the main problem, in the design phase, is the lack of knowledge of the true local mechanical properties in complex-shaped castings, which often means a conservative approach is necessary, with a consequent increase in thickness and therefore in weight. The aim of this research was to study a complex A356 gravity die cast cylinder head, in order to verify the range of variability of the main microstructural parameters and tensile properties, using specimens directly machined from the casting. The component was heat treated at the T6 condition, and the effect of the delay between quenching and aging on the alloy hardness was also evaluated. Simple experimental equations have been proposed, able to successfully predict the local tensile properties in the casting, when only the most important microstructural parameters and alloy hardness are known. These equations allow the designer to predict the local tensile behaviour without any tensile tests; moreover they can also link the post-processing results of the casting simulation software to the pre-processing phase of the structural ones, with an approach of co-engineered design.     

Microstructure and properties of squeeze cast Cu-carbon fibre metal matrix composite

There have been reported attempts of producing Cu based MMCs employing solid phase routes. In this work, copper was reinforced with short carbon fibres by pressure infiltration (squeeze casting) of molten metal through dry-separated carbon fibres. The resulting MMC's microstructure revealed uniform distribution of fibres with minimum amount of clustering. Hardness values are considerably higher than that for the unreinforced matrix. Addition of carbon fibres has brought in strain in the crystal lattice of the matrix, resulting in higher microhardness of MMCs and improved wear resistance. Tensile strength values of MMCs at elevated temperatures are considerably higher than that of the unreinforced matrix processed under identical conditions.     

Microstructure and mechanical properties of laser welded wrought ZK21 magnesium alloy

The as-rolled ZK21 magnesium alloy sheets of 2 mm in thickness were successfully joined by laser welding. The effects of the welding parameters including the laser power and the welding speed on the microstructure and mechanical properties of the joints were investigated. A sound bead, with the ultimate tensile strength (UTS) of 289 MPa and up to 94% of the base metal, was obtained with the optimized welding parameters. The fusion zone (FZ) was characteristic of equiaxed dendritic grains of about 15 μm in size and fine Mg₂Zn₃ precipitates dispersed among the dendritic arms. Besides, a few columnar grains grew from the fusion boundary epitaxially. The fine grains in the heat-affected zone (HAZ) were ascribed to recrystallization.     

Microstructure characterization and tensile properties of squeeze-cast AlSiMg alloys

A research program was conducted to study the effects of squeeze pressure (70, 100 and 160 MPa) and heat treatment T6 on the structure, hardness and tensile properties of cast Al6Si0.3Mg alloys. The influence of squeeze pressure on macro- and microstructures of Al6Si0.3Mg alloys has been investigated. Some of castings were solution treated at 540 °C for various times and others were subjected to aging at 170 °C after solution treatment. The results indicated that precipitation occurred within about 30 min for both cast and squeeze cast alloys. The hardness began to increase and maximum values were observed after about 10 h for as-cast alloy. Increasing of squeeze pressure (70–160 MPa) accelerated strength of the alloys from 8 to 4 h, respectively. Squeeze pressures decreased the percentage of porosity and increased the density, also it decreased the grain size of α-Al and modified the Si eutectic. Hardness and tensile properties increased with both heat treatment and increasing of squeeze pressure.     

Microstructure,tensile properties and fracture behaviour of Al2O3 particulate-reinforced aluminium alloy metal matrix composites

The tensile deformation and fracture behaviour of aluminium alloy 2014 discontinuously-reinforced with particulates of Al₂O₃ was studied with the primary objective of understanding the influence of reinforcement content on composite microstructure, tensile properties and quasi-static fracture behaviour. Results reveal that elastic modulus and strength of the metal-matrix composite increased with reinforcement content in the metal matrix. With increase in test temperature the elastic modulus showed a marginal decrease while the ductility exhibited significant improvement. The improved strength of the Al-Al₂O₃ composite is ascribed to the concurrent and mutually interactive influences of residual stresses generated due to intrinsic differences in thermal expansion coefficients between constituents of the composite, constrained plastic flow and triaxiality in the soft and ductile aluminium alloy matrix due to the presence of hard and brittle particulate reinforcements. Fracture on a microscopic scale initiated by cracking of the individual or agglomerates of Al₂O₃ particulates in the metal matrix and decohesion at the matrix-particle interfaces. Failure through cracking and decohesion at the interfaces increased with reinforcement content in the matrix. The kinetics of the fracture process is discussed in terms of applied far-field stress and intrinsic composite microstructural effects.     

Microstructure,tensile properties and fracture behaviour of aluminium alloy 7150

A study has been made to understand the microstructure, tensile properties and fracture characteristics of aluminium alloy 7150. Detailed optical and transmission electron microscopical observations were used to analyse the intrinsic microstructural features of the alloy in the T77 condition. The alloy was deformed to failure over a range of strain rates in environments of 3.5% sodium chloride solution and laboratory air. The environment was found to have little influence on strength of the alloy. The strength only marginally increased with an increase in strain rate. However, for all strain rates, the ductility of the alloy degraded in the aggressive environment. The ratio of strain to failure in sodium chloride solution to that in laboratory air indicates that the alloy is only mildly susceptible to stress corrosion cracking. The fracture behaviour was different in the two environments. However, in a given environment the fracture behaviour was essentially the same. In the aggressive environment fracture was predominantly intergranular while fracture revealed a ductile transgranular failure in laboratory air. An attempt is made to discuss the kinetics of the fracture process in terms of competing mechanistic effects involving intrinsic microstructural features, matrix deformation characteristics, environment and strain rate.     

Mechanical properties of ABOw+MWNTs/Al hybrid composites made by squeeze cast technique

Multi-wall carbon nanotubes (MWNTs) have been considered a realistic kind of reinforcement for composite materials. In this paper, microstructure and mechanical properties of the aluminum borate whisker (ABOw) and MWNTs hybrid composites were investigated. The results show that MWNTs decrease the compressive deformation of the hybrid preforms and are kept intact in the matrix during squeeze cast processing. A small amount of MWNTs may effectively improve the modulus, strength and elongation of the hybrid composite. Decreasing micropores and strengthening the matrix, high strength MWNTs make the mechanical properties of the hybrid composite superior to the singularly reinforced ones. This makes MWNTs a promising material for novel micro/nanohybrid composite.     

Tensile behaviour of squeeze cast AM100 magnesium alloy and its Al2O3 fibre reinforced composites

Magnesium alloys are increasingly used in automotive and aerospace applications mainly due to their light weight combined with reasonably high tensile properties. In addition to providing a large reduction in weight, magnesium alloys exhibit excellent machinability and good damping capacity. However, their low mechanical properties when exposed to elevated temperatures limit their usage. Making composites out of these magnesium alloys by reinforcing them with ceramic particles or fibres appears to be a viable alternative for improving their thermal stability. The work reported here involved experimental studies on the tensile behaviour of AM100 magnesium alloy and its composites at different temperatures. Fractographic studies justify the effect of temperature on the tensile behaviour.     

Microstructure,tensile properties and fracture behaviour of an Al-Cu-Li-Mg-Zr alloy 8090

The addition of lithium to aluminium alloys has the potential for providing a class of high strength alloys with exceptional properties suitable for aerospace applications. One such candidate is 8090, a precipitation hardenable Al-Li-Cu-Mg alloy. Detailed optical microscopical observations were used to analyse the intrinsic microstructural features of the alloy. It is shown that microstructural characteristics have a pronounced influence on tensile properties and fracture behaviour of the alloy in the peak-aged, maximum strength condition. Tensile test results indicate that the alloy has property combinations comparable with other high strength commercial aluminium alloys. The elongation and reduction in area are higher in the transverse direction of the extruded plate. A change in fracture mode was observed with direction of testing. We rationalize such behaviour based on the grain structure of the material, and the nature, distribution and morphology of the second-phase particles. An attempt is made to discuss the kinetics of the fracture process in terms of several competing mechanistic effects involving intrinsic microstructural features, deformation characteristics of the matrix, brittleness of the grain boundary precipitates and grain boundary failure. The role of stress on particle fracture is highlighted.     

Microstructure,tensile properties and fracture behaviour of an Al-Cu-Mg alloy 2124

The microstructure, tensile properties and fracture behaviour of aluminium alloy 2124 were studied. Detailed optical and electron microscopical observations were made to analyse the as-received microstructure of the alloy. It is shown that microstructural characteristics have a profound influence on tensile properties and quasi-static fracture behaviour of the alloy. Tensile test results indicate that the alloy has uniform strength and ductility in the longitudinal and transverse orientations. The elongation and reduction in area are higher in the transverse direction of the extruded plate. No change in fracture mode was observed with direction of testing. Fracture, on a microscopic scale, was ductile, comprising of void nucleation, growth and coalescence. The fracture process is discussed in terms of competing influences of intrinsic microstructural features, deformation characteristics of the matrix and grain-boundary failure.     

Microstructure of SiCp-reinforced A356 cast Al metal-matrix composite

The microstructure of SiCp-reinforced A356 cast Al metal-matrix composite (MMC) has been investigated by means of transmission electron microscopy and energy-dispersive X-ray analysis. It is found that the MMC contains -SiC, ( + ₂) SiC composite, eutectic Si, GP zones of Si sequence, a new J phase and the amorphous phase Al _x Si₉Ca₂. The ₂-SiC is lathlike with a triclinic crystal structure having a=0.308 nm, b=0.305 nm, c=1.262 nm, = 93.8 °, = 90.0 °, and = 60.0 °. The J phase is bulk-like with a C-face-centred orthorhombic crystal structure having a = 0.680 nm, b = 1.170 nm and c = 0.826 nm.     

The semi-solid tensile deformation behavior of wrought AZ31 magnesium alloy

The semi-solid tensile deformation behavior of wrought AZ31 magnesium alloy has been studied through applying a set of low strain rate (0.001 s⁻¹) hot tension tests at temperature range of 300–500 °C. The results indicated a ductility drop at ∼450 ± 25 °C. This was attributed to the occurrence of eutectic reaction (L → α + γ) and the partial melting of intermetallic γ phase. The ductility was started to improve by increasing the temperature to 500 °C. The latter was explained considering the effect of liquid phase on stress relaxation through accommodation of the grain boundary sliding phenomena. To further investigating the semi-solid tensile deformation behavior of the experimental alloy, the cavitations characteristics of the alloy were also examined.     

Effects of alloy additions on the microstructures and tensile properties of cast Co-Cr-Mo alloy used for surgical implants

The composition of the Co-Cr-Mo alloy has been modified by additions of nickel and some trace elements aluminium, titanium and boron. In this paper, the first part of the present study, the effects of alloy additions on the microstructures and tensile properties of the as-cast Co-Cr-Mo alloy are discussed. The effects of alloy additions on the fatigue behaviour of the alloy will be discussed in the second part of the present study. It is found that alloy additions do not seem to result in changes in the nature of the casting structure. A directional, coarse dendritic cast structure is produced in the modified alloys as in the base alloY. However, the alloy additions affect the carbide precipitations and the formation of some fine details of the microstructure such as dislocations, stacking faults and twins produced during the solidification. A considerable improvement in the transient mechanical properties, especially in the tensile ductility, is achieved by modifying the base alloy with alloy additions.     

电磁铸造5182铝合金锭的组织与性能研究

采用电磁铸造技术铸造了易拉罐用5182变形铝合金，采用光学显微镜和扫描电镜分析了其显微组织，而且对其进行了均匀化处理和对比实验。结果表明，电磁铸造锭有更加良好的内部组织和优良的力学性能，电磁铸造试样的硬度大约是普通连续铸造试样的两倍，疲劳性能是普通连续铸造铸锭的3倍，电磁铸造铸锭还有良好的耐磨性，电磁铸造锭的优良性能得宜于电磁搅拌的作用使整个铸锭获得均匀细小的晶粒组织，电磁铸造技术是一种无模铸造技术，它依靠电磁力约束液体金属成型，液体金属不与铸模接触，铸锭表面光滑如镜；相反，普通连续铸造锭表面因存在振动痕和亚表面偏析等铸造缺陷，力学性能大大降低。     

Microstructure and tensile properties of friction welded aluminum alloy AA7075-T6

Solid-state welding processes like friction welding and friction stir welding are now being actively considered for welding aluminum alloy AA7075. In this work, friction welding of AA7075-T6 rods of 13 mm diameter was investigated with an aim to understand the effects of process parameters on weld microstructure and tensile properties. Welds made with various process parameter combinations (incorporating Taguchi methods) were subjected to tensile tests. Microstructural studies and hardness tests were also conducted. The results show that sound joints in AA7075-T6 can be achieved using friction welding, with a joint efficiency of 89% in as-welded condition with careful selection of process parameters. The effects of process parameters are discussed in detail based on microstructural observations.     

Effect of grain size on the tensile deformation of wrought Mg-MM-Al-Zn-Sn alloy

The effect of grain size on the tensile deformation of Mg-MM-Al-Zn-Sn (EAZT211) alloy sheet has been investigated. Specimens with grain size varying from ~ 10 to ~ 20 μm have been obtained by altering the annealing conditions after rolling. The yield strength of EAZT211 sheet exhibits grain size dependence according to Hall-Petch relationship from room temperature to 200 °C. Occurrence of yield phenomenon and decrease of Hall-Petch slope (k) with increasing strain suggest that non-basal slip system operates during deformation. In addition, deformation mechanism changes from slip mechanism to grain boundary sliding mechanism at ~ 150 °C.     

The properties of a wrought biomedical cobalt-chromium alloy

Two wrought biomedical cobalt-chromium alloys have been developed, and their mechanical properties and corrosion resistance determined by means of tensile and hardness tests and by electrochemical potential-time curves for isolated specimens in a 6.0 wt% NaCl solution at room temperature. In comparison with a current dental alloy, SC-H, and the basic type 18-8 austenitic stainless steel, it is shown that alloy II (chemical composition in wt%:0.11 C, 22.07 Cr, 15.20 Ni, 3.75 Mo, 9.30 W, balance Co) has superior properties. The alloy has a high strength together with a good ductility which permits adequate workability. Also, both cobalt-chromium alloys show a passive behaviour in 6.0 wt% NaCl solution, whereas the basic type 18-8 austenitic stainless steel shows a fluctuating potential and is thus susceptible to pitting, making it unsuitable for surgical implants.     

Compressive properties of high-pressure die cast IP 75 alloy with strengthening Laves phase

The NiAl-2Ta-7.5Cr-0.5Nb alloys (IP 75 alloy) were prepared by high-pressure die cast (HPDC), and tested for compressible strength and fracture behavior in the temperature range 300-1373 K. The fine structures with a homogeneous distribution of Laves phase at the boundary regions created by high-pressure die cast led to improvements in both the compressible yield strength and fracture strain. The high temperature (1373 K) 0.2% compressible yield strength of the HDC IP 75 alloy (160 MPa) is larger than that of the IP 75 alloys prepared by other processes. The room-temperature compressible fracture strain of the HDC IP 75 (14%) is also superior to the IP 75 alloy (5%) prepared by an ingot-casting process. The effects of size refinement and the more homogenous distribution of Laves phase and the formation of a ductile Cr-rich phase due to a rapid solidification contribute to the increments of the compressible yield strength and the fracture strain of the HPDC alloy.     

Microstructural refinement and tensile properties enhancement of Mg-3Al alloy using charcoal additions

Significant grain refinement in Mg-3Al alloy is achieved with the addition of charcoal due to the formation of Al₄C₃ particles, which act as effective nuclei for magnesium grains. Addition of 0.5 wt% charcoal has lead to reduced grain size of Mg-3Al alloy from 500 to 80 μm and no substantial grain refinement is obtained on further addition of charcoal. The results further reveal that the prolonged holding of the melt after the addition of charcoal has not affected the grain refining efficiency of Al₄C₃. Steady increase in tensile properties observed with increasing amount of charcoal addition has been attributed to the grain refinement and the presence of fine Al₄C₃ particles. The strengthening mechanisms due to charcoal addition are discussed in terms of Hall-Petch relation and dispersion strengthening. The predicted values are in good agreement with experimental results.