Influence of melt treatments on sliding wear behavior of Al–7Si and Al–7Si–2.5Cu cast alloys

The microstructures and dry sliding wear behavior of Al–7Si and Al–7Si–2.5Cu cast alloys were studied after various melt treatments like grain refinement and modification. Results indicate that combined grain refined and modified Al–7Si–2.5Cu cast alloys have microstructures consisting of uniformly distributed α-Al grains, eutectic Al– silicon and fine CuAl₂ particles in the interdendritic region. These alloys exhibited better wear resistance in the cast condition compared with the same alloy subjected to only grain refinement or modification. The improved wear resistances of Al–7Si–2.5Cu cast alloys are related to the refinement of the aluminum grain size, uniform distribution of eutectic Al-silicon and fine CuAl₂ particles in the interdendritic region resulting from combined refinement and modification. This paper attempts to investigate the influence of the microstructural changes in the Al–7Si and Al–7Si–2.5Cu cast alloys by grain refinement, modification and combined action of both on the sliding wear behavior.     

The Performance of Zinc Alloy Based Metal Matrix Composites Produced Through Squeeze Casting

Zinc-aluminum cast alloys (ZA alloys) have good castability, mechanical properties and excellent tribological characteristics. Of all the ZA alloys, ZA-27 (containing 27% aluminum) has the highest strength and optimum wear resistance. However all the ZA alloys, including ZA-27, suffer from lack of creep resistance and high temperature stability. One probable solution to improve these properties is to reinforce the alloys with ceramic particles or fibers to result in metal matrix composites (MMCs). MMCs can be economically produced through squeeze casting which involves infiltration. This paper presents the salient features of an experimental study on ZA-27 alloy based MMCs produced through Squeeze Casting.     

Low‐cycle fatigue behavior of a newly developed cast aluminum alloy for automotive applications

The drive for increasing fuel efficiency and decreasing anthropogenic greenhouse effect via lightweighting leads to the development of several new Al alloys. The effect of Mn and Fe addition on the microstructure of Al‐Mg‐Si alloy in as‐cast condition was investigated. The mechanical properties including strain‐controlled low‐cycle fatigue characteristics were evaluated. The microstructure of the as‐cast alloy consisted of globular primary α‐Al phase and characteristic Mg₂Si‐containing eutectic structure, along with Al₈(Fe,Mn)₂Si particles randomly distributed in the matrix. Relative to several commercial alloys including A319 cast alloy, the present alloy exhibited superior tensile properties without trade‐off in elongation and improved fatigue life due to the unique microstructure with fine grains and random textures. The as‐cast alloy possessed yield stress, ultimate tensile strength, and elongation of about 185 MPa, 304 MPa, and 6.3%, respectively. The stress‐strain hysteresis loops were symmetrical and approximately followed Masing behavior. The fatigue life of the as‐cast alloy was attained to be higher than that of several commercial cast and wrought Al alloys. Cyclic hardening occurred at higher strain amplitudes from 0.3% to 0.8%, while cyclic stabilization sustained at lower strain amplitudes of ≤0.2%. Examination of fractured surfaces revealed that fatigue crack initiated from the specimen surface/near‐surface, and crack propagation occurred mainly in the formation of fatigue striations.     

Mechanical properties of hot pressed SiCp and B4Cp/Alumix 123 composites alloyed with minor Zr

In the present study, effect of Zr addition on the microstructure and wear behavior of aluminum alloy composites (AMCs) reinforced with B4Cp and SiCp particles fabricated via hot pressing were investigated. The samples for the study composed of unreinforced aluminum alloy (Alumix 123) and the composites reinforced with 10% B4Cp and % SiCp were prepared by hot isostatic pressing (HIP) method. Similarly, all the samples alloyed with 0.2% Zr were also produced in order to make a comparison. The produced samples were evaluated for microstructural properties and mechanical tests for hardness, tensile and bending strength were performed. Wear test was carried out at 5 mm/s sliding speed under 3.0 N load for the all kind of hot pressed produced samples. The hot pressed composite microstructures have a more uniform distribution of the reinforcements. After HIP process, the composites were successfully produced with high density (>99%). The addition of Zr increased the yield and tensile strength of the samples. The highest strength value was found for the sample Al 123 matrix alloy with Zr. Evaluation of microstructures showed that copper and zirconium dispersed equally within the matrix microstructure without agglomeration. For the composite samples, Al3Zr, appeared as white precipitate, were inspected around B4C and SiC particles. The composite containing SiC particles and Zr had wear resistance value superior to those of the other counterparts.     

Tensile and fatigue properties of a cast aluminum alloy with Ti, Zr and V additions

The development of aluminum alloys for automotive powertrain applications is in high demand due to the required weight reduction and fuel efficiency. The aim of this study was to evaluate the microstructure and mechanical properties of a newly developed Al-7%Si-1%Cu-0.5%Mg cast alloy with further additions of Ti, Zr and V. The microstructure of the alloys consisted of Al dendrites surrounded by Al-Si eutectic structures with Mg/Cu/Fe-containing Si particles, and contained nano-sized trialuminide precipitates in the Ti/Zr/V added alloys. The alloys had a significantly (60-87%) higher yield strength but lower ductility than A356-T6 and 319-T6 alloys. With the addition of Ti/Zr/V both monotonic and cyclic yield strengths increased, but ductility and hardening capacity decreased due to reduced dislocation storage capacity caused by stronger interactions between dislocations and trialuminide precipitates. The Zr/V-modified alloy had a longer fatigue life, and all the alloys exhibited cyclic stabilization at low strain amplitudes and cyclic hardening at higher strain amplitudes. With increasing strain amplitude, the extent of cyclic hardening increased. Both cyclic yield strength and cyclic strain hardening exponent were higher than the corresponding monotonic yield strength and strain hardening exponent, indicating that a stronger cyclic hardening ability of the alloys developed. Fatigue cracks were observed to initiate at near-surface defects, and crack propagation was mainly characterized by the formation of fatigue striations together with secondary cracks.     

Processing and Performance Characteristics of Aluminum-Nano Boron Carbide Metal Matrix Nanocomposites

In this research work, ultrasonic cavitation-assisted casting process was used to fabricate the aluminum alloy-nano boron carbide metal matrix nanocomposites. The optical microscopy results revealed the refined matrix grains in the microstructure of the aluminum alloy-nano boron carbide composites. Boron carbide nanoparticles were uniformly distributed in the aluminum alloy matrix, which can be confirmed by scanning electron microscopic images. The aluminum alloy-nano boron carbide composites show increased dislocation density, compared to the monolithic alloys, which was observed from the transmission electron microscopic images. The addition of nano-boron carbide in aluminum alloy matrix significantly improved its hardness and tensile strength, while the good ductility and impact resistance of the aluminum alloy was almost retained. The results of the dry sliding wear pin-on-disc tests showed improved wear resistance properties of aluminum alloy-nano boron carbide composites compared to the monolithic aluminum alloys.     

Study of particle–matrix interaction in Al/AlB2 composite via nanoindentation

Aluminum diboride (AlB₂) particles enhance wear resistance of functionally-graded aluminum-AlB₂ composites. A critical factor governing the wear resistance of these composites is the mechanical interaction between the diboride particles and the aluminum matrix. To study this interaction nanoindentation experiments were performed on 3–10 µm size AlB₂ particles embedded in the aluminum matrix of an as-received Al–5 wt.%B alloy and a centrifugally cast one. Under large nanoindentation loads (2–8 mN) diboride particles could be pushed into the matrix. The results show that on a per unit area basis, smaller particles are more difficult to push-in than larger particles. Strain gradient plasticity (SGP) theory was used to explain the size dependence of the push-in force.     

Structure and properties of modified compocast microsilica reinforced aluminum matrix composite

A356 aluminum alloy reinforced with 7 wt.% microsilica composites was produced by the three different processing routes viz. liquid metal stir casting followed by gravity casting, compocasting followed by squeeze casting and modified compocasting route and their properties were examined. Microstructure of liquid metal stir cast Al MMC shows agglomeration of particles leading to high porosity level in the developed material. Adopting new route of compocasting followed by squeeze casting process prevents the agglomeration sites with uniform distribution and dispersion of the dispersoids in the matrix metal. Modified compocasting process reduces the segregation of particles in the final composites thus enhancing the mechanical, tribological and corrosion properties of the composites. Superior wear-resistance properties were exhibited by the modified compocast composite compared to the unreinforced squeeze cast alloy and abrasive type wear mechanism was observed in the case of composite. Increasing the sliding speed resulted in the quick evolution of tribolayer and the wear rate of composite gets reduced. The presence of intermetallic phases like MgAl₂O₄, NaAlSi₃O₈ and KAlSi₃O₈ has a favorable effect on increased corrosion resistance of the composite. Microsilica particles significantly enhanced the compressive strength of modified compocast composites compared to the unreinforced squeeze cast Al alloy.     

Cr3C2 incorporation into an Inconel 625 laser cladded coating: Effects on matrix microstructure,mechanical properties and local scratch resistance

There is an increasing industrial demand for metal alloys with high wear resistance under severe operating conditions. Ni-based alloys, such as Inconel superalloys, are an excellent option for these applications; however, their use is limited by their high cost. Ni-based coatings deposited onto carbon steel substrates are being developed to achieve desired surface properties with reduced cost. Laser cladding deposition has emerged as an excellent method for processing Ni based coatings. In this work, microstructure, mechanical properties and local wear behaviour have been investigated in response to the addition of Cr₃C₂ ceramic particles into an Inconel 625 alloy deposited onto a ferritic steel substrate by laser cladding. Using this deposition technique, a homogeneous distribution of Cr₃C₂ particles was observed in the coating microstructure. The addition of ceramic particles to the starting powder resulted in the formation of hard precipitates in the coating microstructure. The partial dissolution of Cr₃C₂ particles during the laser cladding process increased the hardness of the Inconel 625 matrix. Depth sensing indentation and scratch tests were performed to study the local wear behaviour and scratch resistance of the cermet matrix compared with the conventional Inconel 625 alloy. Finally, the effect of Cr₃C₂ on mechanical properties was correlated with the observed microstructure modifications.     

Improving sliding and abrasive wear behaviour of cast A356 and wrought AA7075 aluminium alloys by plasma electrolytic oxidation

An important limitation of aluminium alloys for mechanical applications is their poor tribological behaviour. In this study, surface treatment by plasma electrolytic oxidation (PEO) has been applied to two widely used aluminium alloys: A359 (hypoeutectic Al–Si–Mg) cast alloy and AA7075 (Al–Zn–Mg–Cu) wrought alloy, in order to improve their wear resistance, under sliding and abrasive wear conditions. The main aim of this work was the comparison of the properties and wear resistance of the oxide layers grown under the same PEO treatment conditions on two different aluminium alloys which might be coupled in engineered components. Significant differences in the phase composition, microstructure and mechanical properties measured by microindentation were observed in the oxide layers grown on the two substrates, and were ascribed to the effects of the different compositions and microstructures of the substrate alloys. Abrasion tests were carried out in a micro-scale abrasion (ball-cratering) test, with both alumina and silicon carbide abrasive particles. The results demonstrated the influence of the abrasive material on wear behaviour: whereas relatively aggressive SiC particles gave comparable results for both PEO treated and untreated samples, with the less aggressive Al₂O₃ abrasive the wear rates of the PEO treated samples, for both substrates, were significantly lower than those of the untreated substrates. In unlubricated sliding the PEO treatment significantly increase the wear resistance of both the aluminium alloys, at low applied load. In this condition the wear behaviour of the PEO treated alloys is strongly influenced by the stability of a protective Fe–O transfer layer, generated by wear damage of the steel counterpart. Under high applied loads however, the transfer layer is not stable and the hardness of the PEO layer, as well as the load bearing capacity of the substrate, become the main factors in influencing wear resistance.     

Fabrication and properties of in situ synthesized particles reinforced aluminum matrix composites of Al–Zr–O–B system

A new in situ Al–Zr–O–B system is exploited. The Al–Zr(CO₃)₂–KBF₄ components are used to fabricate the particle reinforced aluminum matrix composites by the direct melt reaction method. The analytical results of XRD and SEM show that the in situ endogenetic particles are ZrAl₃, ZrB₂ and Al₂O₃, which are well distributed in the aluminum matrix. The sizes of reinforced particles are 0.5–2.5 μm. The results of mechanical properties of the composites show that the tensile strength and yield strength are improved with the increase of theoretical volume fraction of particles in matrix in the range of 0–12%, which are much superior to those of aluminum matrix. The best elongation of composites is 33% when the theoretical volume fraction is 3%. The fracture mechanism belongs to a ductile one. The wear resistance properties of the composites are much higher than that of aluminum matrix. The best abradability is got when the theoretical volume fraction of particles is 6%. The wear mechanism of the aluminum matrix is adhesive wear while the wear mechanism of (ZrAl₃ + ZrB₂ + Al₂O₃)_p/Al composites is abrasive wear.     

Effect of grain refinement on mechanical properties and sliding wear resistance of extruded Sc-free 7042 aluminum alloy

The present study deals with an investigation on dry sliding wear behavior of grain refined Sc-free 7042 aluminum alloy by using a pin-on-disc wear test machine. Al–5Ti–1B and Al–15Zr master alloys were used as grain refining agents. The optimum amounts of added Ti and Zr in the alloy were found to be 0.03 wt.% and 0.3 wt.%, respectively. Extrusion was carried out and T6 heat treatment ware applied for all rod specimens before testing. Significant improvement in mechanical properties was obtained with the addition of grain refiners. The worn surfaces were characterized by energy dispersive X-ray spectrometry microanalysis. Results showed that the wear resistance of unrefined alloy increased with the addition of both grain refiners. Furthermore, the worn surface studies showed a mixed type of wear mechanisms; delaminating, adhesive and abrasive which took place at higher applied load.     

Microstructural evolution in ultrasonically processed in situ AZ91 matrix composites and their mechanical and wear behavior

AZ91 alloy matrix composites reinforced with phases formed in situ from the addition of Si particles were fabricated by solidification under ultrasonic vibrations. Application of high-intensity ultrasonic field to the melt resulted in optimized size, morphology and distribution of in situ formed Mg₂Si particles. The amount of Mg₂Si particles increased, its size was refined and the distribution became uniform. Heterogeneous nucleation from the addition of silicon particles and enhanced nucleation from rapid cooling refined the grain size of the matrix in the composites. Hardness and ultimate compressive strength of the composites increased as compared to that of the cast AZ91 alloy. Composites exhibited improved sliding wear behavior of under varying normal loads. Identified dominant wear mechanism at lower sliding velocities is abrasion. Improvement in mechanical and sliding wear properties of the composites is attributed to the refinement of both matrix and reinforcement phases and improved dispersion of the reinforcement under ultrasonic vibrations.     

锌基-结晶硅、陶瓷粒子复合材料的组织与性能

本文研究了用流变铸造工艺制造的锌基-结晶硅,陶瓷粒子复合材料的组织、凝固特点、力学性能,热膨胀系数及摩擦磨损特性.研究发现,SiC粒子可作为高铝锌基合金的初生相的形核衬底,分布在晶内并可细化晶粒;硅粒子的分布与合金的结晶特性有关.而Al₂O₃粒子分布在晶界,对初生相有粗化趋势.也研究了粒子对锌基合金力学性能的影响,发现合金抗拉强度略有下降,硬度提高,粒子含量适当时延伸率及韧性提高.加入粒子后锌基合金热膨胀系数减小,耐磨性明显提高,从而使其能更好的满足模具,轴承材升的要求.     

Mechanical properties of Cu–Cr system alloys with and without Zr and Ag

The effects of addition of Zr and Ag on the mechanical properties of a Cu–0.5 wt%Cr alloy have been investigated. The addition of 0.15 wt%Zr enhances the strength and resistance to stress relaxation of the Cu–Cr alloy. The increase in strength is caused by both the decrease in inter-precipitate spacing of Cr precipitates and the precipitation of Cu₅Zr phase. The stress relaxation resistance is improved by the preferentially forming Cu₅Zr precipitates on dislocations, in addition to Cr precipitates on dislocations. The addition of 0.1 wt%Ag to the Cu–Cr and Cu–Cr–Zr alloys improves the strength, stress relaxation resistance and bend formability of these alloys. The increase in strength and stress relaxation resistance is ascribed to the decrease in inter-precipitate spacing of Cr precipitates and the suppression of recovery during aging, and to the Ag-atom-drag effect on dislocation motion. The better bend formability of the Ag-added alloys is explained in terms of the larger post-uniform elongation of the alloys.     

Microstructure investigations and mechanical properties of an Al‐Al2O3 MMC produced by semi‐solid solidification

The influence of the semi‐solid solidification production parameters (shear rate and agitation time) and the concentration of reinforcing particles on the microstructure formation and mechanical properties of a 520 aluminum alloy reinforced with Al₂O₃ particles was investigated. Depending on the content of reinforcing particles and the stirring conditions different rosette structures were formed. The type of wear mechanism (delamination or adhesion) depends on the size of the rosettes and the distribution of Al₂O₃ reinforcements. Best mechanical properties were obtained for metal matrix composites reinforced with 12 wt% of Al₂O₃ stirred at a shear rate of 2100 s^–1 for 1800 s. These samples showed tensile strength and yield stress similar to the commercial A520 alloy. The hardness and wear resistance were improved by the addition of Al₂O₃ particles, meanwhile the elongation to fracture was reduced.     

Effect of Zr on the microstructure, mechanical properties and corrosion resistance of Mg–10Gd–3Y magnesium alloy

The influence of Zr on the microstructure, mechanical properties and corrosion resistance of Mg–10Gd–3Y (wt.%) magnesium alloy was investigated. The grain size of alloys decreased with Zr content from 0% to 0.93% (wt.%). The addition of Zr greatly improved the ultimate tensile strength (UTS) and the elongation (EL), while slightly improved the tensile yield strength (TYS). The UTS and the EL of the alloy containing 0.93% Zr increased by 125.8 MPa and 6.96% compared with base alloy, respectively. The corrosion resistances were found to decrease with Zr content from 0% to 0.42% and then increase from 0.42% to 0.93%. The differences in the sizes and distributions of the Zr-rich particles have significant effects on the corrosion behaviors. The alloy with 0.42% Zr addition revealed the optimum combination of mechanical properties and corrosion resistance.     

Numerical study of the casting features on the fracture and debonding of Mg17Al12 in AM60B Mg alloy under high cycle fatigue condition

Based on the methods proposed by Gall et al. [Gall K, Horstemeyer MF, McDowell DL, Fan JH. Finite element analysis of the stress distributions near damaged Si particle clusters in cast Al–Si alloys. Mech Mater 2000;32:277–301], Gao et al. [Gao YX, Yi JZ, Lee PD, Lindley TC. A micro-cell model of the effect of microstructure and defects on fatigue resistance in cast aluminum alloys. Acta Mater 2004;52:5435–49], and Horstemeyer et al. [Horstemeyer MF, Ramaswamy S, Negrete M. Using a micromechanical finite element parametric study to motivate a phenomenological macroscale model for void/crack nucleation in aluminum with a hard second phase. Mech Mater 2003;35:675–87], a micro-cell finite element model was developed to study the effect of six important casting features on debonding and fracture of the Mg₁₇Al₁₂ particles in a cast AM60B magnesium alloy under high cycle fatigue condition. The constitutive behaviour of the AM60B alloy and the Mg-2.0～2.5%Al matrix was simulated by employing the Ohno–Wang multilinear kinematic hardening model. The elastic constants of Mg₁₇Al₁₂ particles were determined using conventional ultrasonic measurement technique. To verify the relative influence of each cast feature, a two level design of experiment (DOE) approach was used.     

Grain refinement and wear properties evaluation of aluminum alloy 2014 matrix-TiB2 in-situ composites

A salt base exothermic reaction process has been employed to produce aluminum alloy 2014 matrix-TiB₂ composites using an exothermic reaction process at 850 °C using K2TiF6 and KBF₄ salts. The period of exothermic reaction was varied from a minimum of 15 min to a maximum of 45 min to investigate the relationship between the degree of reaction and the growth behavior of TiB₂ formed. These have been compared with commercially available aluminum alloy 2014 material. Structural and wear properties have been measured. These show that TiB₂ is extremely effective in enhancing wear properties in addition to significantly reducing the coefficient of friction. The microstructure and phase composition of the materials obtained were studied using X-ray diffraction and scanning electron microscopy (SEM). Very fine ceramic particles were obtained in the aluminum alloy matrix.