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.     

Influence of melt treatments and turning inserts on cutting force and surface integrity in turning of Al–7Si and Al–7Si–2.5Cu cast alloys

The microstructures, machinability and surface characteristics 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 machinability and surface characteristics in the cast condition compared with the same alloy subjected to only grain refinement or modification. Performances of the turning inserts (Un-coated, PVD and Polished CVD diamond coated) were evaluated in machining Al–7Si and Al–7Si–2.5Cu cast alloys under dry environment using a lathe. The Polished CVD diamond coated insert outperformed the Un-coated or PVD-coated cutting inserts which suffered from sizeable edge buildup leading to higher cutting force and poor surface finish. The Polished CVD diamond coated insert shows a very small steady wear without flaking of the diamond film during cutting. This paper attempts to investigate the influence of grain refinement, modification and combined action of both on the microstrutural changes in the Al–7Si and Al–7Si–2.5Cu cast alloys and their machinability and surface finish when different turning inserts used.     

Yield stress of SiC reinforced aluminum alloy composites

This article develops a constitutive model for the yield stress of SiC reinforced aluminum alloy composites based on the modified shear lag model, Eshelby’s equivalent inclusion approach, and Weibull statistics. The SiC particle debonding and cracking during deformation have been incorporated into the model. It has been shown that the yield stress of the composites increases as the volume fraction and aspect ratio of the SiC particles increase, while it decreases as the size of the SiC particles increases. Four types of aluminum alloys, including pure aluminum, Al–Mg–Si alloy, Al–Cu–Mg alloy, and Al–Zn–Mg alloy, have been chosen as the matrix materials to verify the model accuracy. The comparisons between the model predictions and the experimental counterparts indicate that the present model predictions agree much better with the experimental data than the traditional modified shear lag model predictions. The present model indicates that particle failure has important effect on the yield stress of the SiC reinforced aluminum alloy composites.     

Effect of secondary processing on the microstructure and wear behavior of spray formed Al–30Mg2Si–2Cu alloy

In the present work, Al–30Mg₂Si–2Cu alloy has been spray formed and subsequently hot pressed for densification. The alloy is then subjected to solutionizing and isothermal aging treatments. The microstructural features, hardness and wear behavior of spray formed and secondary processed alloys have been evaluated individually and compared with that of as-cast alloy. The microstructure of spray formed alloy showed refined and globular shaped primary Mg₂Si intermetallic particles and Al₂Cu precipitate particles uniformly distributed in Al matrix. The microstructure was refined further after hot consolidation. The microstructure after solution heat treatment appeared similar to that of the spray formed alloy but aging led to a further refinement in the microstructure compared to that of the hot pressed alloy. The evaluation of wear behavior of these alloys, under dry sliding condition, showed that the age hardened alloy exhibits maximum wear resistance and minimum coefficient of friction over the entire range of applied load (10–50 N) at a sliding speed of 2 ms⁻¹ followed by hot pressed, spray formed and solution heat treated alloys. The as-cast alloy showed the least wear resistance and highest coefficient of friction. Similar trend has been observed even in their hardness values too. The wear resistance of the alloys is discussed in light of their microstructural modifications induced during spray forming and subsequent secondary processing and also the topography of worn surfaces.     

Mechanical properties of rapidly solidified ribbons of some AI–Si based alloys

Continuous uniform ribbons of Al–16 Si, Al–12.5 Si–1 Ni and Al–12.5 Si–1 Mg were prepared by melt spinning. Microhardness was measured. The as-melt spun values were 1280, 1370 and 1500 MN m-2 which relax on thermal ageing to 700, 700 and 800 MN m-2 for Al–16 Si, Al–Si–Ni and Al–Si–Mg, respectively. The hardness values of the melt spun ribbons are higher than the as-cast rods from which the ribbons were produced by a factor ranging from 1.8–2.2 times. Tensile testing at room temperature shows that the load–elongation curves are linear with a change of slope occurring in some of the specimens. These curves also show serrations in the case of as-melt spun and the intermediately annealed Al–Si specimens, while no serration was observed in the fully annealed samples. No serration was observed in the Al–Si–Ni and Al–Si–Mg alloys. UTS values were 420, 270 and 100 MN m-2 for Al–16 Si, Al–Si–Ni and Al–Si–Mg, respectively. These values show that the rapid solidification process improved the tensile properties significantly in Al–16 Si and Al–Si–Ni alloys while no significant improvement can be detected for Al–Si–Mg alloy. A discussion is given on hardness relaxation and tensile testing results in terms of silicon precipitation. This revised version was published online in November 2006 with corrections to the Cover Date.     

Wear behavior of Al–Cu and Al–Cu/SiC components produced by powder metallurgy

In the present study, the dry sliding wear behavior of some powder metallurgy (P/M) Al–Mg–Cu alloys with different weight percentage of Cu (0, 1, 2, 3, 4, and 5 wt%) and corresponding metal matrix composites reinforced with 5 or 10 vol% silicon carbide particles (SiC) have been carried using pin-on-disk apparatus. The tested specimens were tested against hardened steel disk as a counter face at room conditions (∼20 °C and ∼50% relative humidity). The normal load was 40 N and sliding velocity of counter face disk was 150 rpm (0.393 m/s) and total testing time of 60 min, which corresponds to a distance of 1414 m. Generally, both hardness and wear resistance were enhanced by the addition of Cu and/or SiC to the Al-4 wt% Mg alloy. The formations of mechanically mixed layer (MML) as a result of material transfer from counter face disk to the samples and vice versa were observed in all tested specimens.     

Developing aluminium–zinc-based a new alloy for tribological applications

In order to develop aluminium–zinc-based a new alloy for tribological applications, six binary Al–Zn and seven ternary Al–25Zn–(1–5)Cu were prepared by permanent mould casting. Their microstructure and mechanical properties were investigated. Dry sliding friction and wear properties of the ternary alloys were investigated using a pin-on-disc machine. Surface and subsurface regions of the wear samples were studied with scanning electron microscopy (SEM). The highest hardness and tensile strength were obtained with the Al–25Zn alloy among the binary ones. The microstructure of this alloy consisted of aluminium-rich α and eutectoid α + η phases. Addition of copper to this alloy resulted in the formation of θ (CuAl₂) phase. The hardness of the ternary alloys increased with increasing copper content. The highest tensile and compressive strengths and wear resistance and the lowest friction coefficient were obtained from the ternary Al–25Zn–3Cu alloy. The dimensional change measured on ageing (stabilization) of this alloy was found to be much lower than that obtained from the copper containing zinc-based alloys. Microstructural changes were observed below the surface of the wear samples of the Al–25Zn–3Cu alloy. These changes were related to the heavy deformation of the surface material due to normal and frictional forces, and smearing and oxidation of wear material. Adhesion was found to be the main wear mechanism for the alloys tested.     

Refinement of primary Si and modification of eutectic Si for enhanced ductility of hypereutectic Al–20Si–4.5Cu alloy with addition of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoparticles

It is very difficult to simultaneously refine and modify Si particles in hypereutectic Al–Si–Cu alloys to enhance their ductility. This study investigates how nanoparticles affect Si particles during solidification in hypereutectic Al–Si–Cu alloys. 0.5 wt% γ-Al₂O₃ nanoparticles were added in hypereutectic Al–20Si–4.5Cu alloy melt and further dispersed through an ultrasonic-cavitation-based technique. The as-cast Al–20Si–4.5Cu–Al₂O₃ nanocomposites showed marked enhancements in both ductility and strength. The ductility of Al₂O₃ nanocomposite was more than two times higher than that of the monolithic alloy without the nanoparticles. Microstructural analysis with optical and scanning electron microscopy revealed that both the primary and eutectic Si particles were significantly refined. The primary Si particles were refined from star shapes to polygon or blocky shapes, and their edges and corners were much smoother. The large plate eutectic Si particles were also modified into the fine coralline-like ones. The porosity of alloy was also reduced with the addition of γ-Al₂O₃ nanoparticles. Study suggests that γ-Al₂O₃ nanoparticles simultaneously refine and modify Si particles as well as reduce porosity in cast Al–20Si–4.5Cu, resulting in unusual ductility enhancement that could have great potential for numerous applications.     

Effects of processing parameters on the refinement of primary Si in A390 alloys with a new Al–Si–P master alloy

In this study, a new Al–17Si–2.5P master alloy has been successfully prepared to refine primary Si in hypereutectic A390 alloys. By means of electron probe microanalyzer (EPMA), a large number of AlP particles can be found in the Al–17Si–2.5P master alloy. An orthogonal L₉(3³) test was designed to investigate the integrated effects of refining factors including phosphorus addition level, melting temperature and holding time, and subsequently to optimize the processing parameters. It is found that under the optimized conditions, i.e., phosphorus addition of 375 ppm, melting temperature of 800 °C, and holding time of 30 min, the average sizes of primary Si can be most remarkably decreased from 116 μm to 14 μm with sphere-like morphology. Meanwhile, the Brinell hardness and tensile strength can be significantly increased by 14.1 and 27.8%, respectively. In addition, thermal analysis is also performed with differential scanning calorimeter (DSC) to analyze the solidification process of Al–18Si alloys.     

Influence of additions of Zr, Ti–B, Sr, and Si as well as of mold temperature on the hot-tearing susceptibility of an experimental Al–2% Cu–1%?Si alloy

This study was undertaken to investigate the effects of chemical composition and mold temperature (MT) on the hot-tearing susceptibility (HTS) of an experimental Al–2% Cu–1% Si alloy using a constrained rod casting mold. The HTS results were then compared with 206 (Al–5 wt% Cu) alloys containing the same additions. In general, the Al–2% Cu–1% Si based alloys exhibited higher resistance to hot-tearing than did the 206-based alloys. It was found that an elevated MT is beneficial in reducing the HTS of the Al–2% Cu–1% Si and 206 alloys in that the HTS value decreased from over 21 to less than 5, as the MT was increased from 250 to 450 °C. Increasing the Si content reduced the HTS of the Al–2% Cu–1% Si alloy considerably; this reduction may be attributed to an increase in the volume fraction of eutectic in the structure. The addition of Sr caused deterioration in the hot-tearing resistance of the base alloy due to the formation of Sr-oxides and an extension of the freezing range of the alloy. The refinement of the grain structure obtained with the Zr–Ti–B addition decreased the severity of hot-tearing as a result of an increase in the number of intergranular liquid films per unit volume and a delay in reaching the coherency point. It was also observed that α-Fe intermetallic particles may impede the propagation of hot-tearing cracks. The Al–2% Cu–1% Si alloy with 1 wt% Si addition was judged to be the best composition in view of its low HTS.     

Microstructure modification of Al–17%Si alloy by addition of Mg

The calculated phase diagrams of Al–17%Si alloy with additions of Mg up to 10 wt% Mg have indicated two critical compositions at 4.6 and 6.8% Mg where the liquidus, the binary reaction temperatures as well as the temperature range of the formation of Mg₂Si particles are changed. The eutectic formation temperature is decreased with the addition of Mg up to 4.6% Mg, followed by constant value due to the change of the eutectic formation reaction from the binary (Liq. → Al ± Si) to the ternary (Liq. → Al + Si + Mg₂Si) reaction. This change contributes to the transformation of the eutectic silicon in matrix from long platelet, to fine and compact particles resulting in overall increased hardness of the high Mg alloys even though the hardness of the primary Mg₂Si particles is much smaller than that of the primary silicon particles. A pronounced decrease of both primary and eutectic silicon particles size was also observed for 0.4% Mg alloy when compared to the base alloy showing significant microstructural modification.     

The wear of aluminium-based journal bearing materials under lubrication

The aluminium-based alloys, nowadays, are developed to be used in high performance engine bearings. In this study, new Al-based bearing alloys, which are produced by metal mould casting, were developed; and tribologic properties of these alloys under lubrication were analyzed experimentally. Four different aluminium alloys were carried out on pin on disc wear tester for that purpose. SAE 1040 steel was used as the disc material in the wear tester. Friction tests were carried out at 0.231–1.036 N/mm² pressures and at 0.6–2.4 m/s sliding speeds. Wear tests were carried out at 1.8 m/s sliding speed and at 70 N normal load. Friction coefficients and weight losses of the samples were determined under various working conditions as a result of the experiments. The morphographies of the worn surfaces were analyzed. Hardness, surface roughness, and surface temperature of the samples were measured. The results showed that the friction and wear behaviors of the alloys have changed according to the sliding conditions. The effects of the elements except aluminium composing alloys on the tribologic properties were analyzed. Al8.5Si3.5Cu alloy has a lower friction coefficient value than other alloys. Al8.5Si3.5Cu and Al15Sn5Cu3Si alloys, on the other hand, have the highest wear resistance. Al15Pb3.7Cu1.5Si1.1Fe alloy is the most worn material; and Al15Pb3.7Cu1.5Si1.1Fe alloy has the highest wear rate. As a result of the evaluations conducted, Al–Sn and Al–Si alloys, which include Si and Sn, can be preferred, among the aluminium alloys that will work under lubrication, as the bearing material.     

Effects of modification and heat-treatment on the abrasive wear behavior of hypereutectic Al–Si alloys

In the present study, Al–Si alloys with Si contents of 23, 26, 28 and 31 wt.%, respectively, were modified with a new modifying agent. The results show that the primary silicon size decreased about 8–10 times after modification. The wear rates of the modified and heat-treated Al–Si alloys are lower than those of the unmodified and non-heat-treated Al–Si alloys, respectively. The silicon content in the range of 23–31 wt.% has a significant effect on the wear rates of the same processed Al–Si alloys (modification and heat treatment). Under the same load, the wear rates of the same processed Al–Si alloys decreased with the increasing silicon content. The abrasion took place mainly by cutting and partly by ploughing actions for the non-heat-treated Al–Si alloys, or on the contrary, mainly by ploughing and partly by cutting actions for heat-treated Al–Si alloys.     

Impact toughness in Al–12Si and Al–12Si–3Cu cast alloys—Part 1: Effect of process variables and microstructure

The charpy impact energy of Al–12Si and Al–12Si–3Cu cast alloys was measured in terms of the total absorbed energy. The standard charpy specimens 10×10×55 mm with a 2 mm V-notch were prepared from the castings. Effect of process variables and microstructural changes on the impact toughness of Al–12Si and Al–12Si–3Cu cast alloys was investigated. The results indicate that combined grain refined and modified Al–12Si–3Cu cast alloys have microstructures consisting of uniformly distributed α-Al dendrites, eutectic Al–Si and fine CuAl₂ particles in the interdendritic region. These alloys exhibited better impact toughness in the cast condition compared with the same alloy subjected to only grain refinement or modification.     

Oxidation behavior of a refractory NbCrMo0.5Ta0.5TiZr alloy

Isothermal oxidation behavior of a refractory high-entropy NbCrMo_0.5Ta_0.5TiZr alloy was studied during heating at 1273 K for 100 h in flowing air. Continuous weight gain occurred during oxidation, and the time dependence of the weight gain per unit surface area was described by a parabolic dependence with the time exponent n = 0.6. X-ray diffraction and scanning electron microscopy accompanied by energy-dispersive X-ray spectroscopy showed that the continuous oxide scale was made of complex oxides and only local (on the submicron levels) redistribution of the alloying elements occurred during oxidation. The alloy has a better combination of mechanical properties and oxidation resistance than commercial Nb alloys and earlier reported developmental Nb–Si–Al–Ti and Nb–Si–Mo alloys.     

Wear characteristics of a new type austenitic stainless steel

In order to solve the problem of the poor wear resistance in conventional austenitic stainless steels, a new type austenitic stainless steel was designed based on Fe–Mn–Si–Cr–Ni shape memory alloys in this article. Studies on its wear resistance and wear mechanism have been carried out by comparison with that of AISI 321 stainless steel using friction wear tests, X-ray diffraction, scanning electron microscope. Results showed that the wear resistance of Fe–14Mn–5.5Si–12Cr–5Ni–0.10C alloy was better than that of AISI 321 stainless steel both in dry and oily friction conditions owing to the occurrence of the stress-induced γ → ε martensitic phase transformation during friction process. This article also compared the corrosion performance of the two stainless steels by testing the corrosion rate. Results showed that the corrosion rate of Fe–14Mn–5.5Si–12Cr–5Ni–0.10C alloy was notably lower in NaOH solution and higher in NaCl solution than that of AISI 321 stainless steel.     

The effect of humidity on friction and wear of an aluminium–silicon eutectic alloy

The sliding wear of an aluminium–silicon eutectic alloy against cast iron counterface in 3–100% relative humidity range has been investigated. The results show that the moisture content has a significant effect on the friction and wear of the Al–Si alloy. The wear rate decreases by two orders of magnitude as the relative humidity increases from 3% to 100%. At low humidity conditions adhesive wear is predominant, whilst at high humidity conditions a layer of compacted oxide–metal debris film is formed on the Al–Si slider surface, which reduces the direct metal–metal contact. The friction coefficient is maximum at 3% and 100% relative humidity conditions. This revised version was published online in November 2006 with corrections to the Cover Date.     

Mechanism of synthesizing dense Si–SiC matrix C/C composites

The following technique is known to synthesize C/C (carbon fiber-reinforced carbon) composites. The organic matter in the preformed yarn (plastic straw covered yarn including bundles of long carbon fibers, carbon powder, and organic binder) is pyrolyzed at 500 °C and concurrently hot-pressed. Then, the carbon ingredient is graphitized in an atmosphere of nitrogen at 2000 °C. The authors used the above mentioned C/C composites as a starting material and developed a dense Si–SiC matrix C/C composites in which most long carbon fibers remain without reacting with Si which is infiltrated in argon at 1600 °C and 100 Pa. As a result, production of 1 × 2 m large size plates free from warps and cracks was attained in NGK Insulators, Ltd. This mechanism consists of three steps. First, a trunk-shaped Si–SiC matrix is synthesized between yarn and yarn. Then a trunk-shaped Si–SiC matrix extends a yarn by force. Only differential gap is made in a yarn surface. Finally, branch-shaped Si–SiC matrix is synthesized so that a trunk-shaped Si–SiC matrix leads to the yarn inside.     

Effects of complex modificating technique on microstructure and mechanical properties of hypereutectic Al–Si alloys

In this article, the structure of Al-18Si alloy was modified by thermal-rate treatment technique at 930 °C based on the DSC result. The mechanical properties of Al–18Si alloy were improved remarkable by a complex technique with alloying and thermal-rate treatment. A new treating technique named as complex modificating technique was proposed, and the performance of this technique on Al–18Si–1.5Cu–0.6Mg alloy was investigated. The results show that primary Si can be refined when Al–P master alloy was added into the melt at 770 °C after thermal-rate treatment. Compared with the conventional casting technique by which the melt of alloy was unmodified, better refinement effect can be obtained with the combination of alloying and complex modificating technique: the size of primary Si is decreased from 66 to 16 μm, the tensile strength increased by 75.94% and the brinell hardness by 66.59%. Moreover, the mechanism of the complex modificating technique was also discussed.     

Deformation behavior of spray-formed hypereutectic Al–Si alloys

The deformation behavior of spray-formed hypereutectic aluminum–silicon alloys—AlSix (x = 18, 25, and 35 wt%)—has been studied by means of compression test at various temperatures and strain rates. The flow stress of the spray-formed Al–Si alloys increases with decreasing compression temperature and increasing strain rate. Higher silicon content in the alloys also leads to higher flow stress during deformation. The flow curves determined from the compression tests exhibit that the deformation of the materials is controlled by two competing mechanisms: strain hardening, and flow softening. Particle damage during the deformation may have an influence on the flow curves of the alloys with large silicon particles. Based on the flow curves obtained from the compression tests and knowledge of aluminum extrusion, the spray-formed hypereutectic Al–Si alloy billets have been hot extruded into wires with a high area reduction ratio around 189. Since primary silicon particles were greatly refined and uniformly distributed in the spray-formed materials, the heavy deformations of the spray-formed Al–Si alloys containing high amount of silicon were successfully performed.