Tribological behaviour of alternate hypereutectic Al–Si alloys with different antiwear additives

Abstract

Two alternate hypereutectic Al–Si alloys, varying in composition and processing route, were investigated alongside their commercial equivalents, with two different lubricant antiwear chemistries. A reciprocating sliding wear test was used with a steel nitrided piston ring on substrate configuration. The conditions (average load and speed) replicated those seen in a fired gasoline engine. The oil containing zinc dialky dithiophosphate (ZDDP) antiwear gave the best antiwear protection, determined from the lower specific wear rate and friction coefficient with all alloy types. A zinc–phosphorus–sulphur based protective film was found in the wear track. The poor performance of a zinc free active phosphate ester (ashless antiwear additive) was thought to be due to no phosphorus based film forming. The presence of a calcium based film, originating from the detergent, suggests that it had a higher film forming affinity in the competitive surface interactions. The commercially termed Alusil (A390, Al–18·5Si alloy) is currently used as cylinder material in various gasoline engines. This reference material had a different microstructure to that produced by rheo-die casting. The high shear rate during the rheo-die casting process produced a finer microstructure with smaller but more numerous primary and secondary silicon particles. The increased surface area was envisaged to increase the wear resistance and evenly distribute the applied load. However, the reference Alusil gave considerably better wear performance with all oil types tested. The wear track microstructures showed material loss, void formation with fracture, fragmentation and sinking of the primary silicon particles into the aluminium matrix. The damage was controlled to an extent by use of the ZDDP oil with an organic friction modifier. An uneven wear pattern termed stick slip was occasionally observed. It was found not to be dependent on substrate alloy or oil type. The only differences were greater fluctuation in friction coefficient during the running-in period, higher overall wear track hardness and striated lines on a considerable number of fractured primary silicon particles.     

Chemical characterization and nanomechanical properties of antiwear films fabricated from ZDDP on a near hypereutectic Al–Si alloy

X-ray absorption near edge structure (XANES) spectroscopy was used to monitor the products formed during the breakdown of the engine additive ZDDP during its action as a protective tribochemical agent. This investigation examines the film formed under various physical conditions on a near hypereutectic Al–Si alloy. For the first time, tribochemical films (tribofilms) formed on a high silicon weight content alloy, with virtually no ferrous component have been studied. Phosphorus K- and L-edge spectroscopies show that under typical engine operating conditions, the tribofilms have similar chemical composition over a range of different test conditions. X-ray photoelectron emission microscopy (X-PEEM) reveals that the polyphosphate glasses formed vary in chain length within localized regions. The mechanical properties of the substrate and the tribofilms were acquired using a Triboscope^® from Hysitron Inc. The elastic moduli can be extracted from the indentation curves and show that the tribofilms mechanical properties are similar to those of the tribofilms which form on steel under similar conditions.     

Influence of the microstructure of Al–Si alloy on the surface-layer quality in microarc oxidation

The properties of the hardened surface layers formed by microarc oxidation on Al–Si alloy billet are investigated. The microhardness, thickness, and porosity of the surfaces formed depend on the alloy structure.     

Microstructure and properties of Fe–Al intermetallic coatings on the low carbon steel synthesized by mechanical alloying

Fe–Al coating was obtained on low carbon steel substrates using mechanical alloying and subsequent heat treatment. Light optical microscopy and a scanning electron microscope equipped with energy dispersive spectroscopy were used to conduct the microstructure characterization. Mechanical properties of the coatings were evaluated by microhardness measurements and wear tests. The corrosion behavior was determined by potentiodynamic polarization measurements in 3.5 % NaCl solution. The results of the mechanical and corrosion tests showed that the hardness and wear resistance of the coatings decreased with increasing the milling time, while increase in the milling time resulted in a significant increase in the thickness, porosity level, and corrosion resistance.     

Exploring the hybrid metal extrusion and bonding process for butt welding of Al–Mg–Si alloys

The hybrid metal extrusion and bonding (HYB) process is a new solid-state joining technique developed for aluminum alloys. By the use of filler material addition and plastic deformation sound joints can be produced at operating temperatures below 400 °C. The HYB process has the potential to compete with commonplace welding technologies, but its comparative advantages have not yet been fully explored. Here, we present for the first time the results from an exploratory investigation of the mechanical integrity of a 4-mm AA6082-T6 HYB joint, covering both hardness, tensile and Charpy V-notch testing. The joint is found to be free from defects like pores, internal cavities and kissing bonds, yet a soft heat-affected zone (HAZ) is still present. The joint yield strength is 54% of that of the base material, while the corresponding joint efficiency is 66%. The indications are that the HYB process may compete or even outperform conventional welding techniques for aluminum in the future after it has been fully developed and optimized.     

Structure and microstructure evolution of Al–Mg–Si alloy processed by equal-channel angular pressing

An ultrafine grained Al–Mg–Si alloy was prepared by severe plastic deformation using the equal-channel angular pressing (ECAP) method. Samples were ECAPed through a die with an inner angle of Φ = 90° and outer arc of curvature of ψ = 37° from 1 to 12 ECAP passes at room temperature following route B_c. To analyze the evolution of the microstructure at increasing ECAP passes, X-ray diffraction and electron backscatter diffraction analyses were carried out. The results revealed two distinct processing regimes, namely (i) from 1 to 5 passes, the microstructure evolved from elongated grains and sub-grains to a rather equiaxed array of ultrafine grains and (ii) from 5 to 12 passes where no change in the morphology and average grain size was noticed. In the overall behavior, the boundary misorientation angle and the fraction of high-angle boundaries increase rapidly up to 5 passes and at a lower rate from 5 to 12 passes. The crystallite size decreased down to about 45 nm with the increase in deformation. The influence of deformation on precipitate evolution in the Al–Mg–Si alloy was also studied by differential scanning calorimetry. A significant decrease in the peak temperature associated to the 50% of recrystallization was observed at increasing ECAP passes.     

Nanoscale chemistry and mechanical properties of tribofilms on Al–Si alloy (A383): interaction of ZDDP,calcium detergent and molybdenum friction modifier

Abstract

The interaction of a friction modifier and a calcium phenate detergent additive, with zinc dialkyl dithiophosphates (ZDDPs) in the formation of antiwear films on A383, has been studied using synchrotron radiation and nanoindentation techniques. X-ray absorption near edge structure (XANES) spectroscopy has shown that films prepared from oils containing both ZDDP and detergent, and ZDDP and molybdenum dithiocarbamate (MoDTC), are chemically similar to, but thicker than those made from oils containing only ZDDP. In addition, wear was greatly reduced in the presence of the detergent which was correlated with the basicity and the presence of the friction modifier. The phosphorus K and L edge XANES spectra show that the tribofilms are polyphosphate glasses of similar nature to those found on steel, but characterised by a shorter chain length. The sulphur K edge shows a MoS₂ like film and under certain conditions, the presence of a sulphate species is detected. High resolution topographic images and mechanical properties were determined by atomic force microscopy and imaging nanoindentation. The films formed in the presence of the detergent exhibited similar mechanical responses independent of the conditions tested. The indentation modulus of the films on the Al matrix always appear much softer than the films formed on the Si grains whether or not the lubricant contains only ZDDP, or both ZDDP and MoDTC.     

Effects of Silicon Content on the Mechanical Properties and Lubricated Wear Behaviour of Al–40Zn–3Cu–(0–5)Si Alloys

In this work, one ternary Al–40Zn–3Cu and seven quaternary Al–40Zn–3Cu–(0.25–5)Si alloys were synthesized by permanent mould casting. Their microstructure, mechanical and lubricated wear properties were investigated using appropriate test apparatus and techniques. As the silicon content increased the hardness of the alloys increased, but their elongation to fracture decreased. Tensile strength of the alloys decreased with increasing silicon content following a sharp decrease and a slight increase. Among the silicon-containing quaternary alloys the highest and the lowest tensile strength values (348 and 305 MPa) were obtained with the Al–40Zn–3Cu–2Si and Al–40Zn–3Cu–5Si alloys, respectively, while the base alloy (Al–40Zn–3Cu) exhibited a tensile strength of 390 MPa. However, the volume loss due to wear of the alloys increased with increasing silicon content after showing an initial increase and a sharp decrease. The lowest wear loss was obtained with the alloy containing approximately 2% Si which has the highest tensile strength among the quaternary alloys containing more than 0.25% Si. Wear surfaces of the alloys were characterized mainly by smearing indicating that adhesion is the dominant wear mechanism for the experimental alloys.     

Adhesive wear of stir cast hypereutectic Al–Si–Mg alloy under reciprocating sliding conditions

This paper describes an attempt to enhance the wear properties of hypereutectic cast aluminium–silicon alloys produced by semi-solid metal (SSM) processing technique. The rheological experiments on SSM slurries were performed under continuous cooling condition from liquidus temperature. Wear characteristics of alloy under investigation were studied using pin on flat wear system over a range of normal load (10–40 N) at constant average sliding speed (0.2 m/s) against cast iron and stainless steel counter surface. Stir cast alloy showed lesser weight loss compared to conventional cast alloy. Stir cast and conventional cast alloys showed higher weight loss against the stainless steel as compared to that against cast iron counter surface. Optical microscopy of the conventional cast and stir cast hypereutectic alloy has shown that stir casting causes refinement of primary silicon particles and modification of eutectic silicon compared to conventional cast alloy. The scanning electron microscopy of wear surfaces was carried out to investigate the mode of wear.     

Effects of strain induced martensitic transformation on the cavitation erosion resistance and incubation time of Fe–Cr–Ni–C alloys

The effect of a strain-induced martensitic transformation on the cavitation erosion resistance and incubation time of Fe–10Cr–10Ni–xC (x = 0.3, 0.4, 0.5, and 0.6 wt%) austenitic steels has been studied. As the carbon concentration increased, mass loss in the alloys also increased, while the incubation period and the amount of transformed martensite decreased. In addition, the martensite volume fraction increased with increasing testing time and reached a saturation point for each test alloy. After the saturation point was reached, the martensite volume fraction did not change throughout the remainder of the test, even though the transformed martensite phase was removed. This result indicates that new martensite phases were formed immediately after the removal of the previously formed martensite. Martensitic transformation exerts significant effects on wear resistance and incubation time by steadily absorbing the cavity collapse energy.     

Effect of matrix/reinforcement particle size ratio (PSR) on the mechanical properties of extruded Al–SiC composites

This paper studied the combined effects of matrix-to-reinforcement particle size ratio (PSR) and SiC volume fraction on the mechanical properties of extruded Al–SiC composites. A powder metallurgy technique (PM) of cold pressing at 500 MPa followed by hot extrusion at 580 °C was adopted to produce Al/SiC composite. Aluminum powder of size 60 μm and silicon carbide with different sizes, i.e., 50, 20, and 8 μm, were used. Three different volume fractions of SiC were employed, i.e., 5, 10, and 15 %, for each investigated size using a constant extrusion ratio of 14.36. The effect of matrix-to-reinforcement PSR on the reinforcement spatial distribution, fabricability, and resulting mechanical properties of a PM-processed Al/SiC composite were investigated. It has been shown that small ratio between matrix to reinforcement particle size resulted in more uniform distribution of the SiC particles in the matrix. As the PSR increases, the agglomerations and voids increase and the reinforcement particulates seem to have nonuniform distribution. In addition, the agglomerations increased as the volume fraction of the SiC increased. It has also been shown that homogenous distribution of the SiC particles resulted in higher yield strength, ultimate tensile strength, and elongation. Yield strength and ultimate tensile strength of the composite reinforced by PSR (1.2) are higher than those of composite reinforced by PSR (7.5), while the elongation shows opposite trend with yield strength and ultimate tensile strength.     

Comparative assessment of the mechanical and electromagnetic surfaces of explosively clad Ti–steel plates after drilling process

The present work pertains to the analysis of surface topography of explosively clad material such as titanium plated steel in drilling process. The study was conducted for different types of indexable insert drills with different configuration of the tool coatings and for WC-Co drill tool. In this context, surface topography of the drilled holes especially in the region of contact area was analyzed. Metrological analysis was performed using stylus-based and optical profilometry. In this paper the differences between mechanically and electromagnetically measured surfaces are highlighted. It has been observed that the parameters of the surface topography are dependent upon the type of layers of the clad and the type of drill.     

Effects of cold working and heat treatment on microstructure and wear behaviour of Cu–Be alloy C17200

The effects of cold work process between aging and solution heat treatment on the microstructure, hardness and the tribologic behaviour of a copper–beryllium (Cu–Be) alloy C17200 were investigated. The wear behaviour of the alloys was studied using ‘pin on disc’ method under dry conditions. The results show that the formation of fine grained structure and γ phase particles enhances the mechanical properties of the alloy; nonetheless, they do not reduce the wear rate. This is attributed to the capability of the softer specimens to maintain oxygen rich compounds during the dry sliding test.     

A comparative study on the wear properties of coarse-grained Al6061 alloy and nanostructured Al6061–Al2O3 composites

Wear behavior of nanostructured Al6061 alloy and Al6061–Al₂O₃ nanocomposites produced by milling and hot consolidation were investigated. The samples were characterized by hardness test, pin-on-disk wear test, X-ray diffraction (XRD), and scanning electron microscopy (SEM). Nanocomposites containing 3 vol% Al₂O₃ showed a maximum hardness of 235 HV and optimum wear rate of 4×10⁻³ mg/m. Increasing the amount of Al₂O₃ up to 5 vol% resulted in decrease in hardness values (∼112 HV) and a sharp rise in wear rate (∼18×10⁻³ mg/m).     

Modeling and analysis of the effects of processing parameters on the performance characteristics in the high pressure die casting process of Al–SI alloys

The high pressure die casting (HPDC) process has achieved remarkable success in the manufacture of aluminum–silicon (Al–SI) alloy components for the modern metal industry. Mathematical models are proposed for the modeling and analysis of the effects of machining parameters on the performance characteristics in the HPDC process of Al–SI alloys which are developed using the response surface methodology (RSM) to explain the influences of three processing parameters (die temperature, injection pressure and cooling time) on the performance characteristics of the mean particle size (MPS) of primary silicon and material hardness (HBN) value. The experiment plan adopts the centered central composite design (CCD). The separable influence of individual machining parameters and the interaction between these parameters are also investigated by using analysis of variance (ANOVA). With the experimental values up to a 95% confidence interval, it is fairly well for the experimental results to present the mathematical models of both the mean particle size of primary silicon and its hardness value. Two main significant factors involved in the mean particle size of primary silicon are the die temperature and the cooling time. The injection pressure and die temperature also have statistically significant effect on microstructure and hardness.     

Effects of sliding velocity on tribo-oxides and wear behavior of Ti–6Al–4V alloy

Dry sliding wear tests were performed for Ti–6Al–4V alloy on a pin-on-disc wear tester. The wear behavior of Ti–6Al–4V alloy at sliding velocities of 0.5–4 m/s was studied and the tribo-oxides and their function were explored. Ti–6Al–4V alloy presented a marked variation of wear rate as a function of velocity. With the rise and fall of wear rate, Ti–6Al–4V alloy underwent the transitions of wear mechanisms from the combination of delamination wear and oxidative wear at lower speeds to delamination wear at 2.68 m/s, and then to oxidative wear at 4 m/s. These phenomena were attributed to the appearance and disappearance of tribo-oxides. In spite of trace or a small amount, tribo-oxides would change the wear behavior, and even wear mechanism.     

Effect of silicon content on wear resistance and corrosion behaviour of Al–Si eutectic alloy

Abstract

In the present study, Al–Si eutectic alloys produced at the Aluminium Institution were studied. The alloys were cast and forged into bars of 20 mm diameter. The results obtained were compared with a pure aluminium sample. Metallographic analysis, spectral analysis, SEM and energy dispersive spectroscopy analysis, pin on disk abrasive wear tests and mass loss tests were performed. Wear resistances of alloys with various silicon contents were tested under different loads and constant abrasive speed. SiC abrasive paper of 80 grit size was used. The dry sliding tests were carried out under loads of 10, 20 and 30 N and the testing was conducted under a constant sliding velocity of 36˙8 m min^–1 in a dry air atmosphere. Corrosion rates were determined in 0˙1M NaCl acid solutions. The corrosion tests were performed at 2, 4, 6 and 8 h. The wear and corrosion resistance of all the eutectic alloys increased with increasing silicon content in the matrix.     

Effects of speed sequence on friction properties of sintered Cu–SiO2

Abstract

The effects of the speed sequence and SiO₂ content of Cu–SiO₂, sintered by powder metallurgy method, on friction and wear properties have been investigated at fixed speeds. The results indicate that the sequence of speeds employed in the tests plays great roles in the friction and wear properties. When the tests are executed from a lower speed to a higher speed, friction coefficients decrease and oscillate dramatically as the speed goes up, resulting in a severe wear. On the contrary, as the speed starts from a higher value, the friction coefficients are stable and wear is small. These phenomena can be explained by states of third bodies formed in the friction. The third body formed at lower friction speeds is usually granular, which is responsible for the coefficient oscillations and larger wear loss. At higher speeds, the third body formed is rather dense, leading to stable friction coefficients and lower wear loss.     

The effects of nickel and carbon concentrations on the wear resistance of Fe–Ni–C austenitic alloys

The effects of nickel and carbon concentrations on the wear resistance of Fe–xNi–yC (x = 14–20 wt.%, y = 0.6–1.0 wt.%) were investigated with respect to strain energy initiation of the martensitic transformation and hardness. The strain energy needed to initiate the martensitic transformation increased with increasing carbon and nickel concentrations, except in 1.0 wt.% C alloys. The wear resistance of the material decreased with increasing carbon concentration up to 0.9 wt.% C. This effect is most likely due to decrement of the martensite volume fraction with increasing carbon concentration induced by the incremental strain energy required to begin the martensitic transformation. In the case of 1.0 wt.% C, the improved wear resistance may be due to carbide precipitation.