Tribology at high-velocity impact

The tribological events taking place when a high-velocity projectile hits a SiC particulate reinforced AA 5083 composite material was examined under real conditions. The samples were cast in a disc shape by squeeze casting method. Different volume fractions of SiC particles were used. They were solidified under 180 MPa in a steel mould with a 650–700 °C temperature range. SiC particles with the size of 250–500 μm, and 30% and 45% in volume fraction were incorporated into the matrix material. The composites were machined to ensure a smooth surface and to obtain samples without burrs. The samples had a diameter and thickness of 140 and 20 mm, respectively. The terminal ballistic tests were carried out in an army zone under standard test conditions. An AP 7.62 mm armour piercing projectile with a speed of 710 m/s was used for testing the composite.The frictional characteristics and wear mechanisms caused by high-velocity impacts to the composite were determined by SE microscopy studies. The evaluations of the tribological events on both the hole and projectile tip surfaces resulting from high-velocity friction were carried on. As the projectile moved thorough the composite, some material broke from the matrix body and conglomerated along the path followed by the missile. Then these conglomerated blocks yielded and slided along the hole surface. There were also scratching and local melting on the hole surface. Similarly, some ploughing took place on the hole, some SiC particles were removed from the matrix body by the friction effect of projectile and these particles were conglomareted on the tip surface of the projectile. Thus, the nature of wear mechanism on the projectile surface was predominantly abrasive while those of the friction surfaces of the composites were predominantly abrasion and melt wear.     

The effect of billets extruded by a curved and flat-face die on the semisolid characteristics and tensile properties of thixoformed products

In this study, A356 aluminum billets in different extruded states are used as feedstock for the thixoforming. The extrusion billets were fabricated by a hot extrusion process through a flat-face and a curved die. After the induction reheating of the extrusion billets into a semisolid state, the microstructural evolution was thoroughly investigated. For the extrusion alloy by the flat-face die, there was a large variation in the average grain size (20 %) and the mean roundness (17 %) of equiaxed α-Al grains. This, together with evidence of elongated grains in the interior regions of the billet, indicated that a noticeably nonuniform globular microstructure had been obtained. In contrast, for the extrusion alloy through the curved die, the obtained globular microstructure was more uniform. There were slight variations of 5 % and 7 % in the average grain size and the mean roundness, respectively. By using the extrusion billets, some parts fabricated via the thixoforming process those underwent T6 heat treatment. The tensile test results for the fabricated parts showed that when the extrusion billet through the conventional flat-face die was used as the feedstock, there was a large scattering in the tensile properties throughout the part. In contrast, when the extruded billet through the curved die was used as the feedstock, limited variation was observed in the tensile properties.     

A study on radial flow field for extrusion through conical dies

A generalized expression for the radial flow field for extrusion through a conical die is suggested. The upper bound to the extrusion pressure for a rigid-perfectly plastic material is obtained. Other energy methods which include solutions for work-hardening and composite billets are also obtained for the generalized radial flow field. The results are used to analyse the hydrostatic extrusion of Al, Cu and Al-Cu composite billets. The extrusion pressure and hardness distribution of the product were measured in experiments and they are compared with theoretical results.     

Assessment of ZK60A magnesium billets for forging depending on casting methods by upsetting and tomography

The workability of ZK60A billets fabricated by semi-continuous casting, subsequent extrusion, and die casting were evaluated. To determine the deformation capability of the three different billets, upsetting tests were conducted at elevated temperatures and two different strain rates. The differences in critical height reduction depending on the casting methods were investigated based on inherent defects inside the billets, and variation in the mechanical property with the location within the semi-continuously casted billet was investigated by X-ray tomography and electron backscatter diffraction. Uniformity of density substantially affected the mechanical properties of the billet. The drastic decrease in the workability of the die-casted billet at temperatures higher than 320°C was also discussed.     

Investigation into sliding wear performance of zinc-based alloy reinforced with SiC particles in dry and lubricated conditions

The objective of the present investigation was to assess the influence of SiC particle dispersion in the alloy matrix, applied load, and the presence of oil and oil plus graphite lubricants on the wear behaviour of a zinc-based alloy. Sliding wear performance of the zinc-based alloy and its composite containing SiC particles has been investigated in dry and lubricated conditions. Base oil or mixtures of the base oil with different percentages of graphite were used for creating the lubricated conditions. Results show a large improvement in wear resistance of the zinc-based alloy after reinforcement with SiC particles. The lubrication improved the wear resistance and friction behaviour of both the reinforced and base alloys. It was also observed that there exists an optimum concentration of graphite particles in the lubricant mixture that leads to the best wear performance. The composite experienced higher frictional heating and friction coefficient than the matrix alloy in all the cases except oil lubricated conditions; a mixed trend was noticed in the latter case. The wear rate and frictional heating increased with load while friction coefficient was affected in an opposite manner. Test duration influenced the frictional heating and friction coefficient of the samples in a mixed manner.Examination of worn surfaces revealed a change of predominating wear mechanisms from severe ploughing and/or abrasive wear for base alloy to delamination wear for the reinforced material under dry sliding conditions. The presence of the lubricant increased the contribution of adhesive wear component while reducing the severity of abrasion. This was attributed to the generation of more stable lubricant films on the contacting surfaces. Cross-sections of worn surfaces indicated substantial wear-induced plastic deformation, thereby suggesting adhesive wear to be a predominant wear mechanism in this study. The debris particles revealed deformed flakes and machining chips signifying the involvement of adhesion and abrasion modes of wear respectively.     

The use of generalised deformation boundaries for the analysis of axisymmetric extrusion through curved dies

A generalised kinematically admissible velocity field is derived for axisymmetric extrusion through curved dies by employing rigid-plastic boundaries expressed in terms of arbitrarily chosen continuous functions. The corresponding upper-bound extrusion pressure is related directly to boundary functions for the plastically deforming region when the die shape, lubrication condition and material characteristics of the billet are given. The proposed method of analysis makes it possible to predict the deformation pattern as well as extrusion pressure. In computation a third-order polynomial is chosen for the die boundary and the bounding function for the plastic region is chosen to be a fourth-order polynomial. The workhardening effect is considered in the formulation. The plastic boundaries as well as stream lines are affected by various process parameters. The theory predicts the relatively faster axial flow at the center than near the die boundary for greater friction factor even with the same die shape. The effects of area reduction and die length are also discussed in relation to extrusion pressure and deformation. Experiments are carried out for steel billets at room temperature. Deformation patterns are measured for several area reductions by the photoetching technique and the extrusion pressure is measured using a load-cell. The predicted extrusion pressure is in excellent agreement with the value computed by the finite element method. The deformation patterns agree well with the experimental observation.     

Ductile damage micromodeling by particles’ debonding in metal matrix composites

The elastic–plastic behaviour of particle-reinforced metal matrix composites undergoing ductile damage is modelled using a two-level micro-structural approach. The considered heterogeneous material is a polycrystal containing intra-crystalline elastic particles. Ductile damage is initiated by the matrix/particle interface debonding and the subsequent voids growth with plastic straining of the crystalline matrix. Homogenization techniques are used twice: first at mesoscale to derive the equivalent grain behaviour and then to obtain the macroscopic behaviour of the material. Plastic deformation of the crystalline matrix is due to crystallographic gliding on geometrically well-defined slip systems. The associative plastic flow rule and the hardening law are described on the slip system level. The evolution of micro-voids volume fraction is related to the plastic strain. The elastic–plastic stress–strain response of particle composite is investigated. Predictions of the proposed model are compared to experimental data to illustrate the capability of the suggested method to represent material behaviour. Furthermore, specific aspects such as the stress triaxiality and yield surfaces are discussed.     

Erosion–corrosion degradation mechanisms of Fe–Cr–C and WC–Fe–Cr–C PTA overlays in concentrated slurries

In this investigation the microstructure and erosion–corrosion behaviour of a Fe–Cr–C overlay (FeCrC–matrix) produced by plasma transferred arc welding (PTA) and its metal matrix composite (FeCrC–MMC) were assessed. The FeCrC–MMC was obtained by the addition of 65 wt.% of tungsten carbide (WC). The erosion–corrosion tests (ECTs) were carried out using a submerged impinging jet (SIJ); after the ECTs the surface of the overlays was analysed to identify the damage mechanisms. Two different temperatures (20 and 65 °C) and sand concentrations (10 and 50 g/l) were used in a solution of 1000 ppm of Cl⁻ and a pH value of 8.5; the conditions were chosen to be representative of the recycling water in the tailings line in the oilsands industry. The FeCrC–matrix showed a dendritic structure and a high concentration of carbides in the interdendritic zone. The addition of the WC reinforcing phase promoted the formation of W-rich intermetallic phases, increased the microhardness values of the matrix phase of the FeCrC–MMC overlay and dramatically improved its erosion–corrosion performance as expected. For the FeCrC–matrix overlay the main erosion–corrosion degradation mechanisms were severe plastic deformation and the formation and removal of material flakes due to consecutive impacts. At 65 °C the dendritic zone was severely corroded in the area of low impact frequency. The FeCrC–MMC showed greater attack of the matrix phase compared to the WC grains; at high sand concentration the WC grains were severely fractured and flattened. The anodic polarisation analysis showed active corrosion behaviour of the FeCrC–MMC at both temperatures and sand concentrations; however the temperature dramatically increased the corrosion process of the surface studied under erosion–corrosion conditions. The paper assesses the degradation mechanisms of both FeCrC–matrix and FeCrC–MMC with the aim of understanding what aspects of MMCs must be adapted for optimum erosion–corrosion resistance.     

Effects of extrusion variables on temperature distribution in axisymmetric extrusion process

A numerical method was developed to simulate the non-steady-state temperature distributions during forward extrusion process. The velocity, strain rates, and strain fields within the deformation zones during extrusion were obtained, using upper bound method of analysis to obtain internal heat generations coupled to the necessary heat transfer conduction equations. The computer program written in C++ language essentially simulates the extrusion process and takes into account extrusion variables such as material properties, friction conditions, extrusion velocity, extrusion ratio, die preheat temperature, billet height, percentage reduction in area, and die land length. The effects of billet height and percentage reduction in area on the temperature distributions within the dead metal zone give good agreements with experimental results. It is found that the higher the billet's heights and higher the percentages reduction in areas, the higher the temperature rises during the extrusion process. The die land zone shows increasing temperature rise with increasing friction coefficient, while increasing friction coefficient has no effect on the dead zone temperature. Also, increasing speed of deformation shows an increasing dead zone temperature rise than a more gradual die land temperature rise. It can be stated that the extrusion temperature increases proportionally to the increase of the container temperature.     

Photoelastic investigations on initial plastic straining

Photoelasticity has been used for an investigation of the stress distribution in billets forged on flat dies. The flat models of the billet were made of birefringent material which were loaded to a state approaching the elastic limit.The authors' experiments were based on Nadai's hypothesis that in the first stages of plastic straining the stresses and strains are commensurate with the corresponding extremal values in the elastic range.The stresses were analysed for different values of feed ratio, i.e. relation of (actual) width of contact area between the die and the billet to its height. The experiments showed that if the feed ratio is less than 1·3, tensile stresses are present in the central zone of the billet under the die centre.There are no unfavourable tensile stresses in the central zone of the billet if the feed ratio is more than 1·3. However, for all values of feed ratio at points on the billet axis at a distance of 0·2 of the height of the billet from the die edge, tensile stresses are present.These conclusions lead to practical recommendations, concerning the forging of ingots and bars of complex brittle alloys at a maximum feed ratio.     

Hydrostatic extrusion of composite rods with hard cores

The modes of deformation of composite materials with a hard core during hydrostatic extrusion are examined. The experimental work was carried out for Cu-Al composite rods with the fractional cross-sectional area of the copper being 0.16. It has been found that uniform deformation occurs at low extrusion ratios and the copper rod fails in tension at higher ratios; also the critical extrusion ratio increases as the die angle decreases. A theoretical analysis is developed to predict the mode of deformation by using the upper bound method. The effects of the end of the billet, yield stress ratio, fraction of the harder core, extrusion ratio, die angle and frictional shear factors at the interface and die surface are considered in the analysis. The theoretical results show good agreement with the experimental results.     

Analysis of hydrofilm extrusion of elliptic shapes using perturbation method

An approximate method of solution is proposed for the hydrofilm extrusion of elliptic shapes from round billets through optimized curved dies. A modified upper-bound theorem and hydrodynamic lubrication theory are used in combination, in order to analyze metal deformation and fluid flow respectively. The fluid analysis in hydrodynamic lubrication theory is simplified by use of elliptic transformation and perturbation technique. Strain-hardening effect of billet material and viscosity variation of fluid due to pressure are taken into consideration.For several reductions of area, experiments are carried out at room temperature by using mild steel specimens and caster oil as the lubricant.The experimental extrusion pressures are in good agreement with the theoretical predictions.     

大型钢锭内部空洞性缺陷锻合过程的实验研究

本文采用均匀压缩热钢模拟实验,研究了内部空洞在锻合过程中受应力应变的影响,提出空洞沿最大压应变方向闭合,并经由变形→压合→扩散焊合三个阶段得以消除的机制。对坯料内部三维变形条件下对称面上应变分布的实验测量与计算结果,发现在平砧拔长条件下,应变峰值随砧宽比变化而移动的规律,以及在连续锻造过程中,两砧交界处的坯料中心存在小变形区的现象。     

Numerical and experimental study on the three-dimensional extrusion of square section from square billet through a polynomial shaped curved die

The geometry of die profile plays a major role in reducing the extrusion pressure and ensuring the smooth flow of material. In general, the extrusion process is mostly affected by billet geometry, die geometry, and interface frictional force at the die billet geometry. In the present investigation, an analysis using three-dimensional upper bound method using fifth-order die profile function has been carried out for extrusion of square sections from square billet. The extrusion pressure and optimum die length have been computed by multivariable optimization technique. The present die shape profile is found to be superior to many other profiles. The results obtained will help in design of optimum die profile and investigation of its performance.     

Embeddability behaviour of some Pb-free engine bearing materials in the presence of abrasive particles in engine oil

One of the tribological requirements on engine bearing material is its ability to safely embed contaminant particles onto its surface and minimise damage to both the bearing and crankshaft surfaces. In this work, a journal bearing test rig that operates under constant load has been employed to investigate the embeddability behaviour of selected multi-layered Pb-free engine bearing materials at three different rotational speeds using engine oil contaminated with SiC particles. Experimental results have shown that third-body abrasive wear is influenced by the lubricant film thickness. There was also difference in embeddability of the different materials. Bismuth-based overlay and MoS₂ containing polyamide-imide-based overlay-coated materials show higher wear compared to tin-based overlay and a polyamide-imide-based composite overlay-coated material. Steel counter surfaces sliding against bismuth-based overlay and MoS₂ containing polyamide-imide-based overlay exhibited higher wear than those sliding against tin-based overlay and polyamide-imide-based composite overlay.     

Effect of SiC reinforcement on the deformation and fracture micromechanisms of Al–Li alloys

The effect of SiC reinforcement on the microstructure of a naturally aged 8090 Al alloy as well as on the deformation and fracture micromechanisms was investigated. To this end, the microstructural characteristics (grain and reinforcement morphology, precipitate structure) were determined in the unreinforced alloy and in the composite reinforced with 15 vol.% SiC particles. The materials were tested under monotonic tension and fully reversed cyclic deformation and then carefully analysed through scanning and transmission electron microscopy to find the dominant deformation and failure processes for each material and loading condition. It was found that the dispersion of the SiC particles restrained the formation of elongated grains during extrusion and inhibited the precipitation of Al₃Li. As a result, the plastic deformation in the composite was homogeneous, while strain localization in slip bands was observed in the unreinforced alloy specimens tested in tension and in fatigue. The unreinforced alloy failed by transgranular shear along the slip bands during monotonic deformation, whereas fracture was initiated by grain boundary delamination, promoted by the stress concentrations induced by the slip bands, during cyclic deformation. The fracture of the composite was precipitated by the progressive fracture of the SiC reinforcements during monotonic and cyclic deformation.     

Heat Treated NiP–SiC Composite Coatings: Elaboration and Tribocorrosion Behaviour in NaCl Solution

Tribocorrosion behaviour of heat-treated NiP and NiP–SiC composite coatings was investigated in a 0.6 M NaCl solution. The tribocorrosion tests were performed in a linear sliding tribometer with an electrochemical cell interface. It was analyzed the influence of SiC particles dispersion in the NiP matrix on current density developed, on coefficient of friction and on wear volume loss. The results showed that NiP–SiC composite coatings had a lower wear volume loss compared to NiP coatings. However, the incorporation of SiC particles into the metallic matrix affects the current density developed by the system during the tribocorrosion test. It was verified that not only the volume of co-deposited particles (SiC vol.%) but also the number of SiC particles per coating area unit (and consequently the SiC particles size) have made influence on the tribocorrosion behaviour of NiP–SiC composite coatings.     

High stress abrasive wear behaviour of aluminium hard particle composites: Effect of experimental parameters, particle size and volume fraction

High stress abrasive wear behaviour of aluminium alloy (ADC-12)–SiC particle reinforced composites has been studied as a function of applied load, reinforcement size and volume fraction, and has been compared with that of the matrix alloy. Two different size ranges (25–50 and 50–80 μm) of SiC particles have been used for synthesizing ADC-12–SiC composite. The volume fraction of SiC particles has been varied in the ranges from 5 to 15 wt%. It has been noted that the abrasive wear rate of the alloy reduced considerably due to addition of SiC particle and the wear rate of composite decreases linearly with increase in SiC content. It has also been noted that the wear resistance of composite varies inversely with square of the reinforcement size. The wear rate of the alloy and composite has been found to be a linear function of applied load but invariant to the abrasive size; at critical abrasive size, transition in wear behaviour is noted. This has been explained through analytically derived equations and wear–surface examination.     

Paraffinic mineral oil lubrication for cold forward extrusion: Effect of lubricant quantity and friction

In this paper, Finite Element (FE) and experimental analyses have been developed on the deformation of aluminium billet over a tool. Effect of friction resulted from the use of additive-free ISO460-compliant paraffinic mineral oil with kinematic viscosity of 455.192 mm²/s at 40 °C in amounts of 0.1, 1, 5, and 20 mg were examined. The time behaviour of displacements on the billet in the experiment was used as inputs for the FE model. The FE analysis results for load–displacement behaviour of the extrusion were compared with the experimental results. It was shown that significant differences exist between the four lubricant quantities on friction and contact pressure distribution.     

On the equivalent friction factor in hydrofilm extrusion

Hydrofilm extrusion is a kind of hydrostatic extrusion which uses a minimum amount of oil as lubricant and pressure-transmitting medium. In hydrofilm extrusion energy dissipation in the fluid lubricant between the die and working material corresponds to sliding friction in ordinary lubricated extrusion using solid lubricants. Utilizing the upper-bound theorm, an “equivalent” friction factor is defined so that the overall frictional effect between the die and working material can be conveniently investigated in terms of geometrical parameters and press velocity. On the basis of this definition, the effects of various process parameters on the frictional characteristics in hydrofilm extrusion are discussed. It is consequently found that the dominant contribution to frictional energy dissipation is made by reduction of area and press velocity. Die length is found to have very little influence on the equivalent friction factor in so far as it is longer than billet diameter.