Effect of post-oxidizing temperature on tribological and corrosion behavior of plasma nitrided austenitic stainless steel

In this study, the effect of temperature of post-oxidation process on tribological and corrosion behavior of AISI 316 plasma nitrided stainless steel has been studied. Plasma nitriding was carried out at 450 °C for 5 h with gas mixture of N₂/H₂ = 1/3. The plasma nitrided samples were post-oxidized for 1 h with gas mixture of O₂/H₂ = 1/5 at different temperature of 400, 450 and 500 °C. The structural, tribological and corrosion properties were analyzed using XRD, SEM, microhardness testing, pin-on-disk tribotesting and electrochemical polarization. The results indicated that the nitride layer was composed of S-phase. The amount of S-phase decreased as the treatment temperature rose from 400 °C to 500 °C. In addition, it was found that oxidation treatment reduces wear resistance of plasma nitrided sample. It was demonstrated that the corrosion characteristics of the nitrided sample were further improved by post-oxidation treatment. The difference in corrosion resistance is mainly attributed to the thickness of the oxide top layer, which is governed by the post-oxidizing temperature.     

Microstructural and tribological characterization of plasma- and gas-nitrided 2Cr13 steel in vacuum

Plasma- and gas-nitrided 2Cr13 samples were characterized using optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), and microhardness testing techniques. Nitrogen concentration profiles in the cross-sections of the nitrided samples were obtained by glow discharge optical spectroscopy (GDOS). Residual stress profiles along depth of the nitrided samples were measured using an X-ray stress tester. The tribological behaviour of the plasma- and the gas-nitrided samples in vacuum was investigated in order to analyse the effect of nitriding on wear resistance of the 2Cr13 steel. The results show the tribological properties of the 2Cr13 steel in vacuum are improved considerably by plasma nitriding and gas nitriding resulted from microstructure modification and surface hardening during nitriding. The plasma-nitrided samples have better wear resistance than the gas-nitrided samples under 30 N, while the gas-nitrided samples have higher wear resistance under 90 N. With increasing normal load from 30 N to 90 N, the wear mechanism shows a transition from mild adhesive and abrasive wear to severe adhesive or even delamination wear. The plasma-nitrided sample has thicker compound layer than the gas-nitrided sample, resulting that it exhibits more intensive delamination under high load of 90 N.     

Microstructure and mechanical performance of metal-composite hybrid joints produced by FricRiveting

The mechanical performance and microstructure of friction riveted metallic-insert joints made of polyether ether ketone composite reinforced with 30% short carbon fibers and titanium grade 3 was studied. The metallic-insert joints reached a maximal pull-out tensile force of 10.6 kN, which corresponds to 100% of the titanium base material strength. It was shown the pull-out force increased as the rivet tip widened. Frictional heat during the process was mainly generated by the friction between the tip of the rivet and the composite substrate in the friction zone. Microstructural analyses of the metallic part of the joint revealed the presence of different microstructural zones: a friction zone, and two thermomechanically affected zones 1 and 2. Based on the composite morphology, the composite part of the joint was categorized into three different zones: the stir zone, a thermomechanically affected zone and a heat-affected zone. A study of the material flow showed that the flow of the composite was strongly affected by the rotation and axial movement of the rivet.     

Improvement of wear and corrosion resistances of 17-4PH stainless steel by plasma nitrocarburizing

17-4PH stainless steel was plasma nitrocarburized at 460 °C for improving its mechanical properties without compromising its desirable corrosion resistance. The plasma nitrocarburized layers were studied by optical microscope, X-ray diffractometer, microhardness tester, pin-on-disc tribometer and the anodic polarization method in a 3.5% NaCl solution. The experimental results show that the nitrocarburized layer depths increase with increasing duration time and the layers growth conform approximately to the parabolic law. The phases in the nitrocarburized layer are mainly of γ′-Fe₄N and α′-Fe with traces of CrN phase. The surface hardness of the modified specimen is more than 1200 HV, which is three times higher than that of untreated one. The friction coefficient and corrosion resistance of the specimen can be apparently improved by plasma nitrocarburizing. With the increase of duration time, the surface hardness slightly decreases whereas the friction coefficient and corrosion resistance of the modified specimen are first increase and then decrease. The 8 h treated specimen has the lowest friction coefficient and the best corrosion resistance in the present test conditions.     

Microstructural evolution and mechanical properties of oil jet peened aluminium alloy,AA6063-T6

Grain size refinement by severe surface plastic deformation is one way of improving the surface properties. This paper describes the microstructural evolution due to severe surface plastic deformation by oil jet peening in aluminium alloy, AA6063-T6. Detail characterization of the treated surfaces using X-ray diffraction analysis and transmission electron microscopy revealed the formation of submicron size grains at and near the surface. The nozzle-traveling velocity decides the peening intensity and coverage and affects the surface properties. The specimen peened at low nozzle-traveling velocity exhibited an ultrafine grain size (∼210 nm) with high surface hardness (∼0.88 GPa), compressive residual stress (−102 ± 7 MPa) and dislocation density. The hardness is high at the surface and the depth of hardened layer is ∼400 μm. Formation of high-density dislocations and associated grain refinement resulted in increased surface hardness. Presence of surface modified layer will be beneficial in improving the fatigue and tribo behavior.     

Effect of different whiskers on the physical and tribological properties of non-metallic friction materials

Potassium titanate, magnesium borate and calcium sulfate whisker modified non-metallic friction materials (designated as P, M and C) were prepared by compression moulding process. Results of physical test showed that the addition of whisker greatly improved mechanical properties and slightly increased thermal stability. Particularly, M exhibited the highest tensile strength and C showed the best thermal stability. The tribology properties were investigated against a special counterpart of wire rope under dry and grease lubrication conditions. Results showed that potassium titanate whisker improved the wear-resistance most, and calcium sulfate whisker increased the value and stability of friction coefficient most at high sliding speed.     

Microstructural aspects and mechanical properties of friction stir welded AA2024-T3 aluminium alloy sheet

In the present work, aluminium alloy AA2024-T3 thin sheets were joined by the Friction Stir Welding – FSW – process. Butt joints were obtained in 1.6 mm sheets, using an advancing speed of 700 mm/min. These joints were characterised by optical, scanning electron microscopy, tensile and fatigue mechanical tests. The results showed that the resulting microstructure is free of defects and the tensile strength of the welded joints is up to 98% of the base-metal strength. Fatigue tests result indicates an equivalent stress intensity factor (kt) of approximately 2.0 for the welded samples. Consequently, the FSW process can be advantageous compared to conventional riveting for airframe applications.     

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.     

Characterization of friction stir welded boron carbide particulate reinforced AA6061 aluminum alloy stir cast composite

Development of welding procedures to join aluminum matrix composite (AMCs) holds the key to replace conventional aluminum alloys in many applications. In this research work, AA6061/B₄C AMC was produced using stir casting route with the aid of K₂TiF₆ flux. Plates of 6 mm thickness were prepared from the castings and successfully butt joined using friction stir welding (FSW). The FSW was carried out using a tool rotational speed of 1000 rpm, welding speed of 80 mm/min and axial force of 10 kN. A tool made of high carbon high chromium steel with square pin profile was used. The microstructure of the welded joint was characterized using optical and scanning electron microscopy. The welded joint showed the presence of four zones typically observed in FSW of aluminum alloys. The weld zone showed fine grains and homogeneous distribution of B₄C particles. A joint efficiency of 93.4% was realized under the experimental conditions. But, FSW reduced the ductility of the composite.     

Microstructural evaluation and corrosion properties of aluminium matrix surface composite adding Al-based amorphous fabricated by friction stir processing

A novel aluminium matrix surface composite adding Al_84.2Ni₁₀La_2.1 amorphous, which the layer depth was 5 mm, was fabricated by friction stir processing. The surface composite region shows obvious sandwich structure. The average hardness of the surface composite is about HV97, higher than the base metal is about HV80. The maximum tensile strength of the processed aluminium plate with the surface composite is 410 MPa. The surface composite was mainly composed of phases α-Al, Mg₂Al₃, MnAl₆ and La₃Al₁₁. The surface composite added the amorphous strip had the lower icorr, corrosion current density, and the higher passivation current than the surface composite not added amorphous strip. And there is obvious passivation zone for the surface composite. However, a large number of ultrafine grained which was composed of the α-Al and α-Al amorphous structures can be observed in the surface composite. And the grain size range of them is 90–400 nm. It is conceivable that the existence of these ultrafine grained structures and change of crystal plane would contribute greatly to improve the mechanical properties and corrosion resistance of the aluminium matrix surface composite.     

Preparation and tribological performance of chemically-modified reduced graphene oxide/polyacrylonitrile composites

Interface control and dispersion of graphene base nanomaterials in polymer matrix are challenging to develop high comprehensive nanocomposites due to their strong interlayer cohesive energy and chemical inertia. In this research, an efficient approach is presented to functionalize reduced graphene oxide nanosheets by N-[3-(trimethoxylsilyl)propyl]ethylenediamine, which is dispersed into polyacrylonitrile to prepare N-[3-(trimethoxylsilyl)propyl]ethylenediamine – reduced graphene oxide/polyacrylonitrile nanocomposites. A thermogravimetric analysis technique was employed to evaluate thermal properties of the nanocomposites. The tribological properties of the polyacrylonitrile/graphene nanocomposites were investigated. The morphologies and volume of the worn surface were examined using a 3D profilometer. The impact of loading ratio on friction coefficient, carry-bearing capacity and durability were studied. The N-[3-(trimethoxylsilyl)propyl]ethylenediamine – reduced graphene oxide/polyacrylonitrile nanocomposite with appropriate loading ratio of reduced graphene oxide exhibited a high load-bearing capacity and durability. Therefore, the polyacrylonitrile/graphene nanocomposite shows promising potential to industrial applications involving the lubrication and anti-wear.     

Crack and wear behavior of SiC particulate reinforced aluminium based metal matrix composite fabricated by direct metal laser sintering process

In this investigation, crack density and wear performance of SiC particulate (SiCp) reinforced Al-based metal matrix composite (Al-MMC) fabricated by direct metal laser sintering (DMLS) process have been studied. Mainly, size and volume fraction of SiCp have been varied to analyze the crack and wear behavior of the composite. The study has suggested that crack density increases significantly after 15 volume percentage (vol.%) of SiCp. The paper has also suggested that when size (mesh) of reinforcement increases, wear resistance of the composite drops. Three hundred mesh of SiCp offers better wear resistance; above 300 mesh the specific wear rate increases significantly. Similarly, there has been no improvement of wear resistance after 20 vol.% of reinforcement. The scanning electron micrographs of the worn surfaces have revealed that during the wear test SiCp fragments into small pieces which act as abrasives to result in abrasive wear in the specimen.     

Plasma electrolytic oxidation of aluminium networks to form a metal-cored ceramic composite hybrid material

A commercially-available low density aluminium network material (Duocel™) has been processed by plasma electrolytic oxidation to produce a ceramic hybrid material comprising an assembly of ceramic struts with metallic cores. The architecture and microstructure of this material were studied using X-ray tomography, scanning electron microscopy and densitometry. Conversion fractions were determined from mass gains and by image analysis of cross-sections, and the ceramic density was evaluated by hydrostatic weighing. Tensile and compressive testing of the hybrid material was used to study the toughness, as a function of the conversion fraction. Such material retains some of the beneficial mechanical properties of a metal (ductility and toughness), while also exhibiting a low overall density and a high specific surface area of ceramic. It can thus be considered as a highly permeable ceramic scaffold, with a relatively high toughness.     

Effects of temperature and normal load on tribological behavior of nickel-based high temperature self-lubricating wear-resistant composite coating

NiCr/Cr₃C₂–WS₂–CaF₂ mixed powders were designed and aimed to fabricate high temperature self-lubricating wear-resistant composite coating by laser cladding. The friction and wear properties of the coating were investigated under different temperatures and loads against Si₃N₄ ceramic ball. Results show that friction coefficient decreases with the increasing temperature while the wear rate firstly decreases and then increases with the increasing temperature. Both the friction coefficient and wear rate firstly decreases and then slightly increases with the increasing normal load. The coating exhibits relatively excellent tribological behavior under moderate temperature and moderate normal load.     

Wear characteristics of hybrid aluminium matrix composites reinforced with graphite and silicon carbide particulates

This investigation studies the dry sliding wear behaviour of Al matrix composites reinforced with Gr and SiC particulate up to 10%, to study the effect of % reinforcement, load, sliding speed and sliding distance on stir cast Al–SiC–Gr hybrid composites, Al–Gr and Al–SiC composites. Parametric studies indicate that the wear of hybrid composites has a tendency to increase beyond% reinforcement of 7.5% as its values are 0.0242 g, 0.0228 g and 0.0234 g respectively at 3%, 7.5% and 10% reinforcement. The corresponding values are 0.0254 g, 0.0240 g and 0.0242 g in Al–Gr composites and 0.0307 g, 0.0254 g and 0.0221 g in Al–SiC composites, clearly indicating that hybrid composites exhibit better wear characteristics. Increase of speed reduces wear and increase of either load or sliding distance or both increases wear. Statistical analysis has revealed interactions among load, sliding speed and sliding distance in composites with Gr particulates.     

Tensile behavior of dissimilar friction stir welded joints of aluminium alloys

The heat treatable aluminium alloy AA2024 is used extensively in the aircraft industry because of its high strength to weight ratio and good ductility. The non-heat treatable aluminium alloy AA5083 possesses medium strength and high ductility and used typically in structural applications, marine, and automotive industries. When compared to fusion welding processes, friction stir welding (FSW) process is an emerging solid state joining process which is best suitable for joining these alloys. The friction stir welding parameters such as tool pin profile, tool rotational speed, welding speed, and tool axial force influence the mechanical properties of the FS welded joints significantly. Dissimilar FS welded joints are fabricated using five different tool pin profiles. Central composite design with four parameters, five levels, and 31 runs is used to conduct the experiments and response surface method (RSM) is employed to develop the model. Mathematical regression models are developed to predict the ultimate tensile strength (UTS) and tensile elongation (TE) of the dissimilar friction stir welded joints of aluminium alloys 2024-T6 and 5083-H321, and they are validated. The effects of the above process parameters and tool pin profile on tensile strength and tensile elongation of dissimilar friction stir welded joints are analysed in detail. Joints fabricated using Tapered Hexagon tool pin profile have the highest tensile strength and tensile elongation, whereas the Straight Cylinder tool pin profile have the lowest tensile strength and tensile elongation. The results are useful to have a better understanding of the effects of process parameters, to fabricate the joints with desired tensile properties, and to automate the FS welding process.     

The wear and corrosion resistance of shot peened–nitrided 316L austenitic stainless steel

316L austenitic stainless steel was gas nitrided at 570 °C with pre-shot peening. Shot peening and nitriding are surface treatments that enhance the mechanical properties of surface layers by inducing compressive residual stresses and formation of hard phases, respectively. The structural phases, micro-hardness, wear behavior and corrosion resistance of specimens were investigated by X-ray diffraction, Vickers micro-hardness, wear testing, scanning electron microscopy and cyclic polarization tests. The effects of shot peening on the nitride layer formation and corrosion resistance of specimens were studied. The results showed that shot peening enhanced the nitride layer formation. The shot peened–nitrided specimens had higher wear resistance and hardness than other specimens. On the other hand, although nitriding deteriorated the corrosion resistance of the specimens, cyclic polarization tests showed that shot peening before the nitriding treatment could alleviate this adverse effect.     

Microstructure and microtextural studies of friction stir welded aluminium alloy 5052

The present investigation presents a composite picture of the microstructural developments in a friction stir welded (FSW) AA5052. Optimized, defect free and chemically homogeneous, FS weld was generalized in four regions – base material (BM), nugget, advancing side (AS) and retreating side (RS), using standard nomenclatures. Each region had its signature of microstructural features. AS had clear indications of shear and of grain fragmentation. The nugget region, on the other hand, had nearly equiaxed grains, with strong in-grain misorientation and presence of grain-interior dislocation structure ruling out contributions from static recrystallization. Equiaxed grains of the nugget region had typical onion ring structure – each ring did approximately correspond to one dominant family of orientation. Microstructural developments, as obtained from relative grain refinement, in-grain misorientation development, relative banding, etc., were most significant in nugget followed by AS and then by RS. Heterogeneous plastic deformation and thermal activation through localized heating/friction were the apparent causes. Most of the friction stir welded specimen fractured away from the nugget and showed ductile mode of failure.     

搅拌摩擦加工制备MWCNTs／Al复合材料显微结构及硬度

采用搅拌摩擦加工技术（FSP）制备多壁碳纳米管增强铝基（MWCNTs／A1）复合材料,并对该复合材料的显微结构和硬度进行分析。结果表明：MWCNTs／Al复合材料显微结构为细小的等轴晶,晶粒大小不均匀;MWCNTs与Al基体界面结合良好,界面处分布大量的位错,MWCNTs主要分布在晶内;动态再结晶形核方式为亚晶形核,在...     

Predicting tensile strength,hardness and corrosion rate of friction stir welded AA6061-T6 aluminium alloy joints

AA6061-T₆ aluminium alloy (Al–Mg–Si alloy) has gathered wide acceptance in the fabrication of light weight structures requiring high strength-to-weight ratio and good corrosion resistance. The friction stir welding (FSW) process and tool parameters play major role in deciding the joint characteristics. In this research, the tensile strength and hardness along with the corrosion rate of friction-stir-butt welded joints of AA6061-T₆ aluminium alloy were investigated. The relationships between the FSW parameters (rotational speed, welding speed, axial force, shoulder diameter, pin diameter and tool hardness) and the responses (tensile strength, hardness and corrosion rate) were established. The optimal welding conditions to maximize the tensile strength and minimize the corrosion rate were identified and reported here.