Tribological performance of microwave sintered copper-TiC-graphite hybrid composites

Copper matrix composites are finding many applications due to their inherent properties such as good electrical and thermal conductivity and corrosion resistance. New series of copper-TiC (5-15 vol%)-graphite (5-10 vol%) hybrid composites are fabricated through a novel microwave processing technique. Pin-on-disc is used to evaluate their tribological properties under testing parameters of normal loads 12-48 N and sliding speed of 1.25-2.51 m/s. The formation of mixed smooth layer with higher graphite hybrid composites improves the wear resistance and reduces the friction coefficient. Morphology of worn out surfaces and wear debris were analyzed to understand the wear mechanisms.     

Wear and frictional performance of polymeric composites aged in various solutions

The aim of the present work is to investigate the effect of aging process on the wear and frictional characteristics of polyester composites based on oil palm fibres. Prepared samples of treated oil palm fibre reinforced polyester (T-OPRP) composite were immersed in different types of solutions (i.e. water, salt water, diesel, petrol and engine oil) for three years. The samples were then tested on a Pin on disc (POD) machine subjected to a polished stainless steel counterface under dry adhesive wear at different sliding distances (0-6.72 km). Scanning electron microscopy (SEM) was used to observe the damage features on the worn surfaces. Results revealed that aging process has pronounced influence on the adhesive wear and frictional behaviour of the T-OPRP composite. Immersing the samples in water and salt water demonstrated poorest wear performance as compared to the ones immersed in engine oil and diesel. This was mainly due to the higher viscosities of engine oil and diesel solutions as compared to the rest.     

AZ31B镁合金表面激光合金化Al-SiC涂层制备及其性能研究

针对镁合金表面耐磨性差,采用预置粉末法对AZ31B表面进行激光合金化Al-SiC粉末实验。使用光学显微镜和扫描电子显微镜(SEM)、能谱分析仪(EDS)、摩擦磨损试验机、显微硬度计对合金化涂层的微结构、相组成及性能进行了分析研究。结果表明,强化层与基体呈冶金结合、组织均匀致密;合金化层主要由Mg17Al12、SiC、Mg2Si、Al4C5、Al2O3等相组成。涂层的显微硬度、耐磨损性能都明显高于基体。     

Sliding friction and wear of magnesium alloy AZ91D produced by two different methods

Alloy AZ91D is a leading magnesium alloy used for structural applications. It contains aluminum and zinc as principal alloying elements. This alloy is normally die-cast, but recent developments in semi-solid injection molding (Thixomolding^®), which offers certain processing advantages, produces a slightly different microstructure than die-casting, and it was of interest to determine whether the two processing routes would measurably affect the friction and wear of AZ91D. The present work involved ambient air, room temperature testing of die-cast (DC) and Thixomolded^® (ThM) AZ91D, in both unidirectional and reciprocating sliding motion, using stainless steel type 440C as the counterface. After running-in, the average sliding friction coefficients in both types of test fell into the range of 0.29–0.35, irrespective of processing method. The formation of a built-up edge raised the friction slightly in unidirectional tests compared with reciprocating tests. The average wear rate of the ThM alloys in reciprocating sliding was approximately 25% lower than that for DC alloys. However, the wear rates of the magnesium specimens in unidirectional sliding were comparable for DC and ThM materials. Owing to the transfer of magnesium, there was no measurable wear on the stainless steel 440C balls. The wear mechanism during sliding involves the formation of thin, narrow shards along the edges of wear grooves which break off to produce loose particles.     

Fretting wear behavior of AZ91D and AM60B magnesium alloys

The fretting wear behavior of the AZ91D and AM60B magnesium alloys are investigated using a reciprocating fretting wear machine under dry conditions with different numbers of cycles, different normal loads, slip amplitudes and frequencies. The worn surfaces and wear debris were examined using scanning electron microscopy and optical microscopy in order to understand the predominant wear mechanisms of two magnesium alloys. The results indicate that the AZ91D alloy displays a lower friction coefficient and lower wear quantity than the AM60B alloy. The AZ91D shows a higher capability than AM60B in resisting crack nucleation and propagation. Both AZ91D and AM60B show similar friction and wear characteristics. The wear quantity increases with increasing normal load, but decreases with increasing frequency. The friction coefficient also decreases as the normal load is increased. Fretting frequency had little effect on the friction coefficient. In a long term, the fatigue wear and abrasive wear were the predominant wear mechanisms for AM60B and delamination wear, adhesive wear and abrasive wear for AZ91D.     

Erosive-corrosive wear of aluminum alloy composites: Influence of slurry composition and speed

The Erosion-corrosion behavior of a SiC particle reinforced Al-Si alloy has been studied in two different environments, namely saline and acidic, to simulate sea water and mining atmospheres, respectively. These studies were performed at different sand concentrations (20-40 wt%) and varying rotational speeds (700-900 rpm). It is noted from the present study that the composite exhibited better wear resistance than the alloy in marine and acidic atmospheres irrespective of sand content and speed. It is also noted that the wear rates increased with increasing sand content and speed irrespective of material due to increase in the severity of erosive/abrasive attacks. However, the wear rates decreased at higher speeds (e.g., 1100 rpm) due to increased intercollisions and rebounding and also the decrease in the mobility of the erodant particles. It is observed that erosion is the dominant mode of material removal in these two media. Corrosive attack was more predominant in the acidic media than in the NaCl media at lower sand concentrations (0-20 wt%). However, at higher sand concentrations (30-40 wt%) corrosive attack was more severe in the NaCl medium. Scanning electron microscopic (SEM) observation shows that Al/Si interfaces act as predominant sites for corrosion attack rather than the Al/SiC interfaces. Formation and removal of the passive layer, preferential attack at the Al/Si interfaces, fragmentation and wear of SiC particles were observed as mechanisms of material removal in marine and acidic media. SEM studies of the eroded-corroded surfaces indicated that an increase in the sand content of the slurry and in the rotational speed of the slurry increased resulted in greater damage to the SiC particles and matrix, resulting in an increase in wear rates.     

AZ31B镁合金手机外壳拉深模具设计

以AZ31B镁合金手机外壳的拉深模具设计和实际生产为例,对AZ31B镁合金的拉深成形过程进行研究。实际生产表明,选取合适的模具结构和参数可以改善AZ31B镁合金板材的拉深成形性能;在拉深成形时,通过对坯料温度和模具温度等主要影响因素的控制,可有效地消除AZ31B镁合金在拉深过程中的拉裂缺陷。     

Failure modes during uniaxial deformation of magnesium alloy AZ31B tubes

This study examines magnesium alloy AZ31B circular tubing subject to uniaxial compressive loading and compares their performance to steel (ASTM A106 Grade B) and aluminium alloy AA6061T6 circular tubing at both low and high strain rates. Quasi-static tests were undertaken using a hydraulic testing machine for a range of tube lengths and thicknesses for tubes with an outside diameter of approximately 48 mm. To examine the effects of higher strain rate, a drop test rig was used. It was found that magnesium alloy AZ31B outperforms both the steel and aluminium alloys in terms of energy absorption for equivalent mass when subject to uniaxial compressive loads for the thicker sections. This is further enhanced by alloy AZ31B’s strain rate sensitivity, as there is a dramatic increase in the energy absorption at higher strain rates. However, the AZ31B tubes usually fail by fracturing, which generally involves a shear fracture mode, unlike the aluminium and steel tubes, which generally retained their structural integrity to a higher degree. The greatest energy absorption was obtained when the AZ31B failed via fine sharding. This failure mode appeared to be related to the presence of micro-cracks on the surface of the section obtained by overheating during extrusion. At higher strain rates, much greater plasticity and compaction are present in the fracture modes for the thicker AZ31B tubing. Some of the fracture modes have been discussed and the failure/fracture modes are compared with a typical aluminium alloy tube failure mode classification chart.     

Dynamic mechanical behaviour of AZ31 magnesium alloy

To investigate the dynamic mechanical behavior of AZ31 Mg alloy,dynamic compression was carried out using a split Hop kinson pressure bar(SHPB) apparatus at strain rates up to 2.0×103 s-1,and d ynamic hardness was tested employing a dynamic hardness device at room temperatu re.Microstructural characteristic was analysed by optical microscopy.The dynam ic compression results demonstrate that AZ31 Mg alloy exhibits obvious yield phe nomena and strain hardening behavior at high strain rates.The basically same cu rvature of stress-strain curves shows a similar strain hardening rate.The dyna mic yield strength changes little,and the peak stress increases with the strain rates.The dynamic hardness test results indicate that the dynamic mechanical p roperties of AZ31 alloy sheet are anisotropic.The dynamic hardness increases sl owly with average strain for the 0° and 45° oriented samples.For the 90° ori e nted sample,dynamic hardness with strain increases rapidly first and then decre ases when the strain is more than 0.14.An examination by optical microscopy aft er high strain rate deformation reveals the occurrence of twinning,and the twin area percentage escalates with the strain rate increasing.     

Synthesis and evaluation of mechanical and high temperature tribological properties of in-situ Al-TiC composites

In situ Al-TiC (5, 10 and 15 wt%) composites were produced by using a reaction mixture of K₂TiF₆ and graphite powder with molten metal. The effect of ceramic particulate addition on the high temperature sliding wear resistance of the composites was studied. The sliding wear tests were conducted at room temperature, 120 and 200 °C. The wear rate increases with the increase in applied load and decreases with increase in the weight percentage of TiC. Both monolithic and composites were able to withstand thermal softening effects due to the formation of oxidative protective transfer layer.     

Wear characteristics of titanium alloy Ti54 for cryogenic sliding applications

Cryogenic wear behaviour of Ti-5Al-4V-0.6Mo-0.4Fe (Ti54) alloy sliding against tungsten carbide is investigated at different speeds, loads and distances. Empirical models based RSM are developed to predict wear characteristics of Ti54 alloy as a function of sliding conditions. It is found that experimental and predicted results are in good agreement. Besides, cryogenic wear is substantially lower than dry wear. SEM and EDS analyses of worn surfaces and wear debris reveal that cryogenic sliding is significantly influenced by changing material properties along with boundary lubrication performance. The study has shown that modes in dry sliding are adhesion and delamination whereas in cryogenic sliding they are abrasion and delamination.     

Relations between oxidative wear and Cr content of steels

Dry sliding wear tests at 25-400 °C were performed for 45, 4Cr5MoSiV1 and 3Cr13 steels; the relations between oxidative wear and Cr content of steels were explored. The low and medium-Cr steels had a substantially lower wear rate and increasing tendency than the high-Cr steel at 25-200 °C, but the contrary case occurred at 400 °C. With an increase of ambient temperature, the wear rate of the low and medium-Cr steels first decreased, then increased and reached the lowest value at 200 °C, while the wear rate of the high-Cr steel decreased monotonously with the lowest value at 400 °C. At 25 °C, trace tribo-oxides reduced wear to some extent in adhesive-dominated wear for the low and medium-Cr steels. At 200 °C, a small amount of tribo-oxides formed and reached a thickness of 10 μm on contacting asperities in the low and medium-Cr steels, thus oxidative mild wear prevailed. At 400 °C, a great amount of tribo-oxides appeared in the low and medium-Cr steels; unexpectedly, the high-Cr steel had more tribo-oxides than the low or medium-Cr steels in some cases. Its high wear resistance may be attributed to Cr-strengthened adhesion power of tribo-oxides and matrix.     

Effect of Hot Extrusion on Electrical Tribological Behavior of Ag-Cu/MoS2 Composite under Air and Vacuum Conditions

The different microstructures of silver–copper/molybdenum disulfide (Ag-Cu/MoS₂) composites were manufactured by hot press and hot extrusion processes to investigate the electrical tribological behaviors of both the hot-pressed and hot-extruded composites under air and vacuum. The results showed that microstructures and properties of Ag-Cu/MoS₂ composites were improved by hot extrusion, which decreased the wear rates rapidly in both air and vacuum. In air, hot extrusion could improve the transfer layer and tribofilm, resulting in a significant decrease in contact voltage drop, which goes from more than 70 mV in the hot-pressed composite to 30 mV at the hot-extruded composite. Under vacuum condition, some wear debris was melted on the worn surface and then transferred to the counterface to form the transfer layers, which led to the lower contact voltage drops under vacuum, about 6 mV in hot-pressed composites and 3 mV in hot-extruded composites. In addition, the severe adhesive and abrasive wear were attributed to the molten wear debris and transfer layer, resulting in a dramatic fluctuation in the friction coefficient in a vacuum.     

Dry sliding wear properties of unhybrid and hybrid Al alloy based nanocomposites

Abstract

Nanosize B₄C and/or MoS₂ particles reinforced AA2219 alloy composites were prepared using the stir casting process. The wear properties were evaluated for several speed (3.14–5.65 m s^?1), load (10–50 N) and distance (0–2500 m) conditions. The nanoparticles dispersion, density, wear resistance, morphology of the worn surface and loose wear debris were discussed in detail. The wear resistance improvement results by nanoparticle addition correspond well with the hardness. Between the nanocomposites, hybrid composites show significantly higher wear resistance for all load, speed and sliding distance conditions. The better wear resistance is attributed to the matrix strengthening by nanoparticles and the lubricant-rich tribolayer controlled wear in the hybrid composites. The intensity of abrasive, oxidation and delamination wear mechanisms decide the wear rate at any particular wear testing condition.     

Dry sliding friction and casing wear behavior of PCD reinforced WC matrix composites

The friction and casing wear properties of PCD reinforced WC matrix composites were investigated using a cylinder-on-ring wear-testing machine against N80 casing steel counterface under dry sliding conditions. The results indicate that the friction and casing wear rate of PCD reinforced WC matrix composites are the lowest among the materials. As the applied load and sliding speed steadily increase, the friction coefficients of PCD reinforced WC matrix composites decrease. In addition, the casing wear rates increase with increasing load, but decline with sliding velocity. The dominant wear mechanism of the PCD composite is the micro-cutting wear, accompanied by adhesive wear.     

AZ31B镁合金激光喷丸强化后疲劳裂纹扩展的数值模拟研究

激光喷丸(LSP)是一种先进的材料表面强化工艺,能有效提高零件的机械性能及其使角寿命.建立了以有限元软件ABAQUS和MSC.Fatigue为平台,面向抗疲劳制造的激光喷丸工艺有限元分析模型.结果表日月激光喷丸可以有效抑制疲劳裂纹扩展,延长疲劳寿命,喷丸次数的增加在一定程度上增大残余应力及疲劳寿命.残余应力抑制疲劳裂纹扩展的原因归结为最终断裂尺寸的增大以及裂纹扩展速度的减小.研究结果为LSP抗疲劳效果的预测提供了有效的方法,对于优化工艺参数,减少试验次数,降低成本具有指导意义.     

Enhancement of abrasive wear resistance in consolidated Al matrix composites via extrusion process

Abstract

The present study addresses the dry wear behaviour of aluminium matrix composites under different sliding speeds and applied loads. Values of the friction coefficient of the matrix alloy and composite materials were in expected range for light metals in dry sliding conditions. The higher coefficient of friction was the consequence of established contact between hard SiC particles and the counter body material. The rough and smooth regions are distinguished on the worn surface of the composites similar to the unreinforced Al alloy. Plastic deformation occurred when the applied specific load was higher than the critical value. The high shear stresses on the sliding surface cause initiation and propagation of the cracks in the subsurface, leading to the loss of material from the worn surface in the form of flakes. The debrises of the composites at low wear rate comprise a mixture of the fine particles and small shiny metallic plate-like flakes and are associated with the formation of more iron rich layers on the contact surfaces.