G. Purcek

Dry sliding friction and wear properties of zinc-based alloys

Abstract The friction and wear of two near-eutectoid and two monotectoid permanent-moulded zinc-based alloys and an SAE 660 bronze for comparison were investigated under dry sliding conditions. To determine the wear mechanisms surface and subsurface microstructures of worn test samples were examined by scanning electron microscopy (SEM). All of the zinc-based alloys had lower coefficients of friction and higher wear resistance than the bronze; the best wear resistance and lowest coefficient of friction were obtained for the ZnAl40Cu2Si1 alloy. Wear resistance increased strongly with hardness and tensile strength and decreased with friction coefficient. Correlation of the experimental results showed that the wear behaviour of the zinc-based alloys was consistent with Archhard’s equation. Layered structures were formed under the wear surfaces of the zinc-based alloy samples during testing. The topmost layer, formed by smearing and embedding of oxidised zinc–aluminium alloy was hard and is considered to have contributed to their low wear rates. Surface and subsurface examinations showed that adhesion and smearing was the main wear mechanism for the zinc-based alloys, while abrasive wear dominated in the case of the SAE 660 bronze.

Effect of copper content on the mechanical and sliding wear properties of monotectoid-based zinc-aluminium-copper alloys

Abstract One binary zinc-aluminium monotectoid and five ternary zinc-aluminium-copper alloys were produced by permanent mould casting. Their wear properties were examined using a block-on-ring test machine. Hardness, tensile strength and percentage elongation of the alloys were also determined and microhardness of aluminium-rich α phase was measured. It was observed that the hardness of the alloys increased continuously with increasing copper content up to 5%. Their tensile strength also increased with increasing copper content up to 2%, but above this level the strength decreased as the copper content increased further. Microhardness of the aluminium-rich α phase was also affected by the copper content in a manner similar to that of the tensile strength. It was found that the wear loss of the alloys decreased with increasing copper content and reached a minimum at 2% Cu for a sliding distance of 700 km. However, the coefficient of friction and temperature due to frictional heating were found to be generally less for the copper containing alloys than the one without the element. The effect of copper on the wear behaviour of the alloys was explained in terms of their microstructure, hardness, tensile strength, percentage elongation and microhardness of the α phase.

Sliding wear of cast zinc-based alloy bearings under static and dynamic loading conditions

Abstract The lubricated wear behaviour of cast journal bearings, produced from a series of zinc-based alloys and SAE 660 bronze as a reference material, was investigated under both static and dynamic loading conditions using a bearing test rig. All of the zinc-based alloys had higher wear resistance than the SAE 660 bronze. Among the zinc-based alloys, the wear resistance of the monotectoid-based alloys was superior to those based on near-eutectoid composition, and the best wear performance under both static and dynamic loading conditions was obtained with ZnAl40Cu2Si1 alloy. Copper content affected the wear resistance of monotectoid zinc-based alloys. Under dynamic loading conditions, it increased with increasing copper content up to 2%, but declined thereafter. Tensile properties and hardness of the monotectoid alloys were also affected by their copper content. Loading conditions had a strong influence on the wear rate. Under static loading conditions, as-cast zinc-based alloys showed higher wear resistance than the equivalent heat-treated alloys, but this behaviour was reversed for dynamic loading. Possible reasons for this are briefly discussed.

Effect of Copper Content on the Mechanical and Tribological Properties of ZnAl27-Based Alloys

One binary ZnAl27- and five ZnAl27-based ternary alloys containing 1–5% Cu were produced by permanent mould casting. Their friction and wear properties were examined using a block-on-ring test machine after determining their hardness and tensile strength. It was observed that the hardness and tensile strength of the ZnAl27-based ternary alloys increased with increasing copper content up to 2% due to solid-solution hardening, above which their tensile strength decreased, while hardness continued to increase. This is attributed to the formation of copper-rich ε and T′ phases, which reduce the solid-solution hardening of the alloys. It was found that the wear volume loss of the ZnAl27-based alloys decreased with increasing copper content up to 2% but showed a small increase above this level. Therefore, it was concluded that the wear resistance of ZnAl27-based alloys containing 1 to 5% Cu correlates well with their tensile strength. In addition, smearing and scratches were found to be the main features of the wear surfaces of the ZnAl27-based alloys under static loading and lubricated sliding conditions.

Enhancement in mechanical behavior and wear resistance of severe plastically deformed two-phase Zn-Al alloys

Abstract The microstructural evolution, tensile response and wear properties of a two-phase Zn – Al alloy (Zn-8 wt.% Al, ZA-8) have been studied after severe plastic deformation by equal channel angular extrusion (ECAE). The experimental results reveal that the strength levels of the ECAE processed samples were considerably improved regardless of the processing route. More importantly, the elongation at fracture was dramatically increased after ECAE. The optimum tensile properties (high strength and high ductility) were reached after eight ECAE passes following route BA. It was also found that the wear rate of severe plastically deformed ZA-8 is considerably lower than that of the as-cast alloy, especially under high applied pressures, demonstrating improved wear resistance of ZA-8. Moderate strength and high ductility along with improved wear resistance in the ECAE processed ZA-8 samples in comparison with the brittle as-cast alloy makes these alloys attractive for wear-sensitive structural applications.

Mechanical properties of severely deformed ZA-27 alloy using equal channel angular extrusion

Abstract As cast ZA-27 alloy was subjected to equal channel angular extrusion (ECAE) with up to four passes using three different processing routes, and its mechanical properties (strength, hardness, ductility and extrusion load) were evaluated. The changes in the microstructure were also investigated. The ECAE was found to be quite effective in enhancing the mechanical properties of ZA-27 alloy. The strength and hardness of the alloys increased after the first ECAE pass followed by a gradual decrease with further passes for all processing routes. The elongation to failure, however, exceptionally increased with increase in the number of passes for all processing routes. Combined high strength and good ductility were obtained in the alloy after the first pass. The strength and maximum extrusion load showed similar trends with the number of passes for all processing routes.

Enhancement of Tensile Ductility of Severe Plastically Deformed Two-Phase Zn-12Al Alloy by Equal Channel Angular Extrusion

Tensile properties (mainly the ductility and fracture mode) of two-phase Zn-12Al alloy subjected to severe plastic deformation (SPD) via multi-pass equal-channel angular extrusion (ECAE) following route-Bc were investigated. As a result of ECAE processing, elongation to failure (as a ductility) of the alloy increased substantially and continuously with increasing the number of ECAE passes. However, the majority of the tensile strains are obtained in the state of plastic instability and therefore the uniform strains achieved along the gage length are very limited for this alloy. On the other hand, the strength of the alloy increased with increasing the number of passes up to 2, above which it decreased. The alloy sample after four ECAE passes exhibited 168% total elongation to failure at room temperature, which was 26 times higher than that of the as-cast one. This result indicates that multi-pass ECAE is effective on improving the tensile ductility of binary Zn-Al alloys. The fracture mode of the as-cast alloy samples completely changed after multi-pass ECAE and the brittle fracture behavior of the as-cast alloy was transformed into the ductile mode after processing.

Effect of Natural Aging on RT and HSR Superplasticity of Ultrafine Grained Zn-22Al Alloy

Zn–22Al alloy was processed using a well-designed two-step equal channel angular extrusion/pressing (ECAE/P), and ultrafine-grained (UFG) microstructure with 200 nm grain size was achieved. UFG Zn-22Al was subjected to long-term (up to 60 days) aging at room temperature (RT) and it was seen that natural aging caused limited grain growth in the microstructure. Grain sizes of about 300 nm, 350 nm and 350 nm were measured after 15, 30 and 60 days aging, which mean that UFG Zn-22Al alloy has a good microstructural stability at RT up to 60 days. ECAPed Zn-22Al alloy showed a maximum elongation of about 400% at a high strain rate of 5x10-2 s-1 and maximum elongation decreased with increasing grain size. Elongation to failures of ~375% and ~350% were obtained with the samples having 300 nm and 350 nm grain sizes, respectively. In addition, natural aging slightly decreased the strain rate at which superplastic region formed. While the maximum elongation occurred at the strain rate of 5x10-2 s-1 in ECAPed UFG alloy, it took place at lower strain rate of 1x10-2 s-1 after aging for all time periods. Also, flow stress of the alloy increased with increasing grain size during natural aging.

Microstructural and Mechanical Evolution of a Low Carbon Steel by Friction Stir Processing

A low carbon steel (Grade A) was subjected to friction stir processing (FSP), and the effect of FSP on the microstructure and mechanical properties was investigated systematically. It was found that two distinct zones called stir zone (SZ) and heat-effected zone (HAZ) were formed during FSP. The SZ and HAZ consist mainly of ferrite, widmanstatten ferrite, ferrite+cementite aggregates, and martensite. FSP considerably refined the microstructure of the steel by means of dynamic recrystallization mechanism and formed a volumetric defect-free basin-like processed region. The ferritic grain size of the steel decreased from 25 µm in the coarse-grained state to about 3 µm in the fine-grained state, and the grains formed were separated mostly by high angle of misorientation with low density of dislocations. This microstructural evolution brought about a considerable increase in both hardness and strength values without a considerable decrease in ductility. Ultrafine-grained microstructure formed around and just beneath the pin increased the hardness of the steel from 140 Hv0.3 to about 245 Hv0.3. However, no hardness uniformity was formed throughout the processed zone due to the changes in deformation- and temperature-induced microstructure. Both yield and tensile strength values of processed zone increased from 256 and 435 MPa to about 334 and 525 MPa, respectively.

Improvement of wear behaviour of titanium by boriding

Purpose The purpose of this paper is to achieve a hard and protective borided layer on commercially pure Ti (grade-2) by applying boriding, and to investigate the changes in its microstructure, hardness, friction and wear behaviors. Design/methodology/approach Pack boriding technique was used to form a hard boron diffusion layer on titanium substrate. A powder mixture of amorphous boron and anhydrous borax was used as a solid-state boriding media, and then the boriding was carried out under inert atmosphere. Findings A thick dual boride layer consisting of a monolithic titanium diboride (TiB2) on the top and titanium monoboride (TiB) whiskers beneath that layer formed at relatively low diffusion temperature under pressured inert argon atmosphere in a boriding media containing boron source and activator. With boriding at specified conditions, very hard (4100 Hv0.01) and thick monolithic TiB2 layer formed on the top-most layer which is required for improved tribological applications. Hardness decreased gradually through the TiB whisker layer and finally reached to the hardness of base material. Originality/value This paper investigates the effects of components of boriding mixture and conditions of thermal treatment on the formation of borided layer and its properties. In previous studies, boriding mixtures containing a boron source, an activator and a filler material was generally used at high temperatures around or above 1,050°C to achieve a thick monolithic layer on the top of the surface of titanium. In the present study, no filler material was used to accelerate the boron diffusion because filler materials may inhibit the diffusion of boron atom through the surface of substrate of titanium. Also, diffusion treatment was carried out under pressurized argon atmosphere at relatively low diffusion temperature to achieve boride layer with the improved hardness and durability.