Ilija Bobić

Tribological properties of A356 Al-Si alloy composites under dry sliding conditions

Purpose – Aluminium alloys are frequently applied in automotive and other industries, since they provide mass reduction. Besides positive effects, aluminium alloys have their shortcomings reflected, first of all, in inappropriate tribological properties of these materials. The aim of this research was to enable the production of cheap aluminium alloy matrix composite with favourable combination of structural, mechanical and tribological properties, focusing on the tribological behaviour. Design/methodology/approach – The A356 Al-Si alloy was used as a matrix for producing metal matrix composites in compocasting process. Three different materials, in form of particles, were added to the matrix (Al2O3, SiC and graphite). Hardness and tribological properties (wear, friction and wear mechanism) of heat-treated (T6) samples were examined and compared. Tribological tests were carried out on ball-on-block tribometer under dry sliding conditions. Sliding was linear (reciprocating). Counter body was alumina ball. ...

Microstructural and sliding wear behaviour of a heat-treated zinc-based alloy

The effect of heat treatment on microstructure, hardness, tensile properties, fracture mode and wear behaviour during lubricated and dry sliding of the zinc-based alloy with 25 wt.%Al was studied. Microstructural investigation and chemical analysis of as-cast and heat-treated specimens, the fracture and worn surfaces, as well as wear debris were performed by scanning electron microscopy and energy dispersive spectroscopy. Wear tests were carried out using a disc-on block-type wear machine. By a relatively simple heat treating consisting of a short-term annealing in the single-phase region followed by water-quenching, the elongation has been markedly improved, while the strength was maintained high. The results indicate that the wear rate strongly depends on the microstructure, applied load and sliding conditions. The wear rate increases with load, and under dry sliding conditions the wear rate is approximately two orders of a magnitude higher than under lubricated conditions. During dry sliding the best wear behaviour was displayed by the water-quenched specimens, whereas slowly cooled specimens showed the higher wear rate. Lubrication strongly affects the wear behaviour. Contrary to dry sliding, slowly cooled specimens exhibit the best wear properties under lubricated conditions. The wear mechanisms were proposed for dry and lubricated sliding.

Optimization of tribological properties of aluminum hybrid composites using Taguchi design

This paper analyses the influence of graphite reinforcement, load and sliding speed with constant sliding distance on tribological behavior of A356 aluminum matrix composites reinforced with 10 wt.% silicon carbide and graphite using the Taguchi design. Hybrid composites were produced in the compo-casting process. Tribological tests were performed on a block-on-disc tribometer where the weight percentage of graphite has three variations (0, 3, and 5), as well as load (10 N, 20 N, and 30 N) and sliding speed (0.25 m/s, 0.5 m/s, and 1 m/s), with sliding distance of 300 m. The wear of the composite is investigated under dry sliding condition. The specific wear rate was analyzed using Taguchi method with the aim of finding the optimal parameters. By applying analysis of variance, it was determined that the best tribological properties has A356/10SiC/3Gr hybrid composite. It was also found that the greatest impact on specific wear rate has load with the percentage effect of 69.163%, then sliding speed with 14.426% and the interaction between wt.% graphite and load. The dominant wear mechanism is adhesive wear as confirmed by scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS).

Corrosion behaviour of thixoformed and heat-treated ZA27 alloys in NaCl solution

Abstract The influence of corrosion on the microstructure of thixoformed and heat-treated ZA27 alloys was investigated. The microstructure of ZA27 alloy was affected by heat treatment. The process of electrochemical corrosion occurs in both ZA27 alloys through the area of η phase. According to the results of immersion test and electrochemical measurements, the corrosion rate of the thixoformed ZA27 alloy is at least 50% lower than that of the thixoformed and thermally processed alloy. This indicates the unfavourable influence of applied heat treatment (T4 regime) on the corrosion resistance of the thixoformed ZA27 alloy.

Microstructure and mechanical properties of Zn25Al3Cu based composites with large Al2O3 particles at room and elevated temperatures

Abstract Microstructures and compressive properties of Zn25Al3Cu alloy and Zn25Al3Cu/Al2O3 particulate composites with large reinforcing particles (250 μm) were examined. The composites were obtained by the compocasting technique through infiltration of 3, 8 and 16 wt.% Al2O3 particles into the semi-solid metal matrix. The influence of temperature in the range from 70 to 170°C on compressive yield strength of the matrix alloy (as-cast and thixocast Zn25Al3Cu alloy) and the composites was investigated. Above 70°C compressive yield strength of all materials decreases, but the rate of decrease is different for the matrix alloy (as-cast and thixocast) and composites. It was found that the abrupt decrease in compressive yield strength of the matrix alloy occurred at temperatures higher than 70°C, whereas composites retained relatively high values of compressive yield strength till the end of the testing temperature range regardless of the amount of reinforcing particles.

Corrosion influence on surface appearance and microstructure of compo cast ZA27/SiCp composites in sodium chloride solution

Abstract The influence of corrosion on the surface appearance and microstructure of particulate ZA27/SiCp composites was examined after 30 d immersion in a sodium chloride solution with the access of atmospheric oxygen. The composites with different contents of SiC micro-particles were synthesized via compo casting. Microstructural studies by means of optical microscopy (OM) and scanning electron microscopy (SEM) showed that corrosion occurred in the composite matrices, preferentially in regions of the η phase, rich in zinc. The corrosion processes did not affect the silicon carbide particles incorporated in the matrix alloy. According to the results of electrochemical polarization measurements, an increase in the content of SiC particles in the composite matrice has led to the lower corrosion resistance in the composites.

Artificial aging of thixocast ZA27 alloy and particulate ZA27/SiCp composites

Abstract Thixocast ZA27 alloy and particulate ZA27/SiCp composites were subjected to artificial aging at 80, 120 and 160°C (T5 regime). Composites with 5 and 10 vol.% SiC particles were produced via the compocasting method. The influence of the aging was investigated using different techniques. Differential scanning calorimetry was performed to reveal the presence of phases created in the thixocast ZA27 alloy and the composites. Microstructures and fracture behavior were examined by means of optical microscopy, scanning electron microscopy and X-ray diffraction. Aging processes in the matrix alloy and composites were followed using hardness measurements. A decrease in the hardness values was noticed with the increase in the aging temperature. At higher temperatures diffusion of zinc atoms from supersaturated phases in the matrix alloy was enhanced, which resulted in faster changes in the hardness. Aging processes in the composites were significantly accelerated compared to the matrix alloy, due to the presence of particulate reinforcements. The maximum in hardness was achieved for shorter time in the ZA27/SiCp composite with higher volume fraction of SiC particles.