SLURRY WEAR CHARACTERISTICS OF ZINC - BASED ALLOYS AND COMPOSITE

Abstract

This study deals with observations made during erosion-corrosion testing of cast-zinc-based alloys/composite. Experiments were conducted by the sample rotation method in different environments using a slurry test apparatus. The experimental parameters whose effects were studied included the distance traversed (17-637 km), speed of rotation (4.71 and 7.06 m/sec.), the composition of the slurry (0-60% sand in a solution comprising 4 gm sodium chloride and 5 cc concentrated sulfuric acid in 10 liters of water). Material related variables studied were the effects of the presence of fine silicon particles ' as well as coarse alumina particles on the response of a zinc-based alloy. • The study suggests that the. overall behaviour of the specimens is greatly controlled by factors like the traversal distance, speed of rotation, slurry composition and the microstructural features of the samples. These factors were found to have varying degrees of effects and the predominance of one set of parameters over the other decides their net influence on the wear characteristics of the materials. Microstructural aspects form another variety of variables having a significant impact on the wear response. Microstructure of the zinc-based alloy without silicon/alumina revealed the presence of primary dendrites of aluminium rich solid solution of zinc (a) along with eutectoid a+, (ii being a zinc-rich solid solution of aluminium) in the interdendritic regions. Particles of the metastable copper-zinc containing phase (CuZn4) were also observed along with the eutectoid (a+,~) within the dendritic boundaries. Addition of silicon to the alloy system led to the formation of eutectic and primary silicon particles. Microstructural features of the composites delineated its matrix microstructure to be identical to the •one exhibited by the, alloy without silicon while the presence of alumina particles was also observed. Hardness of the composite was maximum while that oP the alloy without silicon/alumina the least with the silicon containing • alloy attaining an intermediate value. No significant variation was noticed in the densities of the three varieties of •the specimens. The electrical conductivity of the silicon/alumina-free alloy was the highest while that of the composite was the lowest with the silicon containing alloy again showing an intermediate value. The wear rate initially increased with the traversal distance of the samples, attained a maximum and then assumed a practically steady state value or exhibited a marginally declining trend at longer distance. On. the contrary, the weight loss increased with distance, irrespective of the speed of rotation and the composition of the slurry and samples. The wear loss as well as wear rate increased with the speed of rotation. However, the slurry composition had a mixed influence wherein wear loss/rate increased with the sand content of the slurry in the beginning, attained a peak value at an intermediate amount of sand (in the slurry) and finally decreased when the concentration of sand particles was increased further. From microstructural considerations, the incorporation of either hard silicon or alumina phase improved the wear-resistance of the zinc-based alloy. However, the degree of their effectiveness was noted to vary depending upon experimental conditions. For example the presence of fine silicon particles became more beneficial than the coarser alumina particles when the slurry composition was towards the higher concentrations of sand (40% and 60%). Contrary to the above, the trend reversed for slurry composition corresponding to lower concentrations of sand (0% and 20%). An appraisal of the observations made in this investigation suggests that there exist several experimental as well as material related variables which govern the response of the material(s) under a given set of test conditions. The nature of control is greatly dependent on either of the parameters and even a minor change in them can considerably alter the order of the effects observed. It thus becomes imperative to truely assess the same through experimentation in order to develop a better understanding on the subject matter