Microstructure Of Za48 Alloy. Micro-alloying And The Effect Of Different Cooling Rates, The Performance Study

Zinc-aluminum (ZA) alloy with excellent wear resistance, mechanical properties and a significant economy, could replace copper alloy for wear-resistant materials, and instead of bronze for low-, medium-speed medium temperature overloading bearings. Despite the advent of ZA alloy is not long, their relevant scientific research and technological development is very fast and it also has a very wide range of applications. Along with economic development, large and special-shaped pieces of products are more and more widespread; a higher demand has been set to the performance alloy products. The developed countries have developed a series of high aluminum zinc-based anti-friction materials for different conditions to select. However, the majority metallurgical enterprises in domestic are still using conventional brands of ZA27 alloy due to the long-standing traditional value. A large number of studies have shown that high aluminum zinc-based alloy has a more excellent performance than of eutectic or eutectoid ZA alloy. Therefore, the high aluminum zinc alloy was chosen in this paper and the alloying, metamorphic, improving casting technique, different solidification, repair, and means to improve the mechanical properties of the alloy. It would have important theoretical and application value for improving the performance of ZA Alloy. It also expands the scope of application of ZA alloy.The influence of different high aluminum content on the microstructure, tensile and wear properties of zinc-based alloy was studied firstly in this paper. The test results show that the tensile strength and abrasion resistance are increased with increasing aluminum content. Zn-Al alloys basically comprise a mixture of primary phase a and eutectoid (α+η).The increase of aluminum content in the alloys conducted the increase of quantity of primary phase a and the decrease of eutectoid (α+η).Theα-Al rich solid solution has a face-centred cubic structure and acts as a strengthening and toughening phase. Its strength and hardness were higher than of theηphase.According to test results, ZA48 alloy with a better performance was chosen as the main object of study. Si and Ti elements were used to strengthen. The effect of different Ti, Si contents on the mechanical properties of ZA48 alloy was studied and the following results were gotten. Titanium improved the mechanical properties effectively, and when the titanium content reaches 0.04%, the mechanical properties achieve maximum value. The refinement mechanism of Ti is the form of Al66Ti25Zn9 particles in ZA alloy and it can be used as heterogeneous nuclei. Then the primary phase a gets refined effectively and the matrix is strengthened. Mechanical properties of ZA48 alloy decreases with the increase of silicon content, especially as the primary silicon phase appears in alloy. The sharp points or cutting edge of eutectic Si and primary Si phase are prone to produce stress concentration and weaken the combination between silica and matrix. The micro-cracks are formed under the effect of applied stress, reducing the alloy mechanical properties.Effect of cooling rate on the microstructure of conventional casting ZA48 Alloy has been systematic studied. The relationship between the dendrite arm spacing and cooling rate was established. The study found that a large number of small particle dispersion precipitated from a phase at higher cooling rate. The precipitate uniform mass distribution in the a phase, then the strength is improved effectively. The spectrum analysis showed that the proportion of segregation was avoided effectively with the increase of the cooling rate. The microsegregation of various elements was also reduced as the cooling rate increased to a certain degree. It believes that the solute redistribution is the root to generate segregation during solidification. Under the non-equilibrium solidification conditions, the actual solute partition coefficient k* of the solid-liquid interface more tends to 1 with the increase of solidification rate. The solid-phase composition of the solid-liquid interface is more close to the liquid phase composition Co, and thus reduces the dendrite segregation.The rapid solidification ZA48 ribbon was prepared by single-roller spinning. According to the heat transfer characteristics, combined with conduction theory and solidification theory, the mathematical analysis method was applied to calculate the cooling rate of ZA48 ribbon. The cooling rate of a 50μm thick ZA alloy ribbon prepared by single-roller spinning is about 105K/s. The time-related non-homogeneous nucleation theory describes the nucleation characteristics of rapidly solidified ZA48 ribbon. With the increasing cooling rate, the phase selection sequence of rapidly solidified ZA48 ribbon isα-Al→η-Zn phase, aluminum element content also has great effect on the nucleation incubation period of the a-Al phase.Effect of cooling rate on the microstructure of ZA48-Ti alloy and ZA48-Si alloy has been systematic studied. Under the conventional casting conditions, the grains of ZA48-Ti alloy get refined with increasing cooling rate and Ti content. When the cooling rate increased to a certain value, Ti elements had no effect on the grain size and the cooling rate as the main reason for grain refinement. The eutectic silicon phase gets refined and the primary silicon phase morphology has no much changed with the increasing cooling rate. After rapid solidification, Ti and 5% Si completely dissolved intoα-Al solution. The large undercooling suppressed the nucleation and growth of silicon phase, the silicon phase does not precipitate and a large number of silicon and other alloy elements solid solution in the matrix.Simulated working conditions, the wear resistance of ZA48 alloy was analyzed systematically. According to the worn surface and wear sub-surface morphology, its wear mechanism was analyzed. The results show the improving wear resistance is the combined action of a andηphase. The softηphase prior to theαphase was removed and the hard a phases were protruded from matrix and acted as a loading phase. Extensive zinc transfer occurred and helped to act as a natural lubricant in sliding wear situations wherein the smearing behavior is facilitated and a lubricating film on mating surfaces is formed. Meanwhile, the iron transfer from the steel ring to block and forced to recess continuously during sliding wear, which forms a thin film at the contact surface between the composite and the counter face. It is equivalent to a number of reinforced particulates added to ZA48 alloy, and its load bearing capability would be improved. The influence of Ti elements on the wear resistance of ZA48 alloy was also studied. The results showed that whether the lubrication conditions or non-lubricated conditions, the adding Ti by electrolysis improved the wear resistance of ZA48 alloy. The reason can be attributed to the decrease of grain size and the increasing of grain boundary area, which induced the shearing force increased with the decrease of grain sizes.Whether lubrication or no lubrication conditions, Si can improve the wear resistance of ZA48 alloys significantly and the extent of the increase is far greater than that of Ti. The hardness of silicon particles of up to 757HV, far higher than that of ZA48 alloy, silicon particles are added to play a role of hard particles, then the wear resistance is improved effectively. The size and shape of silicon particle also have great effect on the wear resistance of alloy. Thick and uniform silica particles are prone to form cracks, and cracks easily along the brittle silicon phase expansion. It has a negative impact on the wear resistance of the material. Diffuse distribution of short rod-like or granular eutectic silicon can increase the hardness of the matrix and enhance furrow abrasion resistance. It also reduced the hardness difference between substrate and primary phase silicon. Thereby improving the coordination of the two phases, enhance the wear resistance of alloys.