Experimental Simulation of Reduction of Erosion Damage in Dies Used in Aluminium Casting

A review of damage mechanisms in dies used for aluminium casting revealed that erosion due to the molten/semi-solid aluminium droplets entering the die was a major cause of wear. It was also clear that no laboratory tests were available to simulate the die/aluminium interactions. Existing test methods use actual casting machines which is time consuming and expensive. The aim of this work was to develop a laboratory test and then to use it to evaluate current die materials as well as possible treatments/coatings that could be used to reduce the problem. A test was developed that used a shot blaster to propel aluminium balls at flat or cylindrical specimens (to cover possible die geometry variations). It was also possible to heat the specimens. A pulsed method was used for the ball application to simulate multiple castings. Ball motion was evaluated and specimen wear characterised across a range of ball impact velocities. Different impact angles were used for flat specimens and for cylindrical specimens central and eccentric ball flows were used. High speed video was used to investigate the actual impact velocities and also observe the behaviour of the aluminium balls impacting at different angles to see how the wear mechanisms actually occurred. Four regimes of particles behaviour: impact only, impact and sliding, sliding only and pressed only were seen. Wear testing showed that wear increased with impact velocity, but different effects were seen when varying the impact angle as seen previously in studies on erosive wear of spherical solid particles for ductile materials. At 30o impacts, a higher erosion rate was seen than those at 60o and 90o impact angle. Zig zag indentations were seen at high impact angles indicating a higher displacement and removal of material by plastic flow keeps advancing downstream until the individual peaks and valleys meet. Wear rate results highlighted four periods with exposure time: incubation period, acceleration or accumulation period, deceleration period and steady-state period. Tests on cylindrical H13 specimens showed that a high amount of wear occurred at an eccentric position due to increased cutting action. The effects of elevation the temperature in the system on the erosion behaviour were also studied. At high temperature materials soften and the erosion increased. Treated surfaces were also studied to evaluate selected candidate coatings and generate data on their wear resistance. The results showed that coating can have a large effect on erosion damage. The knowledge gained by experiment has contributed to the understanding of die failure and will increase die lives as well as reducing maintenance, machine downtime and labour costs