Solid particle erosion on coatings employed to protect die casting molds

摘要

In this study, the performance of coatings that were subjected to solid particle erosion tests was evaluated. These coatings can be used to protect die casting molds. The main interest of this research project was to find possible alternatives to increase the wear resistance of these mechanical components. Die casting is a metal casting process that is characterized by forcing molten metal under high pressure into a mold cavity, which is machined into two hardened tool steel or coated dies. Most die castings are made from non-ferrous metals, such as aluminium, copper, magnesium, lead, zinc, tin based alloys but also they can be protected by specific coatings that have high wear resistance. The erosive wear damage in die casting molds is caused due to the molten metal is blown into the mold by high pressure dry air. Filling the mold cavities during the blown step, molten metal and sand particles impact the internal surface producing erosive wear damage. Coatings such as chromium nitride (CrN) and titanium aluminium nitride (TiAlN) that exhibit low wear damage in these types of applications due to high abrasion and erosion resistance were tested. In addition, uncoated 4140 steel and 6061 aluminium were also tested. An erosion test rig similar to that shown in ASTM G76-95 standard was designed and built to perform the tests. The abrasive particle used was angular silicon carbide (SiC) with a particle size of 420-450 μm. Tests were carried out using different impact angles (30°, 45°, 60°and 90°) with a particle velocity of 24 ± 2 m/s and an abrasive flow rate of 0.7 ± 0.5 g/min. The particle velocity and the abrasive flow rate were low in all of the tests to reduce the interaction between the incident particles and the rebounding particles in the system. The surfaces were examined with a scanning electron microscope (SEM) to characterize the erosive damage. The wear mechanisms identified were pitting and ploughing action at low impact angles (α ≤ 45°) due to sliding component commonly observed at these incident angles whereas bigger craters, radial cracks and a more roughened surfaces were seen at angles near or at 90°.In addition, it was observed that the damaged area was extended in all of the cases at 30°and 45°reducing considerably at 60°and 90°. The wear scars were characterized by an elliptical shape at 30°and 45°, which is a characteristic feature when the specimens are impacted at low-incident angles (α ≤ 45°) whereas a roughly circular was seen at 60°and 90°.