Erosion Of Die-casting Aluminum Alloy On Mold Steel Surface Layers

During the aluminum die-casting process, mold surface contacts with molten aluminum directly, thermal stress on the surface changes for a long time, washout and friction of solid particles in liquid alloy. After a certain number of cycles, intermetallic compounds were formed and attach on mold surface during the diffusion of Fe, Al atoms and chemical reactions with each other, leading to a soldering phenomenon. In order to reduce the damage of mold, prolong its working life, a serious of surface treatments such as surface coating, oxidation, nitriding, laser hardening and so on spring up, providing theoretical reference for industrial production.To study the influences of different surface treatments for mold steel on its antierosion performance. This paper chooses H11 and die-casting aluminum alloy A356 as experiment materials. Anti-erosion performances for aluminum alloy were studied of oxidation layer and nitrided layer on mold steel surface, skills and methods were used such as SEM（Scanning Electron Microscope）, EDS（Energy Dispersive Spectroscopy）, XRD（X-Ray Diffraction） and DSC（Differential Scanning Calorimetry）. Appearance and microscopic morphologies were observed and analyses on chemical composition, phase species as well as change of heat flow in different groups were carried out respectively. To research the difficulty of aluminum alloy’s reactions with oxide layer and nitrided layer on mold surface, their activation energy was calculated too. Based on these experiment results and theoretical analyses, we can draw conclusions as follows.（1） After the erosion test, there were corrosion traces on the surface of sample A ordinary heat treatment and it’s a little obvious that massive spalling character. Only a small area was corroded on sample B after oxidation and there were consecutive corrosion pits, connected with each other by a way of dense distribution on nitride sample C. The data of weight loss in three groups gradually increased as the dipping time extending. Weight loss rate from big to small orderly are: oxidation < ordinary heat treatment < nitriding. The oxidation layer stops inter-diffusion of Fe and Al atoms, slow down the process of their chemical reactions, improving the anti-erosion performance of mold steel to aluminum alloy.（2） Fe₂Al₅ binary alloys and Fe₃Al₂Si₃, FeAl₃Si₂ ternary compounds were detected on the oxidation sample after immersion in aluminum, but only Fe₃Al and FeAl were found and no ternary alloys on nitriding sample. The driving force to form FeAl is lower than that of Fe₂Al₅, so it’s easier to proceed for the latter. On the other hand, element Si on the surface of oxidation sample decreases the activity and diffusion rate of Al, limiting the formation of intermetallic compounds, which can protect mold surface well and prolong its working life.（3） The heat flux released during chemical reactions between mold steel which was processed in three different ways and aluminum from large to small is that nitriding > ordinary heat treatment > oxidation, indicating that the degree of reactions between nitriding layer and aluminum is more fierce than that of oxidation layer. Three peaks in DSC curves, we can get to know that the activation energies of chemical reactions between different surface treatment layers and aluminum from small to large is nitriding < ordinary heat treatment < oxidation, it’s a good explanation for that the nitriding layer has a prior tendency to oxidation layer to react with aluminum. So when it comes to the performance of anti-hot aluminum erosion, the oxidation process is better than that of nitriding.