| 8418 hot-working mould steel is a kind of excellent high temperature mould steel developed by Switzerland Assab Company. Due to its good ductility, resistance to temper softening and high temperature strength, it has been much favored by the majority of die-casting companies. During die-casting process, the temperature was maintained between 650~700 oC for magnesium alloy, while the temperature for aluminum soup should be exceed 700 oC and injection speed was maintained between 40~90 m/s. On the condition of high temperature aluminum liquid corrosion and erosion, variety of defects were formed on 8418 hot-working mould steel surface, such as die-sticking, cracking. In order to improve the die casting mould life and reduce frequency of maintenance, multi-arc ion plating technology was used to deposit Ti/CrN baitlayer coating and Ti/TiN/CrN three-layer coating on the surface of 8418 hot-working steel. In this paper, 8418 hot-working steel, Ti/Cr N and Ti/TiN/Cr N coatings were used as research object and CrN coating room temperature properties were studied at different process conditions. At the same time, 8418 hot-working steel, Ti/Cr N and Ti/TiN/Cr N coatings’ corrosion resistance in liquid aluminum and thermal fatigue resistance also were comparatively studied. The results are as follows:(1) When substrate negative bias and arc current were maintained at 100 V and 80 A, respectively, optimum performance Cr N coating could be obtained. On this condition, the number and diameter of large particles on the surface of Cr N were in the middle of the state. The max binding force between Cr N and 8418 hot-working steel was 47.5 N and micro-hardness was HV2780.(2) Ti/Cr N and Ti/TiN/CrN coatings were obtained at the condition of substrate negative bias 100 V, arc current 80 A. The result showed that the total thickness of Ti/Cr N and Ti/TiN/Cr N coatings were 2.02 μm and 1.92 μm, respectively. Also, both coatings had a good bonding with 8418 hot-working steel and no obvious defects could be observed.(3) 8418 hot-working die steel was corroded very fast. At interface of 8418 hot-working steel with A356 molten aluminum, Fe2Al5 compound was generated. When molten aluminum diffused into Fe2Al5 phase, Fe2Al5 phase changed into Al4.5FeSi and Al5 FeSi phase, and the etching depth with etching time substantially satisfy the linear relationship.(4) Compared with 8418 hot-working mould steel, corrosion resistance of Ti/CrN and Ti/TiN/Cr N coatings in liquid aluminum were significantly improved. Ti/Cr N coating was destroyed after dipped into melt aluminum for 4 hour,but Ti/TiN/CrN coating did not completely fail. This result showed that TiN intermediate intervening layer had a strengthening effect to Ti/CrN coating in terms of corrosion resistance in liquid aluminum. On the other hand, micro-pits and micro-particles on the surface of coatings were the initial corrosion points.(5) 8418 hot-working die steel was oxidized during the thermal shock process. Because Fe2O3 was generated and peeled off from surface of 8418 hot-work steel constantly, a "peeling" phenomenon appeared during thermal shock process. Ti/CrN and Ti/TiN/Cr N coating cracks propagated in the form of mesh, and the maximum crack depth were 15 μm and 9 μm, respectively. |
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