Analysis of the behaviour of steel components in contact with AZ91D and AM60 magnesium alloys in HPDC
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2014
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World Foundry Organization
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The problems encountered in High Pressure Die Casting of magnesium alloys due to the interaction between steel components and the melt alloys are approached. The performance of different steels as well as surface treatments has been compared through a test consisting of the immersion of steel samples in melt AZ91D and AM60. The final objective is to understand the mechanisms of steel degradation through the analysis of the reaction layer and to select the most optimum steel/coating system for magnesium HPDC applications. The materials that are presently used for applications in contact with melt magnesium are usually low carbon stainless steels that do not contain any nickel or copper. Iron reacts with both aluminium and manganese present in magnesium alloys creating an intermetallic reaction layer. Six different steels were selected and tested. It was seen that the main intermetallic layer is composed of Fe2Al5. It is created when AZ91D alloy attacks the Steel and it is thinner in steels with low Mo and V contents. Furthermore the higher the V content the thicker the layer thickness. Subsequently two steels were selected that showed the best performance in the immersion test with the goal of studying different surface treatments that could enhance this performance. The test was repeated with treated samples in both magnesium alloys and a complete microstructural analysis was carried out to understand the reaction mechanisms and select the most appropriate steel alloys. The details of the procedure and results are presented in this work. Following the main conclusions are summarized: It can be said that both PVD and nitriding treatments are capable of minimizing the formation of the intermetallic layer after 3 hours of contact with melt magnesium alloys. The AlCrN coating applied by PVD presents a very good aspect in the case of the 1.2343 steel. It keeps the original thickness and is totally homogeneous all over the surface in contact with the magnesium alloy. The same coating was doped with yttrium and zirconium but no further improvements were appreciated. In the case of the 1.2888 Steel the AlCrN coating presented some cracks after the immersion trial. When doped with Zirconium the cracks did not appear but its thickness diminished and became irregular. Doping with Ytrium did not provide a good behaviour. The coating thickness was irregular and even disappeared in some areas. In the case of the samples treated with the gaseous nitriding process a large porosity was observed into the coatings. This was more evident in the 1.2343 steel. In service conditions these pores could make it possible that Al and Mn diffused through the coating into the substrate with the subsequent formation of the intermetallic layer and loss of properties in the substrate.
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Egizabal , P , Merchán , M , Corengia , P , García , C & Alberdi , M 2014 , Analysis of the behaviour of steel components in contact with AZ91D and AM60 magnesium alloys in HPDC . in 71st World Foundry Congress : Advanced Sustainable Foundry, WFC 2014 . 71st World Foundry Congress: Advanced Sustainable Foundry, WFC 2014 , World Foundry Organization , 71st World Foundry Congress: Advanced Sustainable Foundry, WFC 2014 , Bilbao , Spain , 19/05/14 .
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