Die casting act as developed production technology in whole manufacturing industry,especially for automobile industry.In trend of light weight of automobile,more and more aluminum die casting parts are used.Hard spot of die casting parts is common defect which cause machining failure.The machining failure include rough surface finishing,unproper tolerance,short life span of machining tools.
Hard spot has two main types.One is non-metallic hard spot which mainly composed by metallic oxide.Its hardness from 200 to 1400HV.The other is metallic hard spot which composed by silicon.Its hardness is quite high.Non-metallic hard spot is big and can be found in normal microscope.Metallic hard spot hardly found in normal microscope.Sometimes,non-metallic hard spot exist together with metallic hard spot in aluminum alloy,which affect machining performance and cause machining tools wear out and failure.
Non-metallic hard spot mainly differed as following categories
- Aluminum oxide,One type with many holes and its hardness can be 600HV to 700HV.The other without holes in dark colour and its hardness can be 1300HV to 1500HV.
- Aluminum Magnesium oxide,its hardness can be about 1000HV.
- Silicate include aluminum oxide,calcium oxide,silicon oxide.Its hardness can be about 1500HV.
- Carbide include aluminum carbide and aluminum carbide compound.Its hardness can be about 700HV.
When aluminum liquid in high temperature and kept in quite long time,aluminum oxide will turn into hard form.To avoid hard aluminum oxide,clean residue of aluminum liquid and wall of furnace.Avoid any oxidation into aluminum liquid.Clean ladle and other equipment regularly.
For metallic hard spot,its hardness can be about 1000HV.AlSMnFe compound can be 700HV in hardness.Some rich Ti chemical compound,the hardness can 900HV.
For AlSMnFe compound,the hard spot will exist if it contain more Fe and Mn in low temperature aluminum liquid for long time.To avoid it,firstly,strictly control Fe,Mn,Ti in aluminum alloy.secondly,use higher temperature.thirdly,apply fire resistance material onto furnace.Fourthly,clean residue regularly.
Hard spot mainly caused by raw material,melting equipment and technology,thermo processing.Therefore,manufactures have to strictly control quality of raw material,melting and thermo processing.Meanwhile,proper management is necessary.
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