Preparation And Investigation On Microstructures Of Monel Alloy With Al Addition

Monel alloy with Al addition is widely used in many fields due to its excellent corrosion resistance in sea water,acid,alkali and salt.With development of industry,the increasing stringent properties are required for Monel alloy with Al addition.A reasonable selection of melting technology and composition is an effective way to improve microstructure and properties.Al Monel alloys with different melting technologies and the content of Al addition were produced by vacuum melting method.The characteristics of the microstructure of the alloy as well as the effect of melting parameters and the content of Al addition on the microstructure of the alloy were studied as well.The following conclusions can be drawn from this investigation:The microstructure of the alloy consists of dendriticγmatrix andγ' phase(Ni₃Al intermetallic compound),whereinγ' phase has two morphologies.One distributes dispersedly in single particle on the dendrite arm and exists in the petalform shape in the transition area between dendrite arm and interdendrite.The other is devoiced eutecticγ' phase which distributes in the interdendrite at different melting parameters and Al additions.Meanwhile,the fraction of eutecticγ' phase increases with increasing Al addition.With increasing melting temperature,the dendriticγphase becomes freer,the fraction of devoiced eutecticγ' phase increases and the size ofγ' phase on the dendrite arm becomes smaller.In addition,with increasing melting temperature,the hardness of the alloy increases and the difference of it at different locations has small deviation.This suggests that the alloy has less macrosegregation.Meanwhile,the variation of microhardness from dendrite arm to the interdendrite becomes smaller.It indicates that microsegregation of this alloy becomes lower as well.At 1450℃,the maximum hardness of the alloy can reach HB272.The deviation of hardness and the macrosegregation along with microsegregation is small. With increasing holding time,γphase becomes freer and then coarser.The fraction of eutecticγ' phase increases then decreases.However,the size ofγ' phase distributed on the dendrite arm has the contrary tendency.When holding for 60min,γphase is freer,the fraction of eutecticγ' phase is larger and the size ofγ' phase is smaller compared with those at other holding times.Besides,when the holding time increases,the hardness of the alloy increases and then decreases.At the holding time of 60min,the maxiumum hardness can reach HB272 in this investigation,there are small deviation of hardness at different locations of the test bar and less macrosegregation.Meanwhile,the variation of microhardness from dendrite arm to interdendrite is smaller.It indicates that this alloy has less microsegregation.The mechanism of corrosion in the simulative sea water in the range of Al contents is pit corrosion.As the fraction of Al increases,corrosion resistance in the simulative sea water increases and then decreases.The Monel alloy with 3.0wt%Al addition has the best corrosion resistance.