Research On The Microstructure And Properties Of Unleaded Si-Mg-P Brass

Silicon,magnesium,phosphorus and aluminum are used to replace lead to develop a kind of free cutting brass.To achieve the purpose,the alloy composition is designed using the orthogonal test table,and combining the zinc equivalent and the alloy phase diagram.The alloy is prepared by smelting,and a proper smelting process is developed according to the characteristics of the added elements.The influences of silicon,magnesium and phosphorus on the structure and properties of alloy are studied by orthogonal experiments.Based on the results,the contents of alloy composition are optimized.The microstructure and properties of this lead-free brass is studied systematically by means of metallographic microscope,the depth of field microscope,scanning electron microscope,energy spectrometer,X-ray diffractometer,synchronization,thermal analyzer,electronic universal material testing machine,brinell hardness tester,horizontal lathe,and other related instruments.Based on the theory of brittle fracture,the critical size of hard point particles is calculated,and the types of intermetallic compound and the mechanism of improving cutting are studied.The following conclusions are obtained:?1?Microstructure of this alloy is consisting of?phase,?phase and intermetallic compound.The composite phase which is composed of Mg₃P₂ and Cu₃P is distributed in?phase and the phase boundary,while Cu₂Mg is distributed in the?phase and the phase boundary.With the increase of Mg and P element content,the hard point particles which distribute in the alloy were continuously increased.The Si and Al elements are not involved in the formation of hard point particles,but dissolve in alloy matrix as solid solution.Most of the Si elements are in the?phase.?2?According to orthogonal experiments,Si element is more likely to cause the increase of hardness of alloy,while the influence of Mg and P element is less.P and Si elements have great influence on the strength of alloy,and Si elements have less influence on the ductility of alloy.The Si and Al elements can improve the corrosion resistance of the material.?3?The addition of Si,Mg and P elements are all beneficial to improve the machinability of the alloy,but Si and Mg have greater influence.The addition of Si elements will increase the hardness of the alloy,so the Mg elements increase the machinability most.The particles which are brittle will break the continuity of the alloy,and then approve the machinability of the alloy.?4?Heat treatment process of this alloy would be:solid solution treatment 2 hours in490?,then aging treatment for 1 hour in 270?.Before the aging treatment alloy should be cooled by water.Hardness of this alloy increase after heat treatment.?5?The mechanical properties of this alloy after optimization are:the tensile strength is higher than 346.5MPa,the hardness is more than 110.6,and the elongation is higher than6.94%.This alloy which has good mechanical properties and great machinability is possible to replace lead brass.