| Powder metallurgy (P/M) parts,which have many excellent and unique properties , such as net shape , economization on material and energy , cost-effective,unpolluted and good integrated properties,have been employed in the manufacture of all kinds of machine accessories.Sintering ferrous P/M products are the most widely used P/M materials . Nowadays , property requirement to sintering ferrous parts becomes much higher than ever before.It is very important and emergent to develop high performance ferrous P/M parts.Liquid sintering is one of the most important method that densify powder material.In this thesis,the behavior of Fe-x(1,1.5,2,6,10,15)Cu PM in sintering have been first studied.After sintering Fe-xCu compacts expand and its expansion ratio first went up then down along with copper content increasing,achieving maximum at 6%Cu.Both its porosity and densification parameter first went down then up along with the copper content increasing.This is caused by copper versus ferrous material sintering behavior's impact.After sintering temperature rise to 1083℃upwards, copper grains started to be molten and were filled into the miscellaneous position of powder green compact.The iron grains expanded after copper infiltrating.And at the same time the iron grains were rearranged to further close packing.Change law of expansion ratio and densification parameter were conduced by both distension and densification effect.After sintering,Fe-xCu compacts interior formed new pore space and the compacts expanded.Changing copper addition manner,adopting copper infiltration mean,the problem could be settled.When infiltrating,liquid copper formed at powder green compact exterior and then filled into green compact interior original pore space under the action of capillary force.So didn't form new pore space at green compact interior and thereby cause iron base powder green compact expansion ratio decrease and densification degree advance.Adopting copper infiltration,bulk expansion degree depress largely,but the bulk expansion degree of the powder green compact were sill a little big,this result from copper atom infiltrating into iron grain interior and causing lattice expansion. According to Fe-Cu-C ternary phase diagrams, along withγ-Fe carbon content increase, solid solubility among copper atγ-Fe constantly decrease and hence decrease the expansion stage of iron grains. Find by experiment that on copper infiltration quantity homology occasion,along with plumbago content increase,the bulk expansion degree of Fe-x(0.4,0.6,0.8,1.0,1.2)plumbago compacts constantly decrease. plumbaginous addition evidently restrained bulk expansion degree. Mechanical properties of iron-based materials sintered are low.They need heat treatment according to physical demand:quenching can improve the intensity and hardness,cryogenic treatment can promote residual austenite decompounding,temper can advance plasticity.Conclusions are listed follow:1. The compacts of Fe-xCu expanded after sintering. Both the volume expansion ratio and porosity increase first then decrease with the copper content increasing and they achieve maxima at 6%Cu.The densification parameter decrease first then increase with the copper content increasing and achieve minima at 6%Cu.Notice from the microstructure of Fe-(1,1.5,2,6,10,15)Cu sintered compact that the iron grains of Fe-xCu specimen are constantly refining after sintering:the grain refining effect achieve maxima at 2%Cu.2. Copper infiltration can reduce the porosity of the iron-based powder material and improve the densification degree:the green compacts with 6.9g/cm3 can achieve 7.3 g/cm3 and 7.85 g/cm3.and their relative density can reach 92.56% and 97.92%.3. The volume expansion ratio of Fe-(0.4,0.6,0.8,1.0,1.2)plumbago green compacts increase first then decrease with plumbago content increase.The density of Fe-1.2 plumbago compacts with 7.0 g/cm3 can reach 7.7g/cm3 after copper-infiltration sintering,and its relative density reach 97.4%,but its volume expansion ratio is only 0.48%.Notice from the microstructure of Fe-(0.4,0.6,0.8,1.0,1.2)plumbago sintered specimen that plumbago can promote the sintering course of iron-based PM basal body.The iron grains of Fe sintered compacts were incompact.when adding plumbago,the iron grains of Fe-x plumbago sintered compacts become more and more compact with plumbago content increase.The basal body become oneness at 0.8% plumbago.4. The hardness of Fe-Cu-Co-Ni-Mn-Mo-C compacts after sintering are between 30-35HRC.After quenching at 900℃,30min,their hardness are between 50-55 HRC.Then after temper the hardness are between 45-50 HRC.5. Cryogenic treatment can increase the hardness of quenching material.The increase degree improve with the cryogenic treatment time increasing:its hardness are between 1.25HRC-3.25HRC.Extending cryogenic treatment time is good for hardness improving,and after 24h the increasing trend became smoothing.Notice from its microstructure,cryogenic treatment can promote the decomposition of residual austenite,carbide precipitation and martensite refining. |
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