MTE 449 Powder MetallurgyLecture 21Lecture 21Chapter 10Pi il fF llD ifi iPrinciples of Full DensificationReading: pp. Homework:121Homework:Mark L. WeaverContinuous Belt Furnaces Sintering of powder compacts does not generally produce full density products (density between 0.9 to 0.98 theoretical di)density) The porosity is a defect and reduces the strength, ductility and fracture toughness of the final product. Removal of residual porosity in sintered products improves mechanical propertiesproducts improves mechanical properties  Because of the uniformity and fineness of the grain structure (micron size) of powder metallurgy product their mechanicalmetallurgy product, their mechanical properties are higher than those produced by casting or mechanical forming techniques such as forgingsuch as forging122Batch Furnaces Full densification of the sintered products requires extra processing steps with added production costs The decision for taking this production route should be based onRequired properties of the finalproductRequired properties of the final product Size, shape and dimensions of the product Equipment capacity and costs Production cycle and production rates Processing difficulties of the material Powder contamination and oxidation Phase separation and precipitation at grain boundaries in heat treatment The cost of the product compared to other manufacturing techniques123Full Densification Approaches The approaches to eliminate porosity on sintered products are1. Infiltrate the pores with molten metal2. Close the pores through plastic deformation of the grains These two approaches requires simultaneous use of heat and pressure The heat is used either to melt the infiltrated material or to soften the grains to facilitate material deformation The pressure in the infiltration approach pppis used to overcome capillary forces which resist the flow of the liquid The pressure required depends surface 124energy and wetting characteristics of the liquid and pore size.2cospPdFull Densification Approaches – cont’dIn solid-state densification the pressure is used toIn solid-state densification, the pressure is used to generate stresses above the material’s yield strength near the pores to force pore collapse Thisis the fundamental principle of all solid-stateThis is the fundamental principle of all solidstate densification processes Because of the dependence of yield strength on temperature, these processes are broadly divided into te pe atu e, t ese p ocesses a e b oad y d v ded tofour categories Low stress and high temperaturePress reassisted sintering processesPressure assisted sintering processes Intermediate stress and temperature Hot pressing and hot isostatic pressing High stress and lower temperature Hot extrusion and forging processes125 Very high stress and low temperature Explosive compaction at room temperatureFull Densification Approaches – cont’dTheaxial applied pressure and capillary forces exertThe axial applied pressure and capillary forces exert lateral stresses in the grain with maximum at the neck When these stresses exceed the material yield strength, the stresses produce plastic deformation when they occur lateral at the neck Pore inhalation by plastic deformation will occur if these stresses exceed plastic deformation  The applied pressure to induce plastic flow in second approach depends on the strength and ductility of theapproach depends on the strength and ductility of the of the material at processing temperature126Contact StressesThe applied pressure transmits through the solid andThe applied pressure transmits through the solid and the porosity of the compact causes pressure intensification around the pores due to the decrease in the cross-section of the solid. At the micro-scale, the increase in the pressure occurs in the grain contact region where the neck is smaller than the grain size. The smaller the neck diameter, i.e., lower fractional density, the higher the contact pressure.The capillary forces on the pore surface alsoThe capillary forces on the pore surface also contribute to the increase in contact pressure. For sintered products ( ≈ 0.9), it is double the applied pressurepressure. The axial contact pressure induces lateral stresses in the contact region, called contact stresses, which causes the material to deform along the grain contact127causes the material to deform along the grain contact area.At intermediate temperatures below 0 5TContact StressesAt intermediate temperatures below 0.5Tmp, sintered products with fractional density above 90% are soft enough to deform and close the pores at relatively moderate pressures. The flow of the material at stresses above the yield stress is a result of bulk and grain boundary diffusion. As the material approaches full density, the rate of deformation decreases due to the decrease of contract stresses caused by grain growth. Density dependence on applied pressure: 1/331AgP3;0.91.331exp ; 0.92AggAP  1282Creep Deformation Grain boundary (i.e., Coble) creep model.Atomic volume3148BEdLDPLdt kTGEffective pressureoLdt kTGAb lGrain sizeGrain boundary diffusion rate = Doexp[-Q/RT]Boltzmann’s constantAbsolute temperatureShrinkage rate = strain rateGrain boundary