Study On Residual Stress And Cracks Damage Of Powder Metallurgy Compacts

Powder metallurgy is an effective process of manufacturing near net shapeproducts. It offers many advantages, including high productivity, low production costs,less energy consumption and environmental friendship, which is widely concern inpowder metallurgy industry. Cracking in green compact has always been theimportant issue for a long time in powder metallurgy part industry. In fact, cracks ingreen compacts may occur in all phases of consolidation and handling process.Besides the traditional experience method, numerical simulation technology whichcombined with die and process design, is today a powerful tool in prediction of crackin powder metallurgy process in order to prevent the defect.There is not a uniform mathematical model as the complex mechanism ofpowder compaction. So, little has been published about quantitative analysis of thecrack in green compacts. Thus, a generalized plastic mechanics model has beendeveloped to research the metal powder densification process in this work. And cracksand damage in metal powder compacts have been studied in different phases ofprocess. These provide an effective scientific method to improve the quality ofpowder metallurgy products and optimize the tool and process parameters. Specificareas of development described in this dissertation include:1. A density-dependent generalized plastic mechanics model named modifiedDrucker-Prager Cap model was developed, which parameters were identified throughderivation by powder compaction experiment and strength tests of green compacts.And the model parameters of diffusion prealloyed iron powder Distaloy AE weredetermined as functions of relative density.2. The constitutive model was implemented in Abaqus by writing a usersubroutine to three dimensional simulate the compaction process of metal powder.The simulation results agree well with the experiment data including the curve ofpressing force and relative density distribution.3. The residual stresses were investigated by X-ray diffraction measurement.Two kinds of metal powder compacts (ASC100.29and Distaloy AE) have beenmeasured the residual stresses in different directions on surfaces based on differentpressing forces (450MPa and600MPa) and lubrication conditions. The test results were ananlyzed to obtain the residual stress distribution law of green compacts, whichhave been the validation criteria of residual stress finite element simulation.4. The ejection process of green compact was simulated to predict the residualstress based on the calibrated constitutive model. The influence of model parameterson residual stress of green compact has been discussed. And the suitable parameterswere determined according to the experimental results. The finite element model wasfurther modified on the aspect of the stress calculation, which was the basis ofresearch on the crack and damage of green compact.5. The ductile damage model was derived based on the modified Drucker-PragerCap model considering the mechanism of cracks on the powder compaction process.Meanwhile, the damage threshold of crack was identified.6. The ductile damage model was implemented in Abaqus by writing a usersubroutine to predict the crack and damage in green compact during the pressing andejection process. Furthermore, the impact of factors including the geometric shape ofthe compacts, the lubrication of die and the taper of the die wall on the damge ofgreen compact was discussed.7. The mechanical properties and fracture process of two kinds of greencompacts (ASC100.29and Distaloy AE) were investigated by the Brazilian disc testof circular loading. The paramenters including tensile strengtht, elastic modulus E,fracture toughness KIand fracture energy Gfas functions of relative density weredetermined according to the theoretical and numerical analysis and experimental data,which provided the reliable data for the damage and fracture simulation of greencompact.8. Two kinds of fracture energy based cohesive zone models for metal powdercompacts were established. These models were analyzed combined with the Braziliandisc experiment results. The cracks and damage of the annular compact in handlingprocess was predict considering the residual stress after ejection by the exponent typecohesive zone model with higer simulation precision.