Study On Internal Friction Of Metal Powder Compact In Sintering Process

Sintering is the most critical aspect of powder metallurgy technology,which plays a decisive role to determine final product quality and properties.Understanding the microstructure evolution and the intermetallic formation of metal powders or powder mixtures compact during sintering,and determining the initial sintering temperature,solid-phase diffusion-reaction temperature,crystallization temperature,and types of intermetallic compounds are crucially important for tailoring the sintering process and optimizing product quality.While internal friction technique is very sensitive to the evolution of material microstructure and defects.It is an important experimental technology to study solid defects and structural transformation.It can provide dynamic information on material microstructure and defect evolution from the atomic scale.Therefore,in this paper,the internal friction behaviors of the unit-system metal powder pure Al,Mg,Cu,Fe,and multi-system Fe-Al,Ni-Al powder mixture compacts in the sintering process are systematically investigated by the internal friction technique.This will be conducive to understanding the microstructure and defect evolution and the formation process of intermetallic compounds during the sintering process,thereby providing an accurate basis for the selection of sintering technology.Two internal friction peaks,named heating peak and cooling peak are respectively observed during the first cycle measuring of pure Al,Mg,Cu,and Fe powder compacts of unit-system metal powders.The peak height decreases with the measuring frequency increasing,the heating peak temperature is independent of frequency,and the cooling peak shifts toward higher temperature with the increase of frequency.According to the activation energy and peak temperature location of the cooling peak,the peak belongs to a typical stable relaxational internal friction peak,which originates from the viscous micro-slip of grain boundary during the cooling process.With the appearance of the heating peak,the relative dynamic modulus and resistance decreased obviously,indicating that the microstructure near the heating peak of the metal powder compact changed significantly.Moreover,the heating peak decreases with the increase of pressing pressure and particle size,which is due to the micro-slip mobility decrease of the weak bonding interface and the decrease of the number of interfaces.In the process of heating measurement,the internal friction mainly originates from the micro-slip energy dissipation of the weak binding interface.With the increase of temperature to the peak temperature,the weak binding interface between particles changes to the grain boundary,and the internal friction decreases rapidly,forming an unstable internal friction peak.Therefore,the appearance of the heating peak indicates the transformation of the microstructure of the metal powder compact and the formation of the grain boundary between the particles during the sintering process,so the corresponding peak temperature can be considered as the initial sintering temperature and crystallization temperature of the unit system metal powder compact.The typical internal friction peaks are observed in the internal friction-temperature spectrum of Fe-Al and Ni-Al powder mixture compact of multi-system metal powders.Five typical internal friction peaks are observed respectively termed P₁,P₂,P₃,P₄,and P₅peak for Fe-43at.%Al ball-milled mixture.While for no ball-milled one,only P₁,P₃,and P₅peaks appear.Only two typical internal friction peak is observed in the internal friction-temperature spectrum of Ni-50at.%Al powder mixture compact.These internal friction peaks decrease with the measuring frequency increasing,but the peak temperature is independent of frequency.Meanwhile,the internal friction peak shifts to the high-temperature region,and the peak height increases as the heating rate increases.The analysis indicates that the P₁ peak of Fe-Al and Ni-Al powder compacts is related to the micro-slip of the weak binding interface between powder particles and the recrystallization process of Al particles,the P₂,P₃,P₄,and P₅ peaks of Fe-Al powder compacts are related to the formation of intermetallic compounds Fe Al₃,Fe₂Al₅,Fe Al₂,and Fe Al,respectively,and the P₂ peak of Ni-Al powder compacts originates from the formation of intermetallic compounds Ni Al₃ and Ni₂Al₃.Moreover,the microstructure of Fe-Al and Ni-Al powder mixtures can be effectively tailored by mechanical ball-milling.The P₂ peak of Fe-Al and Ni-Al powder compact obviously shifts toward lower temperature and the peak height obviously changes with the increase of ball-milling time increasing,which indicates that the solid phase diffusion-reaction can be activated at a lower temperature.This is related to the refinement of powder particles,the lamellar formation of the powder mixture,the enhancement of solid solubility and surface free energy,and the shortened atomic diffusion distance due to the mechanical ball-milling.It is also indicated that the mechanical ball-milling can effectively reduce the initial temperature of solid-phase diffusion reaction and then the sintering temperature.