Influence Of Process Factors On The Microstructure And Performance Of Directionally Solidified Al-4.5%Cu Alloy

Directional solidification theory and technology have been studied and attentions have been concentratively paid to grain refinement,primary phase microstructure,size,distribution of dendrite and property.Metallurgical microscope has been applied to describe qualitatively the microstructure morphology of solidified alloys.Single process factor has been generally considered and integrated and systematic research of process factors' influences on microstructure and mechanical property was kept to be expected.Influences of melt superheat treatment process,temperature gradient and withdrawal rate on microstructure,mechanical property and crystallographic orientation of directionally solidified AI-4.5%Cu alloy were studied.Under certain superheat treatment process, temperature gradient and withdrawal rate,influence of rare earth(La,Ce,composite rare earth)on microstructure and thermal fatigue of directionally solidified Al-4.5%Cu alloy was investigated.Temperature field and thermal stress distributions were simulated according to thermal fatigue test parameters.With universal testing machine,stress-strain curves were obtained for different directionally solidified specimens.Directional solidification microstructure of Al-4.5%Cu alloy and fracture morphology after being streched were analyzed through scanning electron microscope(SEM).Thermal fatigue test of directionally solidified Al-4.5%Cu alloy with different contents of rare earth added in was undertaken on self-restrict thermal fatigue experimental machine.The potential mechanism of process factors' impact on final microstructure,mechanical property and thermal fatigue properties was discussed.Melt superheat treatment has remarkable influences on microstrucure and mechanical properties of directionally solidified Al-4.5%Cu alloy.With increase of melt superheat temperature and prolonging of melt superheat time,primary dendritric spacing decreases,and the strength and elongation increase.With the prolonging of low-temperature holding period, primary dendritic spacing increases,and the strength and elongation decrease.The failure mode of directionally solidified Al-4.5%Cu alloy specimen being superheatedly treated is ductile fracture.The melt superheat treatment changes the melt microstructures.Change of liquid metal structure directly affects the final structure,properties and crystallographic orientation.Under certain superheat treatment process and withdrawal rate,with increase of temperature gradient,primary dendritic spacing decrease clearly.The results are uniform with that deduced by Hunt model.Under certain superheat treatment process and temperature gradient,when withdrawal rate is lower than 92μm/s,the directional solidification microstructure is not favorable.With increase of withdrawal rate,dendrite arrangement becomes more regular and primary dendritic spacing gets obviously narrowed.With higher withdrawal rate(more than 123μm/s),measured primary dendritic spacing is in good accordance with that deduced by Hunt model.Tensile strength and elongation of the specimen subjected to withdrawal rate of 200μm/s are higher than the counter parts of the specimen subjected to withdrawal rate of 62μm/s by 16.7%and 85.9%respectively.Under certain superheat treatment process,temperature gradient and withdrawal rate, with 0.3%La,0.3%Ce and 0.4%composite rare earth added in,the directionaUy solidified Al-4.5%Cu alloy has compact microstructure and minimum primary dendritic spacing.Little rare earth helps to the formation of developed dendrite crystal.Dendritic arrangement gets more regular and primary dendritic spacing gets smaller with more rare earth.Thermal fatigue property of the specimen is excellent with 0.3%La,0.1%Ce and 0.3%composite rare earth added in.Spread of thermal fatigue crack can be divided into different steps and is characterized by initial fast pace and following slower paces.Finite element analysis shows that residual stress exists after one cycle and stress concentration appears at the pre-made notch.Thermal stress extends towards the inside part of the specimen with triangular profde.And then the thermal stress extends sharply with two bifurcations.Finally,the thermal stress variation gets smoothly.With increase of cycle number, residue stress accumulates gradually and reaches certain value which drives the crack to occur at the pre-made notch.