Effects Of Mn Addition And Thermal Exposure On Microstructure And Mechanical Properties Of Al-4Cu Alloy

High temperature and heat resistant aluminum alloys are widely used in aerospace,automobile manufacturing and other fields.Binary Al-Cu alloy is the basis of heat resistant aluminum alloy research,and micro-alloying is an important method to improve the high temperature heat resistance of Al-Cu alloy.Manganese is low in price,abundant in resources,excellent in microalloying effect,and has remarkable effect on improving mechanical properties of Al-Cu alloy at high temperature.In this paper,Al-4Cu-x Mn(x=0,0.1%,0.2%,0.3%,0.5%,mass fraction)alloys were selected as the research object.The microstructure of as-cast,T6 and heat-exposed alloy was observed by optical microscope,scanning electron microscope and transmission electron microscope.The infulencing mechanism of Mn on the precipitated phase of Al-4Cu alloy and its existing form in the alloy were analyzed,and the effect mechanism of Mn on the coarsening of Al₂Cu was analyzed by thermal exposure experiment.The results are as follows:(1)In T6 Al-4Cu alloy,when the Mn content was less than 0.2%,the amount of??-Al₂Cu phase increased with the increase of Mn.The size of??-Al₂Cu phase decreased from 64.67 nm to 17.78 nm.This was because the segregation and heteronucleation of Mn atoms at??-Al₂Cu/Al interface refined the??-Al₂Cu phase and promoted the precipitation of??-Al₂Cu phase.When Mn content was further increased to 0.3%,the microstructure of the alloy changed significantly.The amount of??-Al₂Cu decreased while the size increased to 41.61 nm.Many common Mn rich phase with butterfly contrast appeared in the[1-10]_Al direction of the matrix.This was because the Mn atom was more concentrated in the direction of Al matrix[1-10]_Al.When the content of Mn was 0.5%,there is no??-Al₂Cu phase in the matrix and a coherent/semi-coherent Mn-rich phase with a size of about 428 nm formed.By EDS and HRTEM analysis,the Mn content was about 5.98%and had a 6H structure.These Mn-rich phases appeared because a large number of Mn atoms entered into the lattice of??-Al₂Cu phase.Thus,the size of??-Al₂Cu phase increased and the system free energy increased.Therefore,the??-Al₂Cu phase released high free energy by adjusting its crystal structure and composition,forming the coarse Mn-rich phase.(2)For Al-4Cu alloy exposed 100 hours at 420?,Mn atoms exsited there forms in the alloy.When the Mn content was less than 0.2%,a small Mn-rich phase formed at the interface of Al₂Cu/Al matrix,which reduced the passage of Cu atoms into Al₂Cu and inhibited the coarsening of Al₂Cu.When the Mn content was 0.3%,the large Mn-rich phase with higher Mn concentration formed at the??-Al₂Cu/Al interface.The large Mn-rich phase has a better inhibition effect than the coarsening of Al₂Cu.When the Mn content was 0.5%,the rod-shaped Mn-rich phase appeared in the Cu-rich region of Al matrix,which inhibited the diffusion of Cu atoms in the Cu-rich region and slowed down the coarsening rate of Cu atoms absorbed by Al₂Cu.The Mn-rich phase had a common interface with matrix,low interface energy,low initial driving force of coarsening at high temperature and high thermal stability.(3)The addition of Mn increased the room temperature and high temperature tensile strength of Al-4Cu alloy after T6 state or thermal exposure.The largest tensile values were achieved when 0.5%Mn was added.The increasing rates of Mn on the room temperature and high temperature tensile strength of T6 alloy are 31.84%and 41.72%.The increasing rates of Mn on the room temperature and high temperature tensile strength of thermal exposure alloy are 27.97%and 27.3%.The mechanical properties of the alloy were improved mainly because that with the increase of Mn content,the common interface area between the second phase and the matrix increased,and the strengthening effect of the alloy was gradually improved.The addition of Mn improved the ductility of the alloy.In conclusion,the addition of Mn can provide both strength and plasticity of the alloy.