Diffusion Behavior Of Cu Atom In Al-Si Alloy At High Temperature Aging Condition

Cast multivariate Al-Si alloy has become a widely used piston alloy currently.With the development of the engine industry,higher requirements have been put forward piston alloy materials.Al Cu Ni phase is the main high-temperature strengthening phase in the piston aluminum alloy.The stability study of the Al Cu Ni phase and the phase transformation during the heat treatment process has great significance to the property research of the piston aluminum alloy.Previous research found that after the alloy was subjected to high temperature re-aging treatment,the Al Cu Ni phase was granulated,and the fatigue life of the alloy was increased by four times.The high temperature re-aging process accelerated the diffusion of Cu atoms within the alloy,the changes in the structure and properties of the alloy were related to the diffusion of Cu atoms.In this paper,the diffusion behavior of Cu atoms in Al-Si alloy under high temperature aging conditions was systematically studied,and the effect of high temperature aging process on Al Cu Ni phase was discussed in depth.In this paper,an Al-x Si/Cu diffusion couple was prepared and subjected to diffusion heat treatment to study the diffusion phenomenon of Cu atoms in Al-Si binary alloy and the effect of Si content on the diffusion rate of Cu atoms.Al-Si-Cu-Ni-Mg alloy was a model alloy,which was subjected to thermal exposure treatment,and the diffusion of Cu atoms in the Cu-containing phase was studied.This research can provide theoretical and experimental basis for researching of the high temperature strength of piston alloys.The main research content and methods of this article are given as below:?1?An Al-x Si/Cu?x=0,7,12,18?diffusion couple was prepared by a solid-liquid contact method and was subjected to high-temperature aging treatments at 350?and 425?for 50 h,75 h,and 100 h,respectively.Scanning electron microscopy,energy spectrum analysis and X-ray diffraction were respectively used to study the diffusion interface.It was found that four diffusion layers of Al Cu₄,Al Cu₃,Al₄Cu₉and Al₂Cu appeared at the interface.When the Si content was increased from 0 wt%to 12 wt%,the thickness of the diffusion layer continuously increased,and when the Si content was increased to 18 wt%,the thickness of the diffusion layer is decreased rather than increasing.The increase of the holding temperature and the holding time would increase the thickness of diffusion layer.After fitting,it was found that the thickness of the diffusion layer was proportional to the square of the diffusion time.Combined with the phase diagram drawn by the Pandat software,it was found that there were three diffusion paths of Cu atoms in the Al-Si binary alloy.With the increase of Si content,different diffusion paths dominated and affected the diffusion rate of Cu atoms.?2?Preparation of Al-Si-Cu-Ni-Mg model alloys with different Cu and Ni contents,and thermal exposure treatments at 350?and 425?for different times,respectively.The results of the Brinell hardness test showed that with the With the increase of Cu and Ni content,the hardness of the alloy gradually increased,and the hardness of the alloy gradually decreased with the extension of the heat exposure time.The evolution of Cu-containing phases in the alloy was studied by means of optical metallography,scanning electron microscopy,energy spectrum analysis,and transmission electron microscopy.The results showed that the Al Cu Ni phase in the alloy was granulated after thermal exposure,and the reduction of the Brinell hardness of the alloy was closed related to that of the Al Cu Ni phase.This change is directly related.Through in-situ tracking observations of the Al₃Ni phase,it was found that the Al₃Ni phase in the alloy gradually changed from the boundary to the core during the thermal exposure process to the Al₃Cu Ni phase.In alloys with high Cu and Ni contents,it was found that a large number of jagged new phases precipitated at the interface of Al₃Cu Ni after thermal exposure.The Al₃Cu Ni phase was granulated with time,and the results of TEM analysis showed that the zigzag precipitated at the interface of Al₃Cu Ni phase.The new phase was the Al₇Cu₄Ni phase.There was a clear transition layer between the Al₃Cu Ni phase and the Al₇Cu₄Ni phase,indicating that the transition from the Al₃Cu Ni phase to the Al₇Cu₄Ni phase was controlled by diffusion.At the same time,it was found that the Cu content in the matrix near the newly precipitated Al₇Cu₄Ni phase decreased with increasing distance from the Al₇Cu₄Ni phase.?3?Two types of model alloys with different contents of Cu and Ni were subjected to350?and 425?for 50 h,100 h and 200 h,respectively,and then subjected to creep performance tests at 350?and 25 MPa.Observe and analyze the damaged tissue after creep.The creep test results showed that alloys with higher Cu and Ni content had better creep resistance than alloys with lower Cu and Ni content.After heat exposure treatment,the creep resistance of the alloy decreased,and the creep life of the alloy gradually decreased with the increase of the heat exposure time and temperature.The damaged structure was observed,finding that compared with the as-cast alloy samples,the microcracks in the alloy samples were larger in number and size,and these microcracks were mainly near the silicon phase,indicating that the thermal exposure treatment would reduce the binding ability of matrix and silicon phases,making the alloy more prone to fracture during creep.