Design Analysis And Experimental Study Of In-situ High Temperature Tensile Testing Device For SEM

The microstructure of a material is a key factor affecting its mechanical properties.In-situ mechanical property test research is an important way to reveal the microstructure damage evolution mechanism,understand the failure and damage mechanism,and extract the mechanical test parameters to deeply understand the mechanical properties of materials,improve the manufacturing process,and further enhance the service performance of materials.With the continuous progress of scanning electron microscopy in recent years,in-situ mechanical property testing techniques have emerged and received wide attention from the academic community.In-situ testing refers to the combination of mechanical property testing devices with scanning electron microscopy(SEM),X-ray diffraction(XRD),and atomic force microscopy(AFM)to achieve functional synchronization and real-time acquisition of material microscopic morphological changes and their macroscopic mechanical property parameters.In this paper,we developed an in-situ high-temperature tensile testing device within SEM for TESCAN VEGA4 scanning electron microscope,proposed an algorithm for solving the stress-strain curve of variable-section specimens,and realized the in-situ tensile testing function with simultaneous high-temperature environment construction,microscopic morphology characterization,crystal orientation analysis,and strain distribution testing.And on this basis to carry out the mechanical properties of 7075 aluminum alloy welded joints in situ test analysis and characterization research.The main research work of this paper is divided into three parts as follows:(1)Designed and developed an in-situ high-temperature tensile test apparatus within the scanning electron microscopeIn this paper,we developed an in-situ high-temperature tensile testing device for the TESCAN VEGA4 SEM by using a multifactor coupling design approach to overcome the technical problems of compatibility and high-temperature environment in the SEM due to the limited space.By conducting in-situ tensile testing in the SEM and combining the digital scattering(SEM-DIC)strain analysis technology,the in-situ test in the SEM can obtain a variety of material mechanical property curves,surface microstructure,strain field distribution,micro-zone mechanical properties and other in-situ testing and characterization parameters simultaneously and in real time.By debugging the load and temperature modules of the instrument,the precise closed-loop control of displacement,load,temperature and other test parameters is realized,and the supporting in-situ testing and control software of mechanical properties is developed.The accuracy and repeatability of the instrument were tested using standard specimens,and the sources of measurement errors were analyzed.A stress-strain calibration algorithm involving the stiffness of the instrument and the deformation of the arc transition section of the specimen was proposed,and the validity of the calibration algorithm was verified using the DIC technique.(2)The test method and stress-strain solution model for in-situ tensile testing of variable section specimens within SEM are proposedDue to the material defects and the coaxiality error of the instrument,it is difficult to capture the fracture process simultaneously because of the random fracture location of the conventional iso-sectional tensile specimens in the in-situ SEM experiments.In this paper,an in-situ tensile test and stress-strain solution model for elasto-plastic variable-section specimens are proposed,using the segmental integration method of elasto-plastic deformation to solve the deformation in the low stress concentration region in the middle of the sample and to obtain the stress-strain curve.Finally,in-situ tensile tests are carried out on TC4 titanium alloy specimens,and the SEM-DIC strain measurement technique is used to measure the deformation of each segment of the variable-section specimens to verify the effectiveness of the algorithm.(3)Conducted in-situ mechanical properties of 7075 aluminum alloy welded joints test researchThis paper addresses the poor mechanical properties of TIG welded joints of 7075 aluminum alloy,welding deformation and other problems,the 7075 aluminum alloy FSW and TIG welding process and the mechanical properties of the joint research.In-situ testing and characterization of the mechanical properties,microstructural changes,grain size,crystal orientation of the joint using the in-situ tensile testing device developed in this paper,and analysis of the mechanical properties of the welded joint under different welding process parameters.It was found that the best mechanical properties of FSW welded joints obtained under the parameters of 1400-240(rotational speed-welding speed),the tensile strength of 445.89 MPa,elongation after break of 9.95%,when the tensile strength of 80.5% of the base material,compared to the maximum tensile strength of 321.67 MPa of TIG welded joints obtained at 170 A welding current,FSW tensile strength than TIG welded joints increased by 38.6%.The fracture mode of the FSW welded joint is ductile fracture,and the fracture mode of the TIG welded joint is a mixed tough-brittle fracture,through EBSD crystallographic analysis,the average size of the grain in the weld nucleus region of the FSW welded joint is 3.3 μm,much smaller than the average size of the grain in the weld nucleus region of the TIG welded joint 51.7 μm,revealing that the fine grain reinforcement of the grain is the mechanical properties of the FSW welded joint better than The main reason for the TIG welded joints.