Research On Short-wavelength X-ray Diffraction Method For Internal Residual Stress Determination And Its Application On Friction Stir Welded Al Alloys Plates

The values and distributions of internal residual stresses are directly related to the mechanical properties,dimensional accuracy and service life of the friction stir welds(FSW).In this paper,internal residual stresses of FS welded high-strength aluminum alloys plates were non-destructively measured by the short-wavelength X-ray diffraction(SWXRD)method developed by China.This method overcame the shortcomings of the conventional residual stress testing methods for the materials with strong texture.And an evaluation system of the unity of microstructure,macroscopic physical properties and residual stress analysis was established.Study on precision optimization was carried out for internal residual stress determination with SWXRD in order to overcome the disadvantages of FS welded high-strength aluminum alloys plates,such as large residual stress gradient,the complex microstructure and the strong texture.The effects of stress calculation model,stress-free reference lattice spacing,crystal orientation and selection of crystal plane on the residual stress testing results were studied.A high accuracy residual stress measurement method by SWXRD for high strength aluminum alloy was proposed.Fitting method of asymmetrical diffraction peaks for SWXRD was studied.And a fitting function was obtained to determine the actual positions of the asymmetric diffraction peaks.The effects of rotational speed and natural aging time on residual stress of the FS welded 7075-T651 aluminum alloys pre-stretched plates were studied based on the accurate values and distributions of internal residual stresses.The microstructure,microhardness,and their relationships with residual stresses were studied.The minimum hardness of the joint coincide with the position where the residual stress gradient is the largest,locating at the junction of the thermo-mechanical affected zone and heat-affected zones.After a relative long-term natural aging,the microhardness of weld-nugget zone and thermo-mechanical affected zone increases obviously,but the range of the smaller hardness region does not change.The residual stress distribution does not change much.The relationship between the welding process parameters,internal residual stresses,microstructure and mechanical properties of FS welded 7050-T7451 aluminum alloys pre-stretched plates was studied.The welding process parameters were evaluated by the results of the macroscopic residual stress,and the correlation analysis model of residual stress-organization-property was established.Results show that the FSW welding samples with better overall performance can be obtained with lower rotational speed and higher tool travel speed.Internal residual stress distributions of FSW joints and their microstructure and mechanical properties are related.As a result,the residual stress determination by SWXRD can be used as an important non-destructive evaluation of the quality of FSW.