A Way to Determine the Maximum Inducible Residual Stresses in Steel by Mechanical Surface Treatment

The issue of determining the maximum compressive residual stress that can be induced through mechanical surface treatment is of great significance.There are two possible approaches,namely stress peening and stress rolling,both to determine the limit.Steel with high hardness may be under the yield strength,while for those with lower tensile strength,the hardness is increased,and the limit is above the tensile strength.

Analytical Modelling of Shot-peening Residual Stress on Welding Carbon Steel Surface Layer

The residual stress distribution was studied by an analytical model, due to shot peening on the welding carbon steel surface layer. The initial welding residual stresses before shot peening were taken into consideration in this analytical model. The Hertzian elastic contact theory was used to get the elastic compression stress state after impact on the surface layer. The initial welding stress field and the shot peening stress field would superpose and the welding surface layer would yield based on the elastic-plastic evaluation, then the residual stress after shot peening can be achieved. The influence of initial welding residual stress on the stress distribution after shot peening was analyzed and discussed. A series of experiments were carried out and the residual stress on the welding surface was determined by X-ray diffractometer before and after shot peening. The calculation results of the analytical model are consistent with the experimental results. The critical shot velocities when welding surface layer yielded and reverse yielded were calculated. While the welded joint surface material reversely yielded, the maximum compressive residual stress would not obviously increase with the increase of shot velocity, the thickness of the compressive stress layer would be increased. Welding residual tensile stress can enlarge the thickness of the compressive stress layer at the same shot velocity when reverse yield appeared.

A unifying approach in simulating the shot peening process using a 3D random representative volume finite element model

Using a modified 3D random representative volume（RV）finite element model,the effects of model dimensions（impact region and interval between impact and representative regions）,model shapes（rectangular,square,and circular）,and peening-induced thermal softening on resultant critical quantities（residual stress,Almen intensity,coverage,and arc height）after shot peening are systematically examined.A new quantity,i.e.,the interval between impact and representative regions,is introduced and its optimal value is first determined to eliminate any boundary effect on shot peening results.Then,model dimensions are respectively assessed for all model shapes to reflect the actual shot peening process,based on which shape-independent critical shot peening quantities are obtained.Further,it is found that thermal softening of the target material due to shot peening leads to variances of the surface residual stress and arc height,demonstrating the necessity of considering the thermal effect in a constitutive material model of shot peeing.Our study clarifies some of the finite element modeling aspects and lays the ground for accurate modeling of the SP process.

Investigation of Surface Integrity on TC17 Titanium Alloy Treated by Square-spot Laser Shock Peening

Laser shock peening（LSP） is an innovative surface treatment method,which has been shown to greatly improve the fatigue life of many metallic components.This work investigates surface integrity of TC17 titanium alloy treated by LSP with innovative square laser spot.Nd：glass laser with duration of 30 ns and spot size of 4 mm×4 mm is applied.The surface morphology and surface residual stress of the TC17 titanium alloy,treated with varying peening parameters such as laser power density and overlapping ratio,have been studied in detail.The results show that laser pulse energy greatly influences surface morphology and surface residual stress around single-spot treated areas,and compressive residual stresses are saturated as laser pulse energy is over 55 J.There are significantly different surface morphologies and residual stress distributions at the overlapped areas with different overlapping ratios.A relative smooth surface is produced with uniform compressive residual stress distribution at an overlapping ratio of 8 %.The experiment of residual stress relaxation is implemented by measuring residual stress at the center of four overlapped spots and by four point bending fatigue test at the frequency of 105 Hz.The compressive residual stresses induced by LSP are found to relax quite slowly under cyclic fatigue loading.

Effect of Shot Peening and Plasma Electrolytic Oxidation on the Intergranular Corrosion Behavior of 7A85 Aluminum Alloy

The effects of shot peening（SP） and plasma electrolytic oxidation（PEO） on the intergranular corrosion behavior of the novel high strength aluminum alloy 7A85（AA 7A85） were investigated by electrochemical polarization and electrochemical impedance tests.The intergranular corrosion mechanism of SP,PEO and PEO combined with sealingtreated AA 7A85 was studied by the metallographic analysis,residual stress testing,X-ray diffractometer analysis and scanning electron microscopy.The results show that AA 7A85-T7452 is very sensitive to intergranular corrosion.SP would significantly improve its intergranular corrosion resistance.This is attributed to the combination action of residual compressive stress and grain refinement.PEO would reduce the largest corrosion depth by 41.6%.Moreover,PEO without sealing did not eliminate the intergranular corrosion due to the existence of the micropores and microcracks in the oxide coating.However,PEO combined with the SiO2sol–gel sealing treatment could effectively protect the AA 7A85-T7452 from intergranular corrosion because of the good corrosion resistance and barrier function of the sealed coating.

Effect of Surface Polishing Treatment on the Fatigue Performance of Shot-Peened Ti–6Al–4V Alloy

In this study, shot peening is applied to the titanium alloy Ti–6Al–4V, and the surface treatment effect on fatigue life of shot-peened specimens under high cycle loading is investigated. The induced residual stress is measured by using the orbital hole-drilling method. Surface profilometer and optical microscopy are employed to characterize the surface roughness and morphology. The deformed microstructure layers of the shot-peened specimens are investigated by using scanning electron microscopy. Experiments reveal that the fatigue life of Ti–6Al–4V is improved by the shot peening process, and the surface pre-peening polishing. The combination of pre-and post-peening polishing treatments further improves fatigue life of Ti–6Al–4V specimens. The present work provides useful guidelines for developing more efficient shot peening strategies.