Effects of Laser Shock Processing on Mechanical Properties of Laser Welded ANSI 304 Stainless Steel Joint

With the rapid development of engineering component with integration,high-speed and multi-parameter,traditional techniques haven＇t met practical needs in extreme service environment.Laser welding,a new welding technology,has been widely used.However,it would generate the drop of mechanical properties for laser welded joint due to its thermal effect.Laser shock processing（LSP） is one of the most effective methods to improve the mechanical properties of laser welded ANSI 304 stainless steel joint.In this paper,the effects of LSP on the mechanical properties of laser welded ANSI 304 stainless steel joint have been investigated.The welded joint on the front of the tensile samples is treated by LSP impacts,and the overlapping rate of the laser spot is 50%.The tensile test of the laser welded joint with and without LSP impacts is carried out,and the fracture morphology of the tensile samples is analyzed by scanning electron microscope（SEM）.Compared with the yield strength of 11.70 kN,the tensile strength of 37.66 kN,the yield-to-tensile strength ratio of 0.310 7,the elongation of 25.20%,the area reduction of 32.68% and the elastic modulus of 13 063.876 MPa,the corresponding values after LSP impacts are 14.25 kN,38.74 kN,0.367 8,26.58%,42.29% and 14 754.394 MPa,respectively.Through LSP impacts,the increasing ratio of the yield strength and tensile strength are 121.79% and 102.87%,respectively;the elongation and area reduction are improved by 5.48% and 29.38%,respectively.By comparing with coarse fracture surface of the welded joint,the delamination splitting with some cracks in the sharp corner of the welded joint and asymmetric dimples,LSP can cause brighter fracture surface,and finer and more uniform dimples.Finally,the schematic illustration of dimple formation with LSP is clearly described.The proposed research ensures that the LSP technology can clearly improve the yield strength,tensile strength,yield-to-tensile strength ratio,elongation,area reduction and elastic modulus of the welded joint.The enhancement mechanism of LSP on laser welded ANSI 304 stainless steel joint is mainly due to the fact that the refined and uniform dimples effectively delay the fracture of laser welded joints.

Numerical simulation of residual stress field induced by laser shock processing with square spot

Laser shock processing（LSP）,also known as laser peening,is a novel surface treatment technique in the past few years.Compressive residual stresses which imparted by LSP are very important for improving fatigue,corrosion and wea rresistance of metals.Finite element analysis（FEA） simulation using ABAQUS software has been applied to predict residualstresses induced by LSP on Ti-6Al-4V titanium alloy with laser pulse duration 30 ns and water confined ablation mode.The residual stress field generated by different shape laser spots was studied,and the square laser spot is shown the most suitability for avoiding stress lack phenomenon and overlapping LSP.Surface residual stresses and plastically affected depth within single square spot both increased with the increase of laser intensity and laser shock times.Furthermore,compared with circle and ellipse spot,the residual stress distribution in overlapping square spots is very uniform only with small overlapping ratio.LSP with square spot can process advantageous residual stress field,and this technique will be used widely.

Laser Shock Processing of an Austenitic Stainless Steel and a Nickel-base Superalloy

An austenitic stainless steel 1Cr18Ni9Ti and a solid solution-strengthened Ni-base superalloy GH30 were shock processed using a Q-switched pulsed Nd-glass laser. Microstructure, hardness and residual stress of the laser shock processed surface were investigated as functions of laser processing parameters. Results show that high density of dislocations and fine deformation twins are produced in the laser shock processed surface layers in both the austenitic stainless steel and the nickel-base superalloy. Extensive strain-induced martensite was also observed in the laser shock processed zone of the austenitic steel. The hardness of the laser shock processed surface was significantly enhanced and compressive stress as high as 400 MPa was produced in the laser shock processed surface.

Low-cycle Fatigue Behavior of Ni-based Superalloy GH586 with Laser Shock Processing

Low-cycle fatigue behavior of Ni-based superalloy GH586 with laser shock processing（LSP） was investigated. The residual stress of the specimens treated with LSP was assessed by X-ray diffraction method. The microstructure and fracture morphology were characterized by using an optical microscope（OM）, a scanning electron microscope（SEM）, and a transmission electron microscope（TEM）. The results indicated that the maximum residual compressive stress was at about 1 mm from the shocking spot center, where the residual compressive stress was slightly lower. High density tangling dislocations, dislocation walls, and dislocation cells in the microstructure of the specimens treated with LSP effectively prevented fatigue cracks propagation. The fatigue life was roughly twice as long as that of the specimens without LSP. The fatigue crack initiation（FCI） in specimens treated with LSP was observed in the lateral section and the subsurface simultaneously. The fatigue striation in the fracture treated with LSP was narrower than that in the untreated specimens. Moreover, dimples with tear ridges were found in the fatigued zones of the LSP treated specimens, which would be caused by severe plastic deformation.

Mechanism of Grain Refinement Induced by Laser Shock Processing in AZ31 Magnesium Alloy

In order to study the mechanism of grain refinement induced by laser shock processing （LSP） in AZ31 magnesium alloy, the specimens were processed with Nd：glass pulse laser shocking and the microstructures of LSP specimens near the surface were examined by optical microscopy and transmission electron microscopy. Optical microstructure pictures show that the size of grains formed in the top surface layer is about 4-6 μm, which is obviously different from the original grains （with an average size of 20-30 μm） in the substrate in AZ31 magnesium alloy. Transmission electron microscopic observations show that the grain refinement process of AZ31 alloy by laser shock processing includes three stages. At the early stage of LSP, the lower strain and strain rate activates the three dislocation slip systems which include basal plane system, prismatic plane system and pyramidal plane system, with the deformation governed mainly by dislocation. At the intermediary stage, dislocation slip is hindered at grain boundaries and becomes more difficult to continue during LSP. Then, parallel twins appear, which divide the original coarse grains into finer twin platelets. Finally, high-density dislocation walls are formed and subdivide twins into sub-grains. Dynamic recrystallization occurs in the process of further deformation and forms recrystallized grains when strain energy reaches the value needed by recrystallization, which leads to refinement of the grains in the top surface layer.

Experimental study on laser shock processing of brass

Laser shock processing （LSP） is a new surface treatment technique for improving hardness, wear resistance, and fatigue. In this paper, basic theories were introduced and the influence of laser pulse intensity on the laser shock processing of brass specimens was investigated by experiments. Microhardness, roughness, microstructure, wear resistance, friction coefficient evolution, and residual stress were examined with different laser pulse intensities of LSP. The results show that the microhardness increases after LSP treatment, and the higher the pulse intensity, the higher the microhardness. Though the microstructure shows no remarkable change, the roughness and wear resistance increase with the increase in pulse density. Laser shock processing has great potential as a means to improve the mechanical properties of components.

Effects of laser shock processing,solid solution and aging,and cryogenic treatments on microstructure and thermal fatigue performance of ZCuAl_(10)Fe_(3)Mn_(2)alloy

The materials used in variable temperature conditions are required to have excellent thermal fatigue performance.The effects of laser shock processing(LSP),solid solution and aging treatment(T6),and cryogenic treatment(CT)on both microstructure and thermal fatigue performance of ZCuAl_(10)Fe_(3)Mn_(2) alloys were studied.Microstructure and crack morphology were then examined by scanning electron microscopy(SEM)and energy-dispersive X-ray spectroscopy(EDS).The result showed that,after being subjected to the combination treatment of T6+CT+LSP,the optimal mechanical properties and thermal fatigue performance were obtained for the ZCuAl_(10)Fe_(3)Mn_(2) alloy with the tensile strength,hardness,and elongation of 720 MPa,300.16 HB,and 16%,respectively,and the thermal fatigue life could reach 7,100 cycles when the crack length was 0.1 mm.Moreover,the ZCuAl_(10)Fe_(3)Mn_(2) after combination treatment shows high resistance to oxidation,good adhesion between the matrix and grain boundaries,and dramatically reduced growth rate of crack.During thermal fatigue testing,under the combined action of thermal and alternating stresses,the microstructure around the sample notch oxidized and became loose and porous,which then converted to micro-cracks.Fatigue crack expanded along the grain boundary in the early stage.In the later stage,under the cyclic stress accumulation,the oxidized microstructure separated from the matrix,and the fatigue crack expanded in both intergranular and transgranular ways.The main crack was thick,and the path was meandering.

Effect of laser shock processing on residual stress and fatigue behavior of 6061-T651 aluminum alloy

Laser shock processing is a very new technique and an emerging modern process that generates compressive stresses much deeper into the surfaces of metals or alloys. A brief parametric study of the effect of laser parameters on fatigue behavior and residual stress state generated in 6061-T651 alloy specimens was summarized. Residual stress of 6061-T651 alloy was analyzed both before and after laser processing with multishocks. The material remains in compressive residual stress of approximate 1mm in depth which is approximately 10 times deeper than that can be achieved with the conventional technique, and the maximal compressive residual stress at the surface of the sample is about -350MPa. Near the surface, yield strength and hardness are found to be increased by the laser shock. The ratio of fatigue crack initiation life for the laser-shocked to unshocked specimens is found to be 4.9 for specimens. The results clearly show that LSP is an effective surface treatment technique for improving the fatigue performance of aluminum alloys.

Laser Shock Processing of Metal Sheet and Welded Joints

In order to study the application of laser shock processing(LSP) as a post weld treatment technology and a strengthening technology, a series experiments and analysis were taken in this paper. The hardness of the laser shock processed zone of Al-Li alloy was measured, and the microstructure and mechanical properties of the welded joints of the Ni-based superalloy GH30 and the Austenitic stainless steel !Crl8Ni9Ti were compared with those without LSP in this paper. The results showed that the size of strengthened zone was similar to that of laser spot and strengthened layer was about 1mm deep, and the high intense dislocations and twins produced in the shocked zone. Plastic strain also gained surface residual compress stress, which is benefit for the fatigue properties of welded zones. In this test , the surface hardness of welding zone of the superalloy GH30 improved obviously and tensile strength increased by 12%, but the improvement of fatigue life was not obvious; Martensite phase is formed in plasma welding !Crl8Ni9Ti, which reduced the effect of strain deformation martensite induced by laser shock processing, but the surface residual compress stress gained by laser shock processing can obviously improve the fatigue life of !Crl8Ni9Ti welded joints.

Effects on Microstructure and Properties of Brass Treated by Laser Shock Processing

Laser shock processing(LSP)has been proposed as a new surface treatment for improving hardness,wear resistance and fatigue.In this paper,the effect of LSP on brass is investigated with experiment.Micro-hardness,roughness,microstructure,wear resistance and friction coefficient evolution are investigated for different parameters of LSP.The result shows that the roughness increases after LSP;no ablation is observed;the microstructure has no remarkable change;hardness and the wear resistance increase as the pulse density increases.

A comprehensive study on the oxidation behavior and failure mechanism of(γ’+β)two-phase Ni-34Al-0.1Dy coating treated by laser shock processing

Laser shock processing(LSP),as a novel surface modification technology,exhibits tremendous potential to improve the oxidation resistance of alloys.In this paper,(γ’+β)two-phase Ni-34Al-0.1Dy coatings were prepared by electron beam physical vapor deposition(EB-PVD)and then treated by LSP with different pulse energy(4 and 5 J).Their oxidation behavior(including isothermal oxidation and cyclic oxidation)at 1150°C was compared.In the isothermal oxidation case,the LSP-treated samples exhibited good ox-idation resistance due to high dislocation density and refined grains caused by the LSP.For the cyclic oxidation,however,the LSP-treated samples were subjected to undesirable thermally grown oxide(TGO)spallation.In the early oxidation stage(within 30 min),the residual compressive stress introduced by the LSP rapidly released,resulting in a severe plastic deformation at the coating/TGO interface.As a result,the mismatch between coating and TGO gave rise to the TGO spallation.With the increase in the oxida-tion time,work hardening,which continuously limited the synchronous deformation of the TGO/coating interface,became the dominant mechanism controlling the TGO spallation of 5 J-treated coatings.The continuous TGO spallation affected the Al supply and thus deteriorated the cyclic oxidation behavior.

Microstructure Evolution and Microhardness of Ultrafine-grained High Carbon Steel during Multiple Laser Shock Processing

Surface microstructure and microhardness of （ferrite＋ cementite） microduplex structure of the ultrafine- grained high carbon steel after laser shock processing （LSP） with different impact times were investigated by means of scanning electron microscopy （SEM）, transmission electron microscopy （TEM）, X-ray diffraction （XRD） and microhardness measurements. Equiaxed ferrite grains were refined from 400 to 150 nm, and the cementite lamellae were fully spheroidized, with a decrease of the particle diameter from 150 to 100 nm as the impact times increased. The cementite dissolution was enhanced significantly. Correspondingly, the lattice parameter of α-Fe and microhard- hess increased with the impact times.

Effects of processing parameters on fatigue properties of LY2 Al alloy subjected to laser shock processing

We address the effects of processing parameters on residual stresses and fatigue properties of LY2 Al alloy by laser shock processing （LSP）. Results show that compressive residual stresses are generated near the surface of samples due to LSP. The maximum compressive residual stress at the surface by two LSP impacts on one side is higher than that by one LSP impact. The maximum value of tensile residual stress is found at the mid-plane of samples subjected to two-sided LSP. Compared with fatigue lives of samples treated by single-sided LSP, lives of those treated by two-sided LSP are lower. However, these are higher than untreated ones.

Effects of laser shock processing onθ-Al_(2)O_(3)toα-Al_(2)O_(3)transformation and oxide scale morphology evolution in(γ'+β)two-phase Ni-34Al-0.1Dy alloys

(γ’+β)two-phase Ni-Al is a promising high-temperature protective coating material used on Ni-base superalloys since it has good interfacial compatibility with superalloys due to the low Al content compared to single-phaseβ-NiA l.In this paper,we aim to improve the oxidation resistance,whereby Ni-34Al-0.1Dy,a(γ’+β)two-phase Ni-Al alloy,was treated by laser shock processing(LSP)and the oxidation behavior at 1150℃ was investigated.The results showed that after oxidation,Al_(2)O_(3)scale formed on the originalβphase of the untreated alloy with a small grain size(200-800 nm),while for the LSP-treated samples,the scale grown on the originalβphase was dominantly composed of larger Al_(2)O_(3)grains with a size of 2-3μm.The distinction was attributed to the promotion ofθ-Al_(2)O_(3)toα-Al_(2)O_(3)transformation induced by the LSP,because the dislocation density,as well as surface roughness,were increased during LSP treatment which can provide heterogeneous nucleation sites forα-Al_(2)O_(3).In addition,the larger-size Al_(2)O_(3)particles,derived from the direct conversion of needle-likeθ-Al_(2)O_(3)in the initial oxidation stage,could rapidly overspread the wholeβphase surface thus reducing the scale growth rate.

Effects of Thermally Assisted Warm Laser Shock Processing on the Microstructure and Fatigue Property of IN718 Superalloy

The effects of laser shock processing(LSP)and warm laser shock processing(WLSP)on the microstructure of surface hardening layer and high-cycle fatigue performance at room temperature and high temperature(600°C)of IN718 alloy were investigated.It has been revealed that the grain refined hardening layer with greater residual compression stresses,higher fraction of coincidence site lattice(CSL)boundaries and dislocation densities was formed in WLSP-treated alloy than in LSP-treated alloys.Moreover,microtwins includedγ″phase/high density dislocation complex was found in the surface of WLSP-treated alloy.These characters caused the significant enhancement of the medium value fatigue strength of WLSP-treated alloy at room temperature and elevated temperature.Apparently,the microtwins includedγ″phase/high density dislocation complex formed in the surface hardening layer of LSP-treated alloy has more complicated steric structure and more stable at elevated temperature thanγ″phase/low density dislocation complex formed in LSP-treated alloy,leading to the slow recovery process.Therefore,the surface hardening layer in the WLSP-treated alloy remained more ideal strengthening effect under high-cycle fatigue at elevated temperature than that in LSP-treated alloy.This resulted in the much longer fatigue crack initiation incubation and longer high-cycle life of WLSP-treated IN718 alloy under cycling load at 600℃.This discovery provides a new cognition of fatigue resistance by WLSP treatment of precipitation strengthening superalloy.

Impacts of multiple laser shock processing on microstructure and mechanical property of high-carbon steel

Multiple laser shock processing （LSP） impacts on microstructures and mechanical properties were investigated through morphological determinations and hardness testing. Microscopic results show that without equal channel angular pressing （ECAP）, the LSP-treated lamellar pearlite was transferred to irregular ferrite matrix and incompletely broken cementite particles. With ECAP, LSP leads to refinements of the equiaxed ferrite grain in ultrafine-grained microduplex structure from 400 to 150 nm, and the completely spheroidized cementite particles from 150 to 100 nm. Consequentially, enhancements of mechanical properties were found in strength, microhardness and elongations of samples consisting of lamellar pearlite and ultrafine-grained microduplex structure. After LSP, a mixture of quasi-cleavage and ductile fracture was formed, different from the typical quasi-cleavage fracture from the original lamellar pearlite and the ductile fracture of the microduplex structure.

Effect of Laser Shock Processing on High Cycle Fatigue Properties of 1Cr11Ni2W2MoV Stainless Steel

By means of laser shock processing (LSP) treatment,a nanostructured surface layer was formed on 1Cr11Ni2W2MoV stainless steel.Group contrastive high cycle fatigue tests were carried out in 1Cr11Ni2W2MoV stainless steel before and after LSP treated.The experimental data was analyzed by mathematical statistics method and the influence mechanism of fatigue properties affected by LSP treatment was also researched.The results indicate that LSP treatment can improve the high cycle fatigue life of 1Cr11Ni2W2MoV stainless steel effectively.The ultra-fine grained surface layer with residual compressive stress and nanostructured surface layer make great contributions to the improvement in fatigue properties of 1Cr11Ni2W2MoV stainless steel.

Theory Analysis and Experiment Investigation of Laser Shock Processing on Titanium Alloy Blade

Laser shock processing (LSP) is an effective surface treatment technology to increase the resistance of metallic components to high-cycle fatigue (HCF),stress corrosion cracking (SCC),wear,etc.,through imparting compressive residual stress on the surface.The requirement for aeroengine blade treatment and the principle of LSP were introduced at the beginning of this paper.The key theories and technologies for LSP were analyzed,including improving the pressure model and introducing the technique of eliminating wave reflection at the back of blade during treatment.The nanocrystallation of titanium alloy surface was realized for the first time using the method of laser shocking at the same time no impurity particles were introduced.Through treating one kind of aeroengine blade using LSP,the technology parameters were confirmed.The nature frequency,fatigue life and residual stress were measured.The results showed the blade performance was improved obviously,which established the theory and experiment base of laser shock processing on blisk.

The Influence of Laser Shock Processing to TC4 Titanium Alloy TIG Welding Bead Microstructure and Properties

TC4 titanium tungsten inert gas arc (TIG) weld beads were treated by laser shock processing (LSP) technology.After a single and multiple times LSP introduced,surface hardness,tensile mechanical properties and fatigue life of those weld joints were measured.The results showed that:compared with TC4 titanium alloy TIG joints without LSP treatment,those weld beads surface hardness on weld zone and heat affected zone are nearly uniform.The fatigue life of those TC4 TIG weld joints treated by a single time LSP process is enhanced.With the LSP times increased,TC4 titanium alloy weld beads tensile strength and yield strength are not obviously changed.The special elongation of these welding joints treated by different times LSP treatment will be influenced observably.Special elongation of those joints is decreased with the LSP treatment times increased from one to three.The welding joint special elongation was reduced to the least degree since three times LSP treatment had been introduced.