Effect Of Laser Shock On Surface Residual Stress Of Welded Joint

Welding residual stress is one of the factors that seriously affect the service life of welded structures.

Although there are many methods to change the surface residual stress of welded joints, laser shock treatment, as a new surface strengthening technology, can improve the distribution of residual stress of welded joints, form a residual compressive stress layer on the surface of welded joints, improve the service life of stainless steel welded joints, and achieve the purpose of strengthening stainless steel welded joints.

In this post, 304 stainless steel welded joints were treated by laser shock, and the surface residual stress state of welded joints before and after shock was studied.

The results show that there is tensile stress on the surface of the welded joint before laser shock treatment, and a high residual compressive stress appears on the surface of the impact zone after laser shock treatment, and its influence range has exceeded the impact zone and extended to the periphery of the impact zone.

Welding is a kind of processing technology that uses (in most cases, local) heating or pressurization means, adds or does not add filler materials to connect the components together irrevocably, or deposits a coating on the surface of the substrate.

Due to the highly concentrated instantaneous heat input, considerable residual stress (welding residual stress) will be generated after welding.

Compared with load stress, welding residual stress is the internal force without external force.

Welding residual stress (especially residual tensile stress) will seriously affect the manufacturing process itself and the service performance of welded structure.

When there is complex welding residual stress in the welded joint, it first affects the mechanical properties of the welded joint.

This is because when the structure bears the load and an additional stress (welding residual stress), it is bound to reduce the mechanical properties of the structure, and it is easy to cause brittle fracture of the welded structure;

When the welding structure is in contact with the corrosive medium, the welding residual stress promotes the stress corrosion cracking of the welding structure. The stress corrosion cracking will lead to the sudden fracture of stainless steel components without obvious macro deformation when it is lower than the design allowable stress, resulting in huge economic and social losses and serious harm.

Therefore, various effective measures should be taken to change the state of welding residual stress or reduce welding residual stress.

At present, the widely used methods are:

  • mechanical stretching method
  • explosion method
  • overall high-temperature tempering treatment
  • laser shock treatment

Compared with other processing methods, laser shock processing can freely control the parameters of laser beam and impact mode, and does not damage the sample surface. It has the characteristics of high efficiency, flexibility, heat insulation and non-contact.

Laser shock treatment is to irradiate the metal plate coated with the absorption layer with a high-intensity laser pulse. Due to the short energy deposition time, the absorption layer is vaporized instantly, and the steam temperature rises rapidly to tens of thousands of degrees.

With the continuous absorption of the incident laser beam by the steam, the steam electrons are ionized from the atoms to produce a rapidly expanding plasma flow, The plasma continues to strongly absorb laser energy until the end of deposition time;

The confined high-temperature plasma between the metal target and the transparent constraint layer produces hydrodynamic expansion, resulting in a pressure pulse with high amplitude and short pulse width. The pressure pulse is introduced into the metal material in the form of shock wave and causes the material to undergo plastic deformation, so as to improve the surface hardness of the deformation area and produce beneficial residual compressive stress.

Firstly, the argon arc welding test of 304 stainless steel was carried out to measure the surface residual stress of the welded joint;

Then laser impact is carried out, and then the residual stress on the surface of the welded joint after impact is measured, and the variation law of the residual stress on the surface of the welded joint before and after impact is analyzed.

Test process

Sample preparation

Austenitic 304 stainless steel was selected during the test, with a plate thickness of 2mm.

The original chemical composition and mechanical properties are shown in Table 1.

The sample is welded on the stainless steel plate by TIG welding.

Before welding, polish the welding surface with sandpaper, and then clean and polish the surface with acetone to remove oxide film, dirt, burr and other sundries. Fix the workpiece with fixture during welding to prevent large welding deformation.

The welding torch is fixed on the automatic traveling trolley to ensure the stability of arc length and the invariability of welding speed. Multiple welds are welded under the same welding specification for residual stress control test.

The welding process parameters are shown in Table 2.

Laser shock test

The laser shock treatment test was carried out on a nd glass laser system φ8mm×200mm phosphate neodymium glass is used as oscillating laser rod and primary phosphate laser pre amplification stage (φ14mm×350mm), four stage phosphate laser main amplification stage (divided into two routes, each route φ16mm×350mm and φ20mm×350mm neodymium glass rod (two in series). The wavelength of the output laser is 1.054 μm. The pulse width is 20.50ns, the pulse energy is 10 ~ 50J, and the spot diameter is 3 ~ 7mm.

In order to enhance the absorption of laser energy and protect the surface of the sample from being burned by high-energy laser, aluminum foil tape is used as the energy absorption coating;

In order to enhance the shock wave pressure generated by the laser on the material surface and prolong the action time of the shock wave, the laser transparent water is used as the constraint layer to control the flow of tap water and let it rush to the sample surface.

The water flow velocity parameters have been optimized in advance, and finally the shock treatment is carried out.

After the test, remove the aluminum foil tape, wash the surface sundries with acetone and scrub them with alcohol.

Residual stress test

The blind hole method was used to measure the surface residual stress of welded joints before and after laser shock.

There are often rust spots, oxide layers and oil stains on the surface of the specimen, which is very unfavorable to the adhesion of the deformation sheet.

In order to ensure the bonding quality of the strain gauge, the measured area, that is, the surface of the resistance strain gauge, must be carefully cleaned.

However, attention should be paid to the grinding strength during grinding, and the original residual stress field should not be damaged.

In addition, in order to better ensure the accuracy of measuring residual stress, clean the test point with ethanol cleaner to remove surface dust and impurities.

The sticking direction of the strain gauge is determined according to the direction of the weld and the 45 ° direction of the strain gauge.

Measure the residual stress on the surface of the welded joint after drilling.

Table 1 Chemical composition and mechanical properties of 304 stainless steel

Chemical composition(%) Mechanical property
C Si Mn P S Cr Ni Hardness
/HV
yield strength
/MPa
tensile strength
/MPa
Elongation
(%)
0.058 0.35 1.32 0.032 0.007 17.45 8.28 163 257 1030 70

Table 2 Specification for TIG welding of stainless steel

Current and polarity Tungsten electrode diameter
/mm
Welding current
/A
Welding voltage
/V
Welding speed /
(mm/min)
Gas flow/
(L/min)
DC positive connection 1.6 80 16.5 100 8

Table 3 Test results of transverse residual stress on the surface of welded specimens

Distance between measuring point and weld center / mm/mm 0 1 2 3 4 5
Stress value/MPa Before impact 200 262 -143 -116 -31 -18
After impact -32 -143 -456 -281 -42 -32

Table 4 Test results of longitudinal residual stress on the surface of welded specimens

Distance between measuring point and weld center / mm/mm 0 1 2 3 4 5
Stress value/MPa Before impact 120 -152 一113 -106 -65 -36
After impact -288 -346 -382 -302 -123 -58

Discussion on test results

Take 6 measuring points in the direction perpendicular to the weld, with a spacing of 1mm.

For the single point laser impact specimen, take the weld center of the impact zone as the starting point, and select 6 test points vertically.

The measurement results of transverse residual stress and longitudinal residual stress of stainless steel weld before and after laser impact are shown in Table 3 and table 4 respectively.

Fig. 1 Test results of transverse residual stress on the surface of welded specimen

Fig. 2 Test results of longitudinal residual stress on the surface of welded specimen

From the stress test results, it can be seen that residual tensile stress appears around the weld after TIG welding, while residual compressive stress appears around the heat affected zone, which is mainly due to the thermal expansion of the welding zone in the heating process, which is constrained by the surrounding colder area, resulting in plastic thermal compression in the welding zone;

During the cooling process, the cooling shrinkage of the welding area is constrained by the surrounding area.

The maximum transverse and longitudinal residual tensile stresses obtained after welding are 262mpa and 120MPa respectively.

After laser shock, the surface residual stress of stainless steel weldment is compressive stress, which shows that laser shock treatment can improve the residual stress distribution of stainless steel weld, form a certain residual compressive stress layer in the treatment area, and strengthen the surrounding area.

It can also be seen that there is a central weakening effect of residual compressive stress in the laser shock region, that is, the phenomenon of “residual stress hole”.

This phenomenon is mainly due to the simultaneous generation of surface waves such as Rayleigh wave and shear stress wave at the edge of the impact area. These surface waves converge to the center of the laser impact area, resulting in the reverse plastic strain in this local area and reducing the surface residual compressive stress level here.

In addition, as shown in Fig. 1 and Fig. 2, the residual compressive stress level at the 2#, 3# points on the periphery of the impact zone is significantly higher than that at the 4#, 5# points, which shows that the influence range of the residual compressive stress formed by laser impact treatment is not only within the impact Zone, but also can be extended to the periphery of the impact zone.

Function of coating

The principles for selecting appropriate coatings are:

(1) the coating has high laser absorption rate and is easy to increase the peak pressure of shock wave.

The reflectivity of metal surface to light is very high, and the reflectivity of 304 stainless steel surface to laser is as high as 90% at room temperature

Even if other factors such as oxide film on the metal surface increase the absorption of laser, it can only improve the absorption of light.

The metal surface is pre coated with a coating with high laser absorption rate to increase the absorption of laser.

The addition of organic black paint coating improves the energy coupling efficiency, the pressure effect and the surface residual compressive stress.

(2) it has low thermal conductivity and low vaporization heat, which can increase its own heat absorption and reduce the heat conduction to the thermal target, improve the amplitude and pulse width of stress wave and reduce the heat loss of the target.

Conclusion

Through the measurement of residual stress by blind hole method, it is found that after laser impact, the surface residual stress of stainless steel TIG welding sample is basically compressive stress, and the residual compressive stress exists not only in the impact area, but also extends to the periphery of the impact area.

However, in the laser impact area, the residual compressive stress has a central weakening effect, that is, the phenomenon of “residual stress hole”.

The use of appropriate coating can increase the absorption of metal to laser and greatly improve the residual compressive stress on the metal surface.

At the same time, it can also reduce the heat loss to the metal.

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