4.1 Simulation result
The simulation analysis uses ABAQUS software. Firstly, the dynamic response of H13 steel model under LSP was analyzed in the display integration module, and then the static rebound was carried out in the implicit integration module. In case of multiple LSP, the interval static rebound time is the interval time of machining. The final rebound time is 1000 times of the dynamic response time, which makes the material fully rebound.
In Fig. 6-(a), the trend of residual stress of H13 steel is studied by changing the pressure and times of LSP wave. When the pressure increases from 4 GPa to 12 GPa by LSP once, the maximum residual pressure stress and the maximum residual tensile stress show an increasing trend. The pressure stress increases from -472 MPa to -1204 MPa, and the tensile stress increases from 111.7 MPa to 690.4 MPa. In Fig. 6-(b) shows the residual strain corresponding to LSP once. With the increase of shock pressure, the strain increases from 0.013 to 0.139. When the three-times LSP pressure increases from 4 GPa to 12 GPa, the maximum residual pressure stress and the maximum residual tensile stress show an increasing trend. The pressure stress increases from -744.7 MPa to -1652 MPa, and the tensile stress increases from 190.8 MPa to 1019 MPa. Fig. 6-(b) also shows the residual strain corresponding to three-times of LSP. With the increase of LSP pressure, the strain increases from 0.033 to 0.436. It can be concluded that the residual stress and strain increase with the increase of LSP pressure.
However, H13 steel shows 420 MPa residual pressure stress, when the LSP residual tensile stress is greater than 420 MPa, the specimen will have residual tensile stress, and the occurrence of participating tensile stress will lead to cracks in the material. Therefore, in the LSP simulation, the LSP pressure cannot be greater than 8 GPa (residual tensile stress is 441.6 MPa) in once and 7 GPa (residual tensile stress is 461.8 MPa) in three-times.
In Fig. 7, it has shown the residual stress and strain of different LSP pressure. It can be seen that with the increase of LSP pressure, the influence layer of residual stress increases by about 0.2 mm. With the same residual stress, the deeper the stress layer, the stronger the LSP toughness. In Fig. 7-(a), the residual stresses of LSP three-times by 6 GPa have approximately equaled to the LSP once by 8 GPa. And the pressure layer of LSP once by 8 GPa is about 0.4 mm deeper than LSP three-times by 6 GPa. Its strain is about 0.02, and its smaller than LSP three-times by 6 GPa from the Fig. 7-(b). When it maintains the same LSP pressure, with the LSP times increase, the residual stress and stress layer increase. Therefore, it can be concluded that the LSP pressure is larger and the pressure layer is deeper at one time. The residual stress is larger and the stress layer is deeper at three-times.
4.2 Experimental Result
The residual stress increases with the increase of LSP pressure. Excessive residual stress is easy to cause cracks and reduce the service performance. In order to obtain large residual stress without crack, the required laser energy and spot size are calculated with 6 Gpa as LSP pressure. After LSP processing, the surface morphology of the material as shown in the Fig. 8-(a), (b), (c), and (d) respectively correspond to the raw material, once LSP by black paint absorption, three-times LSP by black paint absorption, and three-times LSP of no absorption. It can be seen that the deformation of specimen (c) is larger than the others.
4.2.1 Micro-hardness
The hardness of the material was tested by micro-hardness tester. The hardness of the surface was randomly taken from 15 points, and the average hardness was obtained by removing the maximum value and the minimum value. In Fig. 9-(a), the a, b, c, and d respectively correspond to the raw material, once LSP by black paint absorption, three-times LSP by black paint absorption, and three-times LSP of no absorption, which hardness are about 550 HV, 616 HV, 650.7 HV, and 629.36 HV respectively. Compared with the hardness, it can be concluded that the strengthening effect is the best when the black paint absorption and LSP for three-times, the hardness is increased by 100.7 HV and the strengthening effect is about 20.13%. In terms of hardness, the scheme of three-times LSP of black paint absorption is the best.
4.2.2 Residual stress
The residual stress of the material was measured by X-ray stress measuring instrument. The residual stress of 15 points was randomly selected on the surface. In Fig. 9-(b), the a, b, c, and d respectively correspond to the raw material, once LSP by black paint absorption, three-times LSP by black paint absorption, and three-times LSP of no absorption. According to the Fig. 9-(b), the residual compressive stress of the a is 420 MPa. The residual compressive stress of the c is the largest, about 911 MPa. The residual stresses of the b and c are approximately equal. The residual stress and simulation stress of the c combined with the residual stress of the a, can be obtained that the experiment and simulation results are approximately equal. The b is the once LSP, its residual stress layer of the specimen is small, and the maximum residual stress appears near the surface layer. The residual stress measured by X-ray residual stress measuring instrument is the maximum residual stress of the material, which is equivalent to the simulation. However, the residual stress of the d has quite different from the simulation results. The reduction of the residual compressive stress in the d is due to the annealing effect of high energy laser irradiation on the sample without absorption layer. From the aspect of residual stress, the scheme of three-times LSP with black paint absorption (the c) is the best.
4.2.3 Friction coefficient
(1) Average friction coefficient
After LSP to strengthen H13 steel, the friction experiments were carried out by the friction and wear testing machine. Each experiment parameter is repeated three-times to obtain the stable friction coefficient and calculate the average friction coefficient. Fig. 10-(a) shows the average friction coefficient of the raw material, once LSP by black paint absorption, three-times LSP by black paint absorption, and three-times LSP of no absorption. And the initial pressure is 75 N and 100 N in fiction. When the initial pressure is 75 N, the friction coefficient of black paint absorption LSP three-times is 0.4091 that is the lowest, and the raw material is 0.4601. The results are consistent with the trend of hardness. The reason for this phenomenon is that the highly hardness of material and relative fine grains. And when the initial pressure is 100 N, the friction coefficients become increase, but the trend is consistent with the hardness. The reason for the increase in friction coefficient is that when the initial pressure increases, the friction type and w
After LSP to strengthen H13 steel, the friction experiments were carried out by the friction and wear testing machine. Each experiment parameter is repeated three-times to obtain the stable friction coefficient and calculate the average friction coefficient. Fig. 10-(a) shows the average friction coefficient of the raw material, once LSP by black paint absorption, three-times LSP by black paint absorption, and three-times LSP of no absorption. And the initial pressure is 75 N and 100 N in fiction. When the initial pressure is 75 N, the friction coefficient of black paint absorption LSP three-times is 0.4091 that is the lowest, and the raw material is 0.4601. The results are consistent with the trend of hardness. The reason for this phenomenon is that the highly hardness of material and relative fine grains. And when the initial pressure is 100 N, the friction coefficients become increase, but the trend is consistent with the hardness. The reason for the increase in friction coefficient is that when the initial pressure increases, the friction type and wear morphology of the experiment piece change.
ange.
(2) Friction coefficient
Figure 10-(b) shows friction coefficient of the friction process. In the friction process, except for early fluctuations, the overall trend is the same as that of hardness. As the friction zone enters the bottom layer of the point region, the hardness of the material in this region is uniform, and the friction coefficient tends to be stable. After initial fluctuation, the friction coefficient is stable at 0.4215. Because the black paint absorption LSP once, the surface hardness of the sample is low, and the friction coefficient is large. At the same time, the wear gradually approaches the strengthening area near the surface layer, where the strengthening points overlap more times, the friction coefficient is lower, and the wear type does not change when the initial pressure is 75N. Therefore, the general trend of the friction coefficient of the sample black paint absorption lsp once by 75N shows that it fluctuates and decreases, and finally tends to be stable.
Figure 10-(b) shows friction coefficient of the friction process. In the friction process, the overall trend is the same as that of hardness, except for the early fluctuation. When the initial pressure is 75 N, after the friction coefficient is stable, there is a big fluctuation of black paint absorption LSP once, because of the lower surface hardness. And its main strengthening layer is near the surface layer, the friction coefficient is low in the area where the number of strengthening spot overlaps is more. After the initial fluctuation, the friction coefficient is stable at 0.4215. Because the friction area enters into the bottom layer of the spot area, the material hardness in this area is uniform, and the friction coefficient tends to be stable.
However, when the initial pressure is 100 N, this phenomenon does not appear. This is because the initial pressure increases, the friction process enters the near surface area with uniform hardness quickly. Therefore, when the initial pressure is 100 N, the friction coefficient curves fluctuate little. From the Fig. 10-(b), it can be seen that when the initial pressure is 75 N, compared with the initial pressure of 100 N, the friction coefficient is lower, and the trend is the same as the average friction coefficient.
4.2.4 Wear morphology
(1) Macro morphology by white light interference
Wear morphology was observed by white light interferometer, and wear volume was calculated. Fig. 11 shows the wear morphology and wear volume of under the 75 N and 100 N. Fig. 11-(a) shows wear morphology of raw materials. The wear surface is rough and has large lumps. The reason for this phenomenon is that the hardness of the material is relatively low, the flake will fall off under high load friction. In Fig. 11-(b) and (c), the depth of wear mark after LSP once is lower than that of three-times with black paint absorption layer. Moreover, there are more furrows in the wear morphology after LSP once. When the impact times are the same, the depth of wear mark with black paint absorption is lower than that no absorption. In Fig. 11-(i), the regularity of wear volume is the same as that of friction coefficient. This shows that the hardness increases, but the friction coefficient and wear volume decrease. When the initial load is 100 N, the regularity of wear morphology is the same as that of 75 N. As shown in Fig. 11-(i), the wear volume increases with the increase of initial pressure.
(2) Micro morphology by SEM
The wear morphology was observed by SEM. There are some oxides and carbides in the wear marks. Due to the deep wear marks, the material oxidizes to form iron oxide. Carbide is mainly the material and the hard phase in the grinding ball falls off and is rolled into the wear mark by the grinding ball. In Fig. 12-(a), there are mainly flake shedding and micro cutting marks in the wear morphology. In Fig. 12-(b), in addition to flake shedding, there are furrows and squashed abrasives in the wear morphology. The detached abrasive grains form three-body wear with the worn parts, thus forming furrow. At the same time, the exfoliated abrasive particles are rolled into the worn surface by GCr15 to form squashed abrasives. In Fig. 12-(b), the wear mark surface is mainly micro cutting, furrows and abrasives. There are only some carbides in the wear marks. And the wear mark surface of LSP three-times no absorption is mainly micro cutting and a small amount of shedding. As the initial pressure increases, the quality of wear morphology decreases. As shown in Fig. 12-(e), (f), (g), and (h), the wear morphology has appeared the massive flakes. Due to the increase of load, there are more adhesive wear in the friction type, resulting in a large number of surface shedding. This is also the reason why the friction coefficient increases with increasing load.
The wear morphology was observed by SEM. There are some oxides and carbides in the wear marks. Due to the deep wear marks and the increase of temperature in the friction process, oxidative wear occurs on the wear mark surface, and the material oxidizes to form iron oxide. Carbide is mainly the material and the hard phase in the grinding ball falls off and is rolled into the wear mark by the grinding ball. In Fig. 12-(a), there are mainly flake shedding and micro cutting marks in the wear morphology. In Fig. 12-(b), in addition to flake shedding, there are furrows and squashed abrasives in the wear morphology. Different from the raw materials, the wear marks of LSP samples are reduced due to the increase of hardness. The phenomenon of peeling oxide layer exists, and the peeling oxide layer is rolled to the wear surface by GCr15, forming crushing abrasive. The oxide layer, which is not completely exfoliated, is separated in the process of friction, resulting in three-body wear between abrasive particles and worn parts, thus forming grooves. In Fig. 12-(b), the wear mark surface is mainly micro cutting, furrows and abrasives. And the wear mark surface of LSP three-times no absorption is mainly micro cutting and a small amount of shedding. Therefore, when the initial pressure is low, abrasive wear is dominant, and oxidation wear is accompanied in the wear marks. As the initial pressure increases, the quality of wear morphology decreases. As shown in Fig. 12-(e), (f), (g), and (h), the wear morphology has appeared the massive flakes. Due to the increase of load, there are more adhesive wear in the friction type, resulting in a large number of surface shedding. This is also the reason why the friction coefficient increases with increasing load.
4.3 Analysis and discussion
Through simulation and experiment, it is found that the hardness and residual stress of the H13 steel after three-times of LSP are higher than that the once LSP. This is due to the plastic deformation caused by high pressure shock wave and the increase of dislocation density caused by shock wave, resulting in grain refinement. Meantime, the effect of black paint absorption is better, especially the residual stress. In the aspect of friction coefficient, for the H13 steel with good strengthening effect, the friction coefficient is relatively low. And the early running in wear stage is relatively short. The friction coefficient is related to the surface quality and material properties of the friction pair, but not to the applied load. However, the friction coefficient is increased by 100 N. Through SEM, when the initial pressure is 100 N, there are more flakes appeared on the surface. The reason for this phenomenon is that as the load increases, the heat generated during the friction process also increases. And the wear type changes from simple abrasive wear to composite wear. The composite wear mainly includes abrasive wear and adhesive wear. The change of wear type, especially the appearance of adhesive wear, directly leads to the deterioration of the friction pair surface, thereby increasing the friction coefficient.