Investigation Of Laser Shock Processing On Corrosion Properties And Micro-plastic Strengthening Mechanism Of Stainless Steel

Laser shock processing (LSP) is a new and promising surface treatment technique, which has four notable feature:high-pressure, high-energy, ultra-fast and ultra-high strain-rate, so it has uncomparable advantages compared with conventional techniques. Although LSP can improve mechanical properties and fatigue lives of metallic components, a clear scenery between micro-structure and macro-properties of the refined layer, especially micro-structural evolution and improvement mechanism of stress corrosion resistance of low SFE metals, is still pending.In this thesis, ANSI304stainless steel is regarded as the study subject. Four different topics were involved, i.e. the surface integrity (including residual stress, nano-hardness, elastic modulus, surface topography) of ANSI304stainless steel before and after LSP, the micro-structural evolution and grain refinement mechanism induced by LSP, stress corrosion resistance of ANSI304stainless steel induced by LSP, and laser hybrid processing of ANSI304stainless steel. Some important conclusions and innovative achievements of this work were listed as follows:Firstly, we investigated micro-structures of ANSI304stainless steel during multiple LSP impacts were systematically, and presented micro-structural evolution along depth direction. The thesis proposed and entirely revealed grain refinement mechanism induced by plastic deformation during multiple LSP impacts on low SFE metal (such as ANSI304stainless steel) by the fact that multi-direction mechanical twins dividing the coarse grain based on the microstructure observations for the first time. Micro-structural evolution and grain refinement in ANSI304stainless steel subjected to multiple laser shock processing (LSP) impacts were investigated by means of cross-sectional optical microscopy and transmission electron microscopy observations. The plastic strain-induced grain refinement mechanism of the face-centered cubic (FCC) materials with very low stacking fault energy was identified. The micro-structure was obviously refined due to the ultra-high plastic strain induced by multiple LSP impacts. The minimum grain size in the top surface was about50-200nm. Multidirectional mechanical twin matrix (MT)-MT intersections led to grain subdivision at the top surface during multiple LSP impacts. Furthermore, a novel structure with submicron triangular blocks was found at the top surface subjected to three LSP impacts. The grain refinement process along the depth direction after multiple LSP impacts can be described as follows:(ⅰ) formation of planar dislocation arrays (PDAs) and stacking faults along multiple directions due to the pile up of dislocation lines;(ⅱ) formation of submicron triangular blocks (or irregularly shaped blocks) by the intersection of MT-MT (or MT-PDA or PDA-PDA) along multiple directions;(iii) transformation of MTs into subgrain boundaries;(iv) evolution by continuous dynamic recrystallization of subgrain boundaries to refined grain boundaries. The experimental results and analyses indicate that a high strain with an ultra-high strain rate play a crucial role in the grain refinement process of fcc materials subjected to multiple LSP impacts. The achievements have been published in the international well-known journal Acta Materialia (2010,58(16):5354-5362). The reviews thought that combined with prior reporting of the work on high-stacking fault energy FCC aluminum, the work on low stacking fault energy stainless steel made this an important collection that will be widely used. These researches has a good influence in the field of laser processing.Secondly, we designed three types of the U-bend samples with diferent grain size and residual stress state, measured and characterized crack initiation time, crack appearance, residual stress and micro-structure for three types of U-bend samples. Based on these results, we gained the influence mechanism in combination with high-level compressive residual stress and grain refinement of LSP on stress corrosion resistance of ANSI304stainless steel. After immersion in the boiling42%MgCl2solution at143±1℃, the first type of sample cracks after an average value of16.06h, while the second type of sample cracks after an average value of110.43h. However, the third type of sample is tested for a total of300h without visible cracks in the LSPed surface, which is attributed to the combined effects of both high-level compressive residual stress and grain refinement induced by massive LP impacts. The improvement of the SCC resistance is caused by compressive residual stress and grain refinement during LP process. The compressive residual stress has a dominated beneficial effect on the SCC resistance, while tensile residual stresses has a negative effect on the SCC resistance. In addition, the refined grain can also effectively retard the SCC initiation. The above-mentioned results have been published in the international well-known journal Corrosion Science (2012,60:145-152). The reviews thought that "The effect of laser shock peening on mitigation of SCC by inducing compressive stress and refined grains is very interesting. This submission is well written and of high quality"Thirdly, we experimentally studied the effects of LSP on surface appearance, nano-hardness, residual stress and wear performance of ANSI304stainless steel. Results showed that LSP cannot induce martensite phase transformation of ANSI304stainless steel. We obtained residual stress distribution in the sample surface and along depth direction after a single LSP impact, and revealed the formation mechanism of hybrid structure induced by LSP. Multiple LSP impacts cannot obviously induce martensite phase transformation of ANSI304stainless steel, which is due to the fact that the process of multiple LSP impacts isnot involved in laser thermal effect. Multiple LSP impacts cannot effectively improve wear performance and decrease friction coefficient of sample surface. We investigated the effects of LSP on surface geometric features of ANSI304 stainless steel. Results showed that LSP can obviously increase surface waviness, and induce dual-direction process texture. Hence, LSP can generate hybrid surface structure on the surface of ANSI304stainless steel. The above-mentioned results have been published in the international well-known journal Materials Science and Engineering:A (2011,528(13-14):4783-4788).Finally, we studied and analyzed macro-peoperties and fracture morphologies of the treated stainless steel welded joint by LSP, and explored laser hybrid processing, and revealed the formation process of dimple in the surface of welded joint treated by multiple LSP impacts. Yield strength of welded joint treated by multiple LSP impacts was increased by21.79%, and elongation was improved by5.48%, which was attribute to more uniformly dimples and brighter without delamination splitting in the fracture surface of the welded joint induced by multiple LSP impacts. Based on the micro-structure features observed at the fracture surface, the following procedures are involved in the dimple formation process:(1) the nucleation of the microvoids;(2) the growth of these microvoids due to the effects of external force;(3) the coalescence of the microvoids;(4) the ductile fracture of the laser welded ANSI304stainless steel by multiple LSP impacts. In addition, we fabricated non-smooth surface of ANSI304stainless steel by laser engraving and LSP, and the non-smooth surface has excellent surface performances, such as micro-hardness, residual stress and surface roughness. The above-mentioned results have been published in the international well-known journal Materials Science and Engineering:A (2011,528(13-14):4652-4657) and Chinese Journal of Mechanical Engineering (2012,25(2):285-292). Three invention patents have been issued in China.In summary, important innovative research results had been acquired in many aspects of this thesis, including the technological principles of LSP on ANSI304stainless steel, the micro-structure evolution, the micro-structural strengthening mechanism of LSP on ANSI304stainless steel by refining the coarse grain, the stress corrosion resistance, and the laser thermal-mechanical hybrid processing, etc.