High-brightness laser welding of thin-sheet 316 stainless steel

Photolytic iodine laser (PIL), a new industrial laser in the market, offers much higher brightness than existing Nd:YAG and CO₂ lasers. PIL has also a unique wavelength (1315 nm) that has not yet been tested for welding applications. In this work, the capabilities of PIL for precision seam welding of 0.1-mm thick sheet of AISI 316 stainless steel in the lap-joint configuration were evaluated. The weld performance data of PIL laser were compared with Nd:YAG and CO₂ lasers. The astounding benefits of PIL weld are narrow seam, extremely fine solidification cell structure, fully austenitic microstructure, and small heat-affected-zone (HAZ). These benefits are attributed to the PIL's high brightness that in turn enables achieving small spot size and energy transport through plasma rather than by heat conduction. In contrast, the welds produced by Nd:YAG and CO₂ lasers exhibited wider seams, coarser solidification structures, duplex microstructures of austenite and ferrite, and larger HAZ due to slow cooling of the melt, and lateral heat diffusion. Despite the narrow seam, the PIL weld carried a high tensile load (92% that of base metal) and was harder than the base metal. Microstructural analysis revealed that PIL welds exhibited fully austenitic structures and were free from hot cracking. These advantages are consequences of the rapid solidification effects including large undercooling, minimal segregation of impurities to the grain boundaries, and fine grain size.     

Hardening of 316L stainless steel by laser surface alloying

The goal of this study is to investigate different hardening routes for 316L stainless steel by laser surface alloying. We have investigated the formation of iron-chromium carbides by SiC or carbon incorporation, the alloying with submicronic particles of TiC and the precipitation of titanium carbide from mixtures of Ti and SiC. For each hardening route we present the microstructures and the hardness of the processed surface alloys and the conditions leading to the best compromise between highest hardness, best homogeneity and lowest occurrence of cracks. From these results it can be reasoned that hardening by iron-chromium carbides is the best hardening route and that this surface alloy might be a good candidate for tribological applications.     

水导激光加工对316L不锈钢微观形貌的影响规律

水导激光加工是一项利用水光纤将激光引导到材料加工表面的新颖加工技术,具有几乎无微裂纹、热影响区小、无污染、重熔层少、加工精度高和光束平行等优点。为研究不同水导激光加工工艺参数对微观形貌的影响,探索水导激光与物质的相互作用机理。本文采用自主研发的水导激光加工系统对316L不锈钢薄片试件进行切槽和打孔实验;使用Zeiss Vert.A1金相显微镜观察加工试件的二维形貌;使用Leica DVM6超景深显微镜和Bruke Contour Elite I白光干涉仪观察试件的三维微观形貌。实验结果表明：无论是对试件进行切槽还是打孔实验,均会在加工区域产生一定宽度的沉积层,且沉积层的大小不随加工时间和加工次数变化,其宽度约为13.5 µm;通过观察试件加工区域的二维形貌,发现打孔试件的dr和切槽试件的wl也不随加工试件和加工次数变化;通过观察切槽试件加工区域的三维形貌,其截面呈倒梯形。     

Microstructure and mechanical behavior of laser additive manufactured AISI 316 stainless steel stringers

Laser additive manufacturing of stainless steels is a promising process for near net shape fabrication of parts requiring good mechanical and corrosion properties with a minimal waste generation. This work focuses on high aspect ratio AISI 316 steel structures made by superposition of sequential layers. A special nozzle for precise powder delivery together with a monomode fiber laser allowed producing high quality steel stringers on AISI 316 steel substrates. Although the stringers average compositions were inside the austenite plus ferrite range, only austenite phase was verified. The cladded structure presented some internal pores and cracks, responsible by the low Young’s moduli. However, the tensile properties were similar to the base material and other literature results. The three-point flexural tests also produced good results in terms of formability. The fabricated structures proved to be useful for use in mechanical construction.     

Density analysis of direct metal laser re-melted 316L stainless steel cubic primitives

This paper reports on the density of cubic primitives made by Direct Metal Laser Re-Melting, a process variant of selective laser sintering (SLS). Here, stainless steel 316L powder fractions are scanned and fused by a 90 W Nd:YAG laser in consecutive 100 m layers in order to build a 3-Dimensional object. The effects of Q-Switch pulsing frequency, scanning speed and scan spacing on sample density are described. The samples are measured by two methods: a weight/volume analysis and a xylene impregnation technique. The results are supported by microscopy analysis for qualitative arguments. The results show the significant influence of pulsing the beam on the density of the fabricated material. Also reported is the relationship of material density with energy density (as a function of the process parameters; power, scan speed and scan spacing). Optical analysis of material cross sections shows a periodic occurrence of porosity across the whole range of samples. Causes for this are discussed.     

飞秒激光不锈钢表面陷光微结构的制备与性能研究

利用飞秒激光在高真空环境下,在316L不锈钢表面两次交叉扫描制备了周期性微纳结构,并研究了微纳结构对波长范围200~900nm的光波的吸收增强能力。样品表面微结构形貌与成分采用扫描电子显微镜(SEM)和X射线衍射仪测试。第1次扫描采用高能流激光,获得了微米级锥状钉结构,表面覆盖了典型的激光诱导周期性表面结构(LIPSS)。然后将样品旋转90°,采用能流为0.02J/cm2的激光进行第2次扫描,路径与第1次扫描相交。第1次扫描的结构中的LIPSS被第2次低能流激光打断纳米颗粒,从而与锥状钉结构结合形成双尺度微结构。反射率测试结果表明,这种双尺度微结构表面的平均反射率约为2.28%,为光滑表面平均反射率的3.42%。结合XRD分析结果,不锈钢表面获得强陷光性能主要归因于飞秒激光制备的微结构。     

模具钢表面激光沉积316L不锈钢的组织转变及差异性

为了揭示Cr系注塑模具钢表面采用异种材料沉积过程的组织演变机制,在两种模具钢表面沉积316L不锈钢,利用扫描电镜、电子探针、X射线衍射仪等表征微观组织及硬度特征,并结合熔凝动态过程及Schaeffler相图分析界面及沉积层区组织转变机理。结果表明:由于基体元素组成及组成相分布的差异,导致在沉积层界面的组织转变呈现明显差异性。高Cr莱氏体钢组织过渡区厚度为200μm,过渡区组织形态和组成相明显区别于沉积层其他区域,碳化物在过渡区经历了先局部聚集性长大、之后枝晶间均匀分布等复杂变化;而P20钢沉积层的组织过渡区厚度小于20μm,组织以枝晶方式生长且与沉积层非过渡区连通,同时枝晶内发现了弥散分布的颗粒状碳化物。在熔池中C,Cr,Ni等元素的动态变化作用下,两种钢沉积层的非过渡区组成相也呈现明显差异,高Cr莱氏体钢和P20钢沉积层的基体相分别为奥氏体相与马氏体相,由此导致其硬度值变化范围分别为295~325HV_0.2和500~575HV_0.2。     

316L不锈钢选区激光熔化成型非水平悬垂面研究

为了提高选区激光熔化直接成型非水平悬垂面的能力,通过实验研究了影响悬垂面极限成型角度的工艺因素.实验使用316L不锈钢粉末,设计了不同倾斜角度的悬垂平面模型和悬垂曲面模型.结果表明:提高预置粉末密度可以提高悬垂面成型的极限角度;在25 μm的成型厚度下,激光扫描能量输入在0.15～0.2 J/mm时悬垂面成型精度最高,...     

Advanced lightweight 316L stainless steel cellular lattice structures fabricated via selective laser melting

This paper investigates the manufacturability and performance of advanced and lightweight stainless steel cellular lattice structures fabricated via selective laser melting (SLM). A unique cell type called gyroid is designed to construct periodic lattice structures and utilise its curved cell surface as a self-supported feature which avoids the building of support structures and reduces material waste and production time. The gyroid cellular lattice structures with a wide range of volume fraction were made at different orientations, showing it can reduce the constraints in design for the SLM and provide flexibility in selecting optimal manufacturing parameters. The lattice structures with different volume fraction were well manufactured by the SLM process to exhibit a good geometric agreement with the original CAD models. The strut of the SLM-manufactured lattice structures represents a rough surface and its size is slightly higher than the designed value. When the lattice structure was positioned with half of its struts at an angle of 0° with respect to the building plane, which is considered as the worst building orientation for SLM, it was manufactured with well-defined struts and no defects or broken cells. The compression strength and modulus of the lattice structures increase with the increase in the volume fraction, and two equations based on Gibson–Ashby model have been established to predict their compression properties.     

Joining MoSi2 to 316L stainless steel

The feasibility of joining MoSi2 to 316L stainless steel using active brazing techniques was investigated using two interlayer systems: cusil/Nb/cusil and cusil/Ni/cusil (where cusil is a commercially available Cu–Ag eutectic). Dense, uniform joints were obtained with the cusil/Nb/cusil interlayer system, because the coefficient of thermal expansion (CTE) of niobium closely matched that of MoSi2 over a wide temperature range. Matching the CTEs of MoSi2 and the interlayer material shielded the low-toughness MoSi2 from residual stresses formed during cooling from the joint-processing temperature (830°C). The cusil/Ni/cusil interlayer, however, failed to produce adequate joints because of the large CTE difference between nickel and MoSi2.     

Wedge crack nucleation in Type 316 stainless steel

Type 316 austenitic steel has been heat-treated to produce a range of grain sizes and then creep-tested at 625° C at various stresses so as to examine the nucleation and the factors which effect the nucleation of grain-boundary triple point or wedge cracks. An internal marker technique was used to evaluate the extent of the grain-boundary sliding in relation to the total creep strain. Triple point crack nucleation occurred over the entire range of grain sizes and stresses examined when the product of the stress and grain-boundary displacement reached a critical value; the effective surface energy for grain boundary fracture, estimated using an expression derived by Stroh, was in approximate agreement with the surface free energy value indicating that only limited relaxation occurred by plastic deformation. The first cracks were observed to form along grain boundary facets perpendicular to the applied stress direction and with the sliding grain boundaries at high angles (60 to 80°) to the crack growth direction. Subsequent cracking occurred under conditions which deviated slightly from this initial condition, and the increase in crack density with strain was expressed in terms of geometrical factors which take account of the orientation effects.     

Strain-aging in highly worked 316L stainless steel

The room temperature burst pressure of 316L stainless steel burst discs exhibited increases of about 10% over 90 days. This increase may be associated with a strain-aging phenomenon requiring the presence of carbon since tensile property instability in worked austenitic stainless steels has been reported.^[1–5] The cold worked material directly beneath the score root on the burst disc could undergo the strain aging process, thus causing the observed increase in burst strength. Characterization and analysis were therefore undertaken to identify the controlling phenomena in the small heterogeneous volume that controls rupture of the burst disc. Optical metallography and magnetic measurements confirmed the presence of martensite. Nanoindentation hardness measurements were correlated with finite element simulation of the as-formed mechanical properties. A representative portion of the microstructure was then recreated through cold rolling, and subjected to real-time and accelerated thermal aging treatments and mechanical activation analysis. Saturation of strengthening was observed, and a low temperature martensite reversion anneal was found to prevent or reverse the aging process. The results are consistent with previous observations of strain aging, although in this instance the effects are observed over a 10,000-fold greater aging time. Aging mechanisms are discussed, incorporating the phenomenologies of activation enthalpy and aging kinetics. A model explaining the sensitivity of aging rate to extreme cold work-induced dislocation densities and cold work-induced vacancy content is proposed.     

Hydrogen-assisted cracking of sensitized 316L stainless steel

Thin tensile specimens of 316L-type austenitic stainless steel were tested either at room temperature after cathodic charging or whilst undergoing cathodic charging. Throughout this study we have compared solution-annealed samples with samples given the additional sensitization treatment. The results of the tensile tests show that the room temperature yield and ultimate strengths were not much affected by sensitization, whilst significant reduction in the ductility was observed depending on the heat treatment and the method of charging. The specimens tested while undergoing cathodic charging showed 21% reduction of elongation at-fracture for the annealed specimens and 49% reduction of elongation for the sensitized specimens. The fracture surfaces of the specimens tested while cathodically charged show considerable differences between the annealed and the sensitized specimens. The sensitized specimens were predominantly intergranular, while the fracture of annealed specimens showed massive regions of microvoid coalescence producing ductile rupture. The results were correlated with the presence or absence of the martensite phases determined by means of a ferrite detector and transmission electron microscopy studies.     

Recovery of a creep-deformed Type 316 stainless steel

The recovery of the dislocation structures produced in a Type 316 steel during creep has been examined by annealing over a range of temperatures and times, both in the presence and in the absence of stress. The influence of dislocation recovery on subsequent reloading behaviour has also been examined.Initial dislocation recovery occurs rapidly but the rate of recovery subsequently decreases as precipitate effects become more important. Dislocation recovery in the early, rapid stage appears to be controlled by vacancy diffusion between the dislocation links. The application of stress during recovery leads to an enhancement of the recovery rate in agreement with the network coarsening model whilst the incremental strains observed on reloading after recovery correlate well with the changes in dislocation structure produced during the recovery periods.List of symbols and appropriate values l dislocation link length - D _s self diffusion coefficient - b Burgers vector (2.5×10^–1 m) - C _j equilibrium jog concentration - dislocation link tension - k Boltzman's constant (1.38×10^–23 J atom^–1 K^–1) - T absolute temperature - t recovery time - M mobility term - Z frictional term associated with particles - _d dislocation density determined from micrographs - N _d number of dislocation intersections on test line - p length of test line - S foil thickness - ¯l mean dislocation link length - _c mean intragranular particle (carbide) spacing - r ₀ mean intragranular particle radius at timet=0 - r _t mean intragranular particle radius at timet - D solute diffusion coefficient - B solubility of M₂₃C₆ in austenite - particle-matrix interface energy - atomic volume (10^–29m³) - change in dislocation density during recovery period - incremental strain associated with reloading after recovery period - K constant - dislocation density - ₀ dislocation density at timet=0 - _t dislocation density at timet - ₀ friction stress associated with particles - constant (1) - shear modulus - angle between dislocation segments as dislocation breaks through a particle - A 1 cos (/2) - E constant - creep rate - F Taylor factor - L mean slip distance of dislocations - rate of dislocation recovery - stress - _y yield stress - J strength coefficient - _p plastic strain     

Irradiation behavior of nanostructured 316 austenitic stainless steel

In order to get information about radiation resistance of ultrafine grained austenitic stainless steels, a 316 steel was deformed by high pressure torsion. The mean diameter of the grain after deformation was 40 nm. This material was annealed at 350 °C for 24 h or irradiated with 160 keV iron ions at 350 °C. Changes in the microstructure during annealing or irradiation were characterised by transmission electron microscopy (grain size) and laser assisted tomographic atom probe (solute distribution). Results indicate that this annealing has no influence on the grain size whereas the grain diameter increases under irradiation. Concerning the solute distribution, atom probe investigations show evidence of radiation-induced segregation at grain boundaries. Indeed, after irradiation, grain boundaries are enriched in nickel and silicon and depleted in chromium. On the contrary, no intragranular extended defects or precipitation are observed after irradiation.     

Spectral-directional emittance of thermally oxidized 316 stainless steel

The spectral-directional emittance of thermally oxidized stainless steel is measured for angles from normal to grazing, wavelengths between 2 and 10m, and temperatures between 773 and 973 K. The oxide is grown by holding the steel substrate at a high temperature over a long period while exposed to normal atmospheric conditions, until the measured emissive power of the surface achieves an asymptotic level. It is found that the emittance decreases with angle away from the surface normal at the lower end of the measured spectral range and increases with angle at the higher end. The emittance decreases with wavelength, although there is evidence of a peak near 2 pill. The variation with temperature within the measured range is insignificant. Overall higher values for the oxidized steel are measured than those reported in previous work.     

Characterization of sputter-deposited 316L stainless steel films

Sputter-deposited 316L stainless steel films deposited on various substrates were characterized using transmission electron microscopy and X-ray diffractometry. The deposits were found to be fine-grained and the phases present in the films depended on the nature of the substrate. Films of various thicknesses deposited on microscope slides or oxidized stainless steel substrates contained a mixture of two phases: a body centre cubic (b c c) and a modified hexagonal -phase. The hexagonal phase appeared to be an ordered phase, as suggested by the a _O value of the structure, which is twice that for the -martensite found in many deformed stainless steels. These films were hard and brittle, as indicated by microhardness measurements. Films deposited on oxide-free austenitic stainless steel substrates, on the other hand, were mostly b c c and exhibited a dominant 2 00 texture. These films were softer and less brittle than those deposited on oxidized substrates. In situ high-temperature X-ray diffractometry revealed that the -phase transformed to b c c when the films were annealed at 773 K. On annealing at 873 K, the b c c phase transformed to face centre cubic, which remained stable on cooling to room temperature. These results agree with published data which suggest stability of the b c c phase up to 840 K. Some discrepancies from earlier published reports are discussed in the light of the present results.     

Influence of inclusions and heat treated microstructure on hydrogen assisted fracture properties of aisi 316 stainless steel

An attempt has been made to assess the influence of nonmetallic inclusions and heat treated microstructure on hydrogen assisted cracking of AISI 316 austenitic steel. The steel obtained in two levels of cleanliness was given solution annealing treatment in the temperature range of 1173–1473 K, and additional sensitization treatment of 973 K for 26 hours. Hydrogen embrittlement of this steel has been studied by charging Charpy and Compact Tension specimens of ASTM specification, with hydrogen through cathodic polarization. It is found that hydrogen embrittlement susceptibility increases with the presence of large size inclusions, larger grain and sensitized microstructure.     

A preliminary study of laser cladding of AISI 316 stainless steel using preplaced NiTi wire

NiTi wire of diameter 1 mm was preplaced on AISI 316 stainless steel samples by using a binder. Melting of the NiTi wire to form a clad track on the steel substrate was achieved by means of a high-power CW Nd:YAG laser using different processing parameters. The geometry and microstructure of the clad deposit were studied by optical microscopy and scanning electron microscopy (SEM), respectively. The hardness and compositional profiles along the depth of the deposit were acquired by microhardness testing and energy-dispersive spectroscopy (EDS), respectively. The elastic behavior of the deposit was analyzed using nanoindentation, and compared with that of the NiTi wire. The dilution of the NiTi clad by the substrate material beneath was substantial in single clad tracks, but could be successively reduced in multiple clad layers. A strong fusion bonding with tough interface could be obtained as evidenced by the integrity of Vickers indentations in the interfacial region. In comparison with the NiTi cladding on AISI 316 using the tungsten inert gas (TIG) process, the laser process was capable of producing a much less defective cladding with a more homogeneous microstructure, which is an essential cladding quality with respect to cavitation erosion and corrosion resistance. Thus, the present preliminary study shows that laser cladding using preplaced wire is a feasible method to obtain a thick and homogeneous NiTi-based alloy layer on AISI 316 stainless steel substrate.