Effect of process parameters on residual stress in selective laser melting of AlSi10Mg

ABSTRACT

Residual stress is a key indicator for measuring complex additive components, and its control method has received extensive attention. In this study, the finite element simulation of selective laser melting of AlSi10Mg was performed. It is found that the opposite laser scanning strategy can reduce the final residual stress of the sample. The effect of preheating the substrate to control the residual stress within a certain temperature range is obvious, and the laser scanning speed has the greatest influence on the Z-direction residual stress of the sample. The results show that the sag phenomenon is easy to occur at the laser scanning starting position, and the formed layer is the maximum residual stress region at the junction with the substrate.     

Depth-resolved residual stress analysis of thin coatings by a new FIB-DIC method

A new methodology for the measurement of depth sensitive residual stress profiles of thin coatings with sub-micrometer resolution is presented. The two step method consists of incremental focused ion beam (FIB) ring-core milling, combined with high-resolution in situ SEM-FEG imaging of the relaxing surface and a full field strain analysis by digital image correlation (DIC). The through-thickness profile of the residual stress can be obtained by comparison of the experimentally measured surface strain with finite element modeling using Schajer's integral method. In this work, a chromium nitride (CrN) CAE-PVD 3.0 μm coating on steel substrate, and a gold MS-PVD 1.5 μm on silicon were selected for the experimental implementation. Incremental FIB milling was conducted using an optimized milling strategy that produces minimum re-deposition over the sample surface. Results showed an average residual stress of σ = −5.15 GPa in the CrN coating and σ = +194 MPa in the Au coating. These values are in reasonable agreement with estimates obtained by other conventional techniques. The depth profiles revealed an increasing residual stress from surface to the coating/surface interface for both coatings. This observation is likely related to stress relaxation during grain growth, which was observed in microstructural cross sections, as predicted by existing models for structure-stress evolution in PVD coatings. A correlation between the observed stress gradients and the in-service mechanical behavior of the coatings is proposed. Finally, critical aspects of the technique and the influence of microstructure and elastic anisotropy on stress analysis are analyzed and discussed.     

A transient finite element simulation of the temperature and bead profiles of T-joint laser welds

Laser welding is a high power density welding technology, which has the capability of focusing the beam power to a very small spot diameter. Its characteristics such as high precision and low and concentrated heat input, helps in minimizing the micro-structural modifications, residual stresses and distortions on the welded specimens. In this study, finite element method (FEM) is adopted for predicting the bead geometry in laser welding of 1.6 mm thick AISI304 stainless steel sheets. A three-dimensional finite element model is used to analyze the temperature distribution in a T-joint weld produced by the laser welding process. Temperature-dependent thermal properties of AISI304 stainless steel, effect of latent heat of fusion, and the convective and radiative boundary conditions are included in the model. The heat input to the model is assumed to be a 3D conical Gaussian heat source. The finite element code SYSWELD, along with a few FORTRAN subroutines, is employed to obtain the numerical results. The T-joint welds are made using a Nd:YAG laser having a maximum power of 2 kW in the continuous wave mode. The effect of laser beam power, welding speed and beam incident angle on the weld bead geometry (i.e. depth of penetration and bead width) are investigated. Finally, the shapes of the molten pool predicted by the numerical analysis are compared with the results obtained through the experimentation. The comparison shows that they are in good agreement.     

Interaction of residual stress with mechanical loading in a ferritic steel

A well-defined residual stress field was introduced into modified single edge notched bend, SEN(B), specimens by the ‘in-plane compression’ procedure in order to investigate the interaction between residual stress and applied mechanical loading. Numerical predictions of the residual stress field arising from the in-plane compression procedure are given along with details of the numerical fracture modelling and experimental fracture test results made on A533B ferritic steel specimens in the lower transition region at −150 °C. Use was made of a recently developed finite element post-processor capable of determining path-independent J-integral values in the presence of residual stress fields. The paper compares the experimental results to predictions made using a probabilistic ‘global approach’ based on the conventional crack-tip parameters K and J and predictions made using a well-known structural integrity assessment code, R6 (Revision 4). It is shown that obtaining more accurate estimates of the crack driving force created by residual stresses leads to better correlation between experiments and predictions, and less conservatism in the assessment code.     

Producing nanostructured super-austenitic steels by friction stir processing

In the present work, friction stir processing (FSP) was used to produce the nanostructured super-austenitic steel. After preheating, the specimens were subjected to FSP using the rotation and traverse speed of 2600 rpm and 30 mm min⁻¹, respectively. The specimen temperature during FSP was about 950 ± 2 °C. The results show that a nanostructured layer of about 91 μm thick was produced on the specimen surface. The formed nanograins ranged from 50 to 90 nm. Besides, the hot severe deformation applied during FSP led to significant fragmentation of the coarse sigma particles to nanosize ones.The produced nanostructured layer was then characterized using field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), and scanning electron microscopy (SEM). The formed nanostructure led to a twofold increase in the hardness. The formation of nanostructure resulted in an increase in hardness up to 350 Hv, comparing to 185 Hv pertaining to base structure of super austenitic steel.     

Prediction of Melt Pool Dimension and Residual Stress Evolution with Thermodynamically-Consistent Phase Field and Consolidation Models during Re-Melting Process of SLM

Re-melting process has been utilized to mitigate the residual stress level in the selective laser melting (SLM) process in recent years. However, the complex consolidation mechanism of powder and the different material behavior after the first laser melting hinder the direct implementation of the re-melting process. In this work, the effects of re-melting on the temperature and residual stress evolution in the SLM process are investigated using a thermo-mechanically coupled finite element model. The degree of consolidation is incorporated in the energy balance equation based on the thermodynamically-consistent phase-field approach. The drastic change of material properties due to the variation of temperature and material state is also considered. Using the proposed simulation framework, the single-track scanning is simulated first to predict the melt pool dimension and validate the proposed model with the existing experimental data. The obtained thermal histories reveal that the highest cooling rate is observed at the end of the local solidification time which acts as an important indicator for the alleviation of temperature gradient. Then, the scanning of a whole single layer that consists of multiple tracks is simulated to observe the stress evolution with several re-melting processes. After the full melting of powder material in the first scanning process, the increase of residual stress level is observed with one remelting cycle. Moreover, the predicted stress level with the re-melting process shows the variation trend attributable to the accumulated heat in the tracks. The numerical issues and the detailed implementation process are also introduced in this paper.     

Residual stresses and fracture mechanics analysis of a crack in welds of high strength steels

This study presented the characteristics of residual stresses in welds of high strength steels (POSTEN60, POSTEN80) whose tensile strengths were 600 MPa and 800 MPa, respectively. Three-dimensional thermal elastic-plastic analyses were conducted to investigate the characteristics of welding residual stresses in welds of high strength steels through the thermal and mechanical properties at high temperatures obtained from the elevated temperature tensile tests. A finite element analysis method which can calculate the J-integral for a crack in a residual stress field was developed to evaluate the J-integral for a centre crack when mechanical stresses were applied in conjunction with residual stresses.The results show that the volumetric changes associated with the austenite to martensite phase transformation during rapid cooling after welding of high strength steels significantly influence on the development of residual stresses in the weld fusion zone and heat-affected zone. For a centre crack in welds of high strength steels where only residual stresses are present, increased tensile strength of the steel, increased the J-integral values. The values of the J-integral for the case when mechanical stresses are applied in conjunction with residual stresses are larger than those for the case when only residual stresses are present.     

Effect of laser shock processing on fatigue crack growth of duplex stainless steel

Duplex stainless steels have wide application in different fields like the ship, petrochemical and chemical industries that is due to their high strength and excellent toughness properties as well as their high corrosion resistance. In this work an investigation is performed to evaluate the effect of laser shock processing on some mechanical properties of 2205 duplex stainless steel. Laser shock processing (LSP) or laser shock peening is a new technique for strengthening metals. This process induces a compressive residual stress field which increases fatigue crack initiation life and reduces fatigue crack growth rate. A convergent lens is used to deliver 2.5 J, 8 ns laser pulses by a Q-switched Nd:YAG laser, operating at 10 Hz with infrared (1064 nm) radiation. The pulses are focused to a diameter of 1.5 mm. Effect of pulse density in the residual stress field is evaluated. Residual stress distribution as a function of depth is determined by the contour method. It is observed that the higher the pulse density the greater the compressive residual stress. Pulse densities of 900, 1600 and 2500 pul/cm² are used. Pre-cracked compact tension specimens were subjected to LSP process and then tested under cyclic loading with R = 0.1. Fatigue crack growth rate is determined and the effect of LSP process parameters is evaluated. In addition fracture toughness is determined in specimens with and without LSP treatment. It is observed that LSP reduces fatigue crack growth and increases fracture toughness if this steel.     

Analysis of thermal effect on transparent conductive oxide thin films ablated by UV laser

Transparent conductive oxide thin films are applied to many computer, communication and consumer electronics products including thin film transistor liquid crystal displays, organic light emitting diodes, solar cells, mobile phones, and digital cameras. The laser direct write patterning of the indium tin oxide (ITO) thin film processing technique produces a heat affected zone that has an enormous effect on the electro-optical efficiency of transparent conductive oxide films. This is because direct laser writing patterning in thermal machining process can create debris and micro-cracks in the substrate. Therefore, this study establishes the ultraviolet (UV) laser ablation of temperature model on the polycarbonate and soda-lime glass substrates using the finite element analysis software ANSYS, and measures the temperature field based on the laser micro-patterning process. The meshing model determines the structure of the pre-processors and parameters were set with ANSYS parameter design language. This study also simulates the Gaussian distribution laser irradiation on the pre-processor structure. A UV laser processing system made micro-patterning on ITO thin films to analyze which conditions damaged the substrates. Comparing the simulation and experiment results reveals the minimum laser ablation threshold of the ITO thin films with the melting and vaporization temperatures. Simulation results show that the temperature distribution on PC and soda-lime glass substrates after laser irradiation of 1.05 μs with a laser output power of 0.07 W produces temperatures of approximately 52 °C, 54 °C and 345°Cand 205 °C at the laser output power of 0.46 W. The experiment results show that the patterning region is similar to the simulation results, and the lower laser power does not damage the substrates.     

Analysis of cold expanded fastener holes subjected to short time creep: Finite element modelling and fatigue tests

The beneficial effects of cold expansion have been well documented in previous studies, yet the performance of cold expanded plates exposed to elevated temperatures is an area of technical interest. In this research, finite element (FE) simulations along with experimental fatigue tests have been carried out to investigate the effect of exposure to elevated temperature on residual stress distribution and subsequent fatigue life of cold expanded fastener holes. According to the obtained results, creep stress relaxation occurs due to exposure to 120 °C for 50 h. FE results demonstrate a non-uniform residual stress relaxation regime through the plate thickness around the cold expanded hole and the fatigue test results show that the subsequent fatigue lives have significantly decreased.     

SLM成形薄壁件尺寸偏差预测与控制研究

目的 针对选区激光熔化成形薄壁件过程中存在的变形较大、精度低等问题,通过获得最优工艺参数区间来减小薄壁件的变形。方法 利用有限元软件分析薄壁件成形过程中温度场和应力场的演化规律;建立形变量预测模型并进行试验验证,研究工艺参数对薄壁件尺寸偏差的影响,得到激光功率、扫描速度与形变量之间的关系,实现对形变量的预测和控制。结果 随着扫描层数的增加,熔池的最高温度和热影响区也随之增大,等温线越密集,温度梯度越大,最终趋于稳定;薄壁件成形过程中,出现两侧壁边缘向内倾斜、上侧边缘出现内凹的现象,薄壁件的最大应力随层数的增加而减小,最大热应力主要分布在薄壁件底层的两端;形变量随激光功率的增大而增大,随扫描速度的增大而减小,薄壁件的形变量最小约为0.02 mm;试验验证所建立的数学模型误差在10%左右,误差较小,可以对形变量进行良好的预测和控制。结论 激光功率100～200 W、扫描速度800～1 000 mm/s为最优参数区间;降低能量密度可以有效降低薄壁件形变量,提高其精度。     

Numerical and experimental analysis for solidification and residual stress in the GMAW process for AISI 304 stainless steel

Gas Metal Arc Welding (GMAW) process was analyzed by combining a finite element thermomechanical model for temperature and stress with solidification model. Model prediction was compared with experimental data in order to validate the model. The effects of welding process parameters on these welding fields were analyzed and reported. The effort to correlate the residual stress and solidification was initiated, yielding some valuable results. The solidification process was simulated using the formulation based on the Hunt-Trivedi model. Based on the temperature history, solidification speed and primary dendrite arm spacing were predicted at given nodes of interest. Results show that the variation during solidification is usually within an order of magnitude. The temperature gradient was generally in the range of 10⁴–10⁵ K/m for the given welding conditions (welding power = 6 kW and welding speed = 3.39 to 7.62 mm/sec), while solidification speed appeared to slow down from an order of 10^–2 to 10^–3 m/sec during solidification. SEM images revealed that the Primary Dendrite Arm Spacing (PDAS) fell in the range of 10¹–10² m. The range of predicted sizes was in agreement with the experimental values. It was observed that the average size of the PDAS was dependent upon the welding speed. The PDAS fell between 7.5 to 20 m for columnar and 10 to 30 m for equiaxed dendrites, for welding speeds between 3.39 to 7.62 mm/sec. When the welding speed increased, it was observed that the average size of the PDAS decreased, as the model had predicted. For grain growth at the Heat Affected Zone (HAZ), Ashby's model was employed, and the prediction was in agreement with experimental results. For the residual stress calculation, the same mesh generation used in the heat transfer analysis was applied to make the simulation consistent. The analysis consisted of a transient heat analysis followed by a thermal stress analysis. An experimentally measured strain history was compared with the simulated result. The relationship between microstructure and the stress/strain field of welding was also obtained.     

Finite element simulation of the temperature and stress fields in single layers built without-support in selective laser melting

Overhanging and floating layers which are introduced during the build in selective laser melting (SLM) process are usually associated with high temperature gradients and thermal stresses. As there is no underlying solid material, less heat is dissipated to the powder bed and the melted layer is free to deform resulting undesired effects such as shrinkage and crack. This study uses three-dimensional finite element simulation to investigate the temperature and stress fields in single 316L stainless steel layers built on the powder bed without support in SLM. A non-linear transient model based on sequentially coupled thermo-mechanical field analysis code was developed in ANSYS parametric design language (APDL). It is found that the predicted length of the melt pool increases at higher scan speed while both width and depth of the melt pool decreases. The cyclic melting and cooling rates in the scanned tracks result high VonMises stresses in the consolidated tracks of the layer.     

40Cr激光熔凝硬化组织形态及硬度研究

利用CO2轴流激光加工机对40Cr钢表面进行激光熔凝硬化处理。利用扫描电子显微镜、金相显微镜和显微硬度计研究了不同工艺下熔凝硬化层及基体的显微组织和硬度分布特征。实验表明：熔凝硬化层由熔化区、相变硬化区和热影响区组成;由表及里组织分别为极细隐晶马氏体＋少量残余奥氏体、隐晶马氏体＋碳化物＋残余奥氏体、马氏体＋回火屈氏体＋铁素体。硬化层最高硬度约是基体的3倍;随着扫描速度的增加表层硬度先增加后减小,当扫描速度为2.5m·min^-1时,表层硬度最大,为1097.9HK。     

Experiments and predictions of the effects of load history on cleavage fracture in steel

A series of experiments conducted on two steels, A533B and A508, are summarised. Tests were conducted to explore the influence of different room temperature pre-loading cycles on subsequent low temperature (−150 °C and −170 °C) cleavage fracture. In all cases the low temperature fracture toughness was modified, with tensile pre-loading increasing the toughness and precompression reducing the toughness.Results from finite element simulation of the pre-loading cycles are illustrated. Tensile pre-loading created compressive residual stresses and precompression generated tensile residual stresses. The residual stresses were adopted in a stress based local approach to fracture model using Weibull statistics and applied to the experimental results. The parameters in the Weibull model were calibrated for the virgin steels prior to its application to prior loading cases. The model is found to be successful in predicting the change in toughness relative to the virgin material for pre-loading in tension of A533B steel. The model underestimated the change in toughness for tensile pre-loading of A508 steel and overestimated the toughness change for precompression of both steels.     

Wear assessment of plasma nitrided AISI H11 steel

Plasma nitriding is one of the effective methods for improvement of the hardness, wear and corrosion resistance of steels. In this research AISI H11 hot working tool steel was plasma nitrided in various gas mixtures for different times and temperatures. The morphology, size and composition of nitride nanoparticles formed on the surface of the specimens were investigated using scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD). The wear behavior of plasma nitrided samples was studied by means of unlubricated pin-on-disc method under constant load of 80 N, sliding speed of 1 m/s, sliding distance of 2000 m at room temperature. The results showed plasma nitriding process improved the wear behavior of H11 steel. The increase in time and temperature of plasma nitriding decreased the hardness and increased the wear weigh loss of the specimens.     

On selective laser melting of ultra high carbon steel: Effect of scan speed and post heat treatment

Selective laser melting is a laser‐based additive manufacturing process applying layer manufacturing technology and is used to produce dense parts from metallic powders. The application of selective laser melting on carbon steels is still limited due to difficulties arising from carbon content. This experimental investigation aims at gaining an understanding of the application of the process on ultra high carbon steel, which is a special alloy with remarkable structural properties suitable for different industrial applications. The feedstock ultra high carbon steel (2.1% C) powder, 20 μm to 106 μm was prepared by water atomizing technique. This powder was used for the selective laser melting to build specimens 10×10×40 mm in dimensions. To decrease the thermal stresses during layer by layer building, laser scanning was done through 5×5 mm random island patterns while layer thickness was 30 μm. Laser beam diameter, maximum power output, layer thickness and scan speed range were 0.2 mm, 100 W, 30 μm and 50–200 mm/s respectively. The process was done inside high purity nitrogen environment, with less than 0.5% oxygen content. The results illustrate the influence of scan speed from 50 to 200 mm/s on product geometry and dimensions, surface roughness, internal porosity and cracks, microstructure and surface hardness. The effect of post heat treatment by heating and holding for one hour (annealing) at different temperatures of 700°C, 750°C, 950°C is studied. The results indicate that selective laser melting is able to produce near to 95% density of ultra high carbon steel parts with acceptable geometry and surface quality. Porosity cracks, and microstructure formed during the process could be controlled through proper selection of process parameters and post heat treatment. Industrial ultra high carbon steel products can be rapidly fabricated by selective laser melting.     

Microindentation study of Ti–6Al–4V alloy

In order to study the micromechanical behavior of Ti–6Al–4V alloy, microindentation experiments were performed with five different maximum loads of 100, 150, 200, 250 and 300 mN, and with three loading speeds of 6.4560, 7.7473 and 9.6841 mN/s respectively. The experimental results revealed that loading speed has little influence on microhardness and Young’s modulus. Microindentation hardness experiments showed strong indentation size effects, i.e. increase of indentation hardness with the decrease of indentation load or depth. Then microindentation constitutive equation that described the stress as a function of the strain was proposed through dimensional analysis. And the finite element simulation results showed that the predicted computational indentation data from developed constitutive equation can track the microindentation experimental data of Ti–6Al–4V alloy.     

Deformation rates effects in softwoods: Crack dynamics with lattice fracture modelling

Present study contributes towards understanding crack toughness against the intrinsic deformation rate sensitivity. A methodology for characterizing fracture dependence in softwoods through experimental and numerical analysis has been developed. Time-dependence was found to be the characterising parameter. Image analysis of fracture data acquired with high-speed camera showed that the crack speed histories are stochastic and erratic. In the higher rate range, crack dynamics is characterized as episodic and locally heterogeneous, with irregular jumps and arrests. Critical crack propagation speed at the highest rate tested of 200 mm/min was found to be between 0.7 m/s and 4 m/s (14.3 km/h). Fracture toughness decreased at both slow static and high loading rates, with the mean maximum at 1 mm/min, which is a static deformation rate specific to short-term standard tests. At 200 mm/min deformation rate, inertial effects suggested dynamic fracture response. Explanations of loading rates effects relate to the micro-processes in the fracture process zone (FPZ) and fracture mechanisms, which are simulated with discrete lattice fracture model (LFM). The model included viscous bi-linear stress relaxation into the softening relation and random stochastic finite element properties. Novel characterisation of softwoods is crucial for sensible numerical modeling in seismic structural situations.     

Numerical simulation of residual stresses in diamond coating on Ti-6Al-4V substrate

In this paper, we present numerical simulations of the residual stresses developed between diamond coatings and Ti-6Al-4V substrates when using chemical vapour deposition technique. The large difference in thermal expansion coefficients of diamond and titanium alloys results in high residual stresses in the diamond film. This could lead to interfacial cracking and material failure. The finite element method was used to simulate the cooling process of diamond films at various thicknesses and deposited at temperatures ranging from 600 °C to 900 °C. The influence of different parameters such as temperature, film thickness, material characteristics, geometry and edge effects are investigated for different case geometries. The film debonding and cracking is discussed and numerical results are compared with existing experimental and numerical results. Finally, some propositions are made to enhance the experimental process in order to reduce the residual stress intensities and the possible material degradation.