Stainless Steel Deposits Research Articles

Laser directed energy deposition of AISI 316L stainless steel: The effect of build direction on mechanical properties in as-built and heat-treated conditions

The purpose of this study was to evaluate the effect of build direction on the mechanical properties of AISI 316L stainless steel using the Laser Directed Energy Deposition (L-DED) process, in the as-built and heat-treated conditions. The test specimens were manufactured in vertical and horizontal directions for tensile and impact tests. In addition, analysis test specimens cube-shaped were manufactured for microstructural and microhardness evaluation. The microstructure in the as-built condition showed a combination of cellular, equiaxial dendritic, cellular dendritic and columnar dendritic, while the microstructure in the heat-treated condition showed a homogeneous characteristic, composed by differently sized grains. The microhardness evaluation in the heat-treated condition presented a reduction of 26.52% compared to the as-build condition. The ultimate tensile strength of horizontal test specimens in the as-built condition was 4.86% higher than the heat-treated condition, whereas the ultimate tensile strength of vertical test specimens in the as-built condition was 5.55% higher than the heat-treated condition. The impact resistance of horizontal test specimens in the heat-treated condition was 12.36% higher than the as-built condition, whereas the impact resistance of vertical test specimens in the heat-treated condition was 18.92% higher than the as-built condition. Briefly, the build direction directly affects the mechanical properties of components manufactured through the L-DED process, and it is possible to improve certain mechanical properties, such as ductility and toughness, through heat treatment.

Grain refinement by dynamic recrystallization during laser direct energy deposition of 316L stainless steel under thermal cycles

During direct energy deposition (DED) process, the deposited material undergoes complex thermo-mechanical processing from subsequent thermal cycles. Dynamic recrystallization (DRX) can be activated under suitable strain and temperature conditions, which can induce grain refinement and homogenization. In this work, the evolution of temperature and thermal strain are experimentally measured, and the corresponding evolution of grain morphology, geometric dislocation density (GND), dislocation morphology and misorientation map under multiple thermal cycles during laser-DED of 316L stainless steel are investigated. Results show that DRX is activated during the 8th to 85th thermal cycles, during which temperature evolves in the range of 730 K – 800 K – 700 K and the corresponding strain evolves in the range of 4.5% – 7.0% – 5.3%. Nucleated grains grow by consuming grain boundary energy, with original columnar grains swallowed. Equiaxed grains are formed at the end of 90 thermal cycles. Dynamic recovery (DRV) would occur during the thermal cycling process, with the formation of sub-grains, the increased GNDs and the random grain misorientations through dislocation movement and entanglement promoted by high temperature and accumulated strain.

The selection of laser beam diameter in directed energy deposition of austenitic stainless steel: A comprehensive assessment

One focus of laser directed energy deposition (L-DED) is to increase productivity, and this can be achieved by using a larger beam diameter and higher laser power. However, it is difficult to build complex metallic components with fine structures using a laser beam of large diameter. Under the trade-off between formability and productivity, the choice of beam diameter needs to be thoroughly evaluated. Herein, three beam diameters (0.75 mm, 1.5 mm, and 3 mm) in building austenitic stainless steel parts have been investigated and compared in four aspects: microstructure, mechanical properties, powder utilization, and as-built part surface quality. Specifically, cooling rate and powder splashing in the building process, and phase composition, grain size and dislocation density of as-built parts are revealed by means of numerical simulation, high-speed camera, electron backscattered diffraction, and X-ray diffractometry. Results show that for a small beam diameter, the microstructure formed has finer grains and less amount of ferrite due to higher cooling rate of the molten pool, and grain refinement and high density of dislocations lead to superior mechanical properties of the as-built part; conversely, for a larger beam diameter, more stable molten pool and less amount of powder splashing brings higher material utilization rate and better surface quality. A brief evaluation of beam diameter selection is accordingly proposed, and a novel hybrid beam diameters building strategy is presented for building complex metallic components with high quality and efficiency.

High-speed-rate direct energy deposition of Fe-based stainless steel: Process optimization, microstructural features, corrosion and wear resistance

Different types of the Fe-based stainless steel coatings were prepared by a new laser additive manufacturing technology, i.e. high-speed-rate direct energy deposition (HDED), at different deposited speeds. Subsequently, typical microstructural features, corrosion and wear behaviors were investigated. Results indicated that all the coatings were dominantly composed of ultra-fined structures with the grain size less than 2 μm. The HDEDed coating manufactured at the deposited speed of 60 m/min exhibited superior corrosive resistance with the ranking Rt (about 463.185 kΩ·cm2), which is about four time that of coating manufactured at the deposited speed of 30 m/min (about 123.910 kΩ·cm2). In addition, the lowest repassivation current density (0.16 μA/cm2) also indicates the superior secondary passivation capability. The micro-hardness of the coatings manufactured at higher deposited speeds (50 and 60 m/min) was 20% higher than that of the coatings manufactured at lower deposited speed (30 and 40 m/min). However, the wear resistance of the coatings would reach a threshold value when the deposited speed attained a certain level with the other process parameters remaining constant.

Modeling of laser aided direct metal deposition of stainless steel using supervised deep learning algorithms

Laser Direct Metal Deposition (LDMD) is a very promising additive manufacturing methodology which provides metal cladding layer over the substrate with good corrosion resistance, desired surface finish and very accurate geometry. In this work, an attempt has been made to develop a forward and reverse model of laser aided direct metal deposition utilizing deep learning algorithms. Forward model aims at predicting the deposited bead height and width from the known set of LDMD process parameters such as laser power, scanning velocity, and powder flow rate. Reverse modelling shows the effectiveness of each input parameter over the chosen response factors. Therefore, in the reverse modelling, the optimized input parameters values have been tried to predict to get the desired deposition characteristics during experimentation. Three different neural networks based deep learning architectures, namely, Multi-Layer Feed Forward Neural Network (MLFFNN), Recurrent Neural Networks (RNN) and Radial Basis Function Neural Network (RBFNN), have been put forward to develop the said predictive models. The supervised learning algorithms are used to train the neural networks. The performances of MLFFNN, RNN and RBFNN models are compared among themselves. The results show that all the models are capable of making better predictions and the models can be effectively used in shop floor in optimal selection of most influential parameters for the desired outputs.

Numerical and Experimental Investigations on Deposition of Stainless Steel in Wire Arc Additive Manufacturing

This paper presents numerical and experimental investigations on wire arc additive manufacturing for deposition of 430L ferritic stainless steel. Finite element analysis was used to predict temperature distribution for deposition of multiple layers in wire arc additive manufacturing. The transient temperature distribution and predicted by finite element simulation was in good agreement with the experimental results. A wall type structure was fabricated by deposition of multiple layers vertically, and deposited material was characterized by tensile testing and microstructure study. The microstructure of the deposited wall structure was investigated through optical microscopy and scanning electron microscopy (SEM) with EDS. The microstructure of deposited material was changed from fine cellular grains structure to columnar dendrites structure with the formation of secondary arm. It was found that the YS, UTS, and EL of the deposition direction were better than the build direction. The mechanical properties of the WAAM manufactured material was found comparable to that of the wire metal.

Transformation law of microstructure evolution and mechanical properties of electron beam freeform fabricated 321 austenitic stainless steel

AISI 321 austenitic stainless steel deposition was manufactured by electron beam freeform fabrication, which was to analyze the changes in microstructure evolution and mechanical properties along deposited direction, and to explore their underlying causes in view of the change in thermal process of deposited layers. The microstructure was transformed from columnar crystals to dendrites along fabricated direction. Both δ-ferrite and acicular α′-martensite were formed within the deposition due to the non-equilibrium solidification. More than one kind of M23C6 particles precipitated inside δ-ferrite. The content of all phases varied in different regions, corresponding to the increased density of δ-ferrite but decreased density of α′-martensite along the fabricated direction. Transformation law of microstructure evolution was analyzed. The change in heat dissipation condition in different deposited layers and the component fluctuation especially for Cr and Ni were the main reasons for the microstructural variation. The fabricated 321 austenitic stainless steel showed the decent mechanical properties, of which the maximum microhardness of 198 HV and the maximum tensile strength of 765 MPa. Considering the decrease in α′-martensite, both microhardness and tensile strength were decreased along fabricated direction.

Microstructure and mechanical properties of hybrid additive manufactured dissimilar 17-4 PH and 316L stainless steels

Hybrid metal additive manufacturing is a novel way of combining the advantages of different manufacturing methods. It offers the freedom of fabricating functional parts with varying requirements such as size and complexity, overcoming the need for additional joining operations. On the other hand, combining dissimilar materials enables the route for obtaining different properties such as mechanical strength, high temperature and corrosion resistance in a single part body. In this study, wire arc additive manufacturing (WAAM) and laser powder bed fusion (L-PBF) technologies were combined utilizing dissimilar metals for each process. Deposition of AISI 316L stainless steel on previously L-PBF manufactured 17-4 PH stainless steel part was successfully executed using WAAM process. Microstructural observations showed five different zones in the as-built state whereas a uniform microstructure was obtained following the heat treatment as confirmed by hardness measurements. Tensile tests were conducted on the specimens extracted from the L-PBF/WAAM interface and the results showed a ductile fracture from the WAAM region, validating the strength of the interface. The tensile test results exceeded the standard strength requirements with a minor decrease in ductility.

Closed-loop deposition of martensitic stainless steel during laser additive manufacturing to control microstructure and mechanical properties

A closed-loop laser additive manufacturing (CLAM) system was developed to monitor and control layer-by-layer fabrication via directed energy deposition (DED). The well-modified laser additive manufacturing (LAM) package utilizes a thermal infrared camera and thermal image processing algorithms to extract and control the thermal dynamics of the process, including (i) cooling rate, (ii) melt pool temperature, and (iii) heating rate. Martensitic stainless steel of grade S410-L was used to cumulatively deposit 5 layers on top of each other at different processing conditions without closed-loop control. Significant deviations of cooling rate and melt pool temperature are observed within different layers of a thin-wall structure without feedback control in the open-loop state. To assess the effects of cooling rate control on final LAM properties, three samples with different setpoint cooling rates of 550, 1100, and 1750 °C/s are tested under closed-loop conditions. Microstructural and microhardness maps and profiles of open-loop and closed-loop samples reveal highly uniform and controlled hardness profiles when closed-loop control is applied, in contrast to highly varying properties in samples produce in open-loop operation. The current research results illustrate how one can achieve controlled microstructural characteristics and mechanical properties through real-time control of the cooling rate during the LAM process.

Effect of heat treatment on microstructure and hardness of a worn rail repaired using laser powder deposition

• Laser powder deposition is a highly recommended technique to repair worn rails. • 304L deposition steel gives a standard, martensite-free microstructure. • Heat treating the repaired rail increases hardness and eliminates the micropores. • Preheating may mitigate the developed microcracks at the rail-deposition interface. The frequent replacement of worn rails on tracks brings an immense economic burden on the railroad industry, and also causes significant interruptions to railroad operation. Restoration of worn rails via laser powder deposition (LPD) can considerably reduce the associated maintenance costs. This study was focused on the use of LPD to repair the worn profile of a standard U.S. rail. The microstructure of the 304L stainless steel deposits with a minimum hardness of 85 HRB was composed of austenite, δ-ferrite, and sigma. Micropores were dispersed throughout the deposit, and microcracks were found at the rail-deposition interface. The pearlitic rail substrate showed a moderate hardness of 94 HRB. The fine-grain, pearlitic-ferritic heat affected zone had the maximum hardness of 96 HRB, which was still below the minimum required hardness of 97 HRB for a typical rail. To increase the hardness to or above 97 HRB and to mitigate the microstructural defects, the as-repaired rail went through a heat treatment process. The average hardness of the as-treated rail was increased significantly, i.e., to 103 HRB. Besides, the porous and coarse-grain deposition materials were transformed into an impermeable and fine-grain microstructure. However, heat treatment intensified the microcracks at the rail-deposition interface and also led to the formation of martensite and augmentation of the micropores in the parent rail. Isolation of the base rail during heat treatment and preheating were suggested as solutions for the problematic results. The LPD process ultimately was found to be a promising technique for repairing rails.

Features of the Formation of Structure and Geometry during Direct Laser Deposition of 321L Stainless Steel on Complex Groove Using the LENS Method

Additive manufacturing is a new approach to solving the problems of creating high-performance instrument components. This article is devoted to the solution of the narrowly directed problem of surfacing metal powder 321L using the LENS method on the solenoid valve core. The core is made of electrical steel, in which a complex groove is machined. The difficulty of this work was to accurately perform the geometry of the melt layer that is not standard for this method. As a result, the product was obtained according to a completely new approach to its design, which subsequently gave increased operational characteristics of the final product. The article presents the problems that can be encountered when performing complex crevice surfacing on a complex groove. The structure of the deposited layer is considered, defects characteristic of this method in the form of interlayer pores, unalloyed powder particles are identified and described. The LENS process control modes are presented in order to reduce defects and improve the development of surfacing geometry.

Hot-Wire Laser-Directed Energy Deposition: Process Characteristics and Benefits of Resistive Pre-Heating of the Feedstock Wire

This study investigates the influence of resistive pre-heating of the feedstock wire (here called hot-wire) on the stability of laser-directed energy deposition of Duplex stainless steel. Data acquired online during depositions as well as metallographic investigations revealed the process characteristic and its stability window. The online data, such as electrical signals in the pre-heating circuit and images captured from side-view of the process interaction zone gave insight on the metal transfer between the molten wire and the melt pool. The results show that the characteristics of the process, like laser-wire and wire-melt pool interaction, vary depending on the level of the wire pre-heating. In addition, application of two independent energy sources, laser beam and electrical power, allows fine-tuning of the heat input and increases penetration depth, with little influence on the height and width of the beads. This allows for better process stability as well as elimination of lack of fusion defects. Electrical signals measured in the hot-wire circuit indicate the process stability such that the resistive pre-heating can be used for in-process monitoring. The conclusion is that the resistive pre-heating gives additional means for controlling the stability and the heat input of the laser-directed energy deposition.

Finite Element Modeling of Residual Stress at Joint Interface of Titanium Alloy and 17-4PH Stainless Steel

Residual stress is a crucial element in determining the integrity of parts and lifetime of additively manufactured structures. In stainless steel and Ti-6Al-4V fabricated joints, residual stress causes cracking and delamination of the brittle intermetallic joint interface. Knowledge of the degree of residual stress at the joint interface is, therefore, important; however, the available information is limited owing to the joint’s brittle nature and its high failure susceptibility. In this study, the residual stress distribution during the deposition of 17-4PH stainless steel on Ti-6Al-4V alloy was predicted using Simufact additive software based on the finite element modeling technique. A sharp stress gradient was revealed at the joint interface, with compressive stress on the Ti-6Al-4V side and tensile stress on the 17-4PH side. This distribution is attributed to the large difference in the coefficients of thermal expansion of the two metals. The 17-4PH side exhibited maximum equivalent stress of 500 MPa, which was twice that of the Ti-6Al-4V side (240 MPa). This showed good correlation with the thermal residual stress calculations of the alloys. The thermal history predicted via simulation at the joint interface was within the temperature range of 368–477 °C and was highly congruent with that obtained in the actual experiment, approximately 300–450 °C. In the actual experiment, joint delamination occurred, ascribable to the residual stress accumulation and multiple additive manufacturing (AM) thermal cycles on the brittle FeTi and Fe2Ti intermetallic joint interface. The build deflected to the side at an angle of 0.708° after the simulation. This study could serve as a valid reference for engineers to understand the residual stress development in 17-4PH and Ti-6Al-4V joints fabricated with AM.

Thermal field prediction in DED manufacturing process using Artificial Neural Network

In the last decade, machine learning is increasingly attracting researchers in several scientific areas and, in particular, in the additive manufacturing field. Meanwhile, this technique remains as a black box technique for many researchers. Indeed, it allows obtaining novel insights to overcome the limitation of classical methods, such as the finite element method, and to take into account multi-physical complex phenomena occurring during the manufacturing process. This work presents a comprehensive study for implementing a machine learning technique (artificial neural network) to predict the thermal field evolution during the direct energy deposition of 316L stainless steel and tungsten carbides. The framework consists of a finite element thermal model and a neural network. The influence of the number of hidden layers and the number of nodes in each layer was also investigated. The results showed that an architecture based on 3 or 4 hidden layers and the rectified linear unit as the activation function lead to obtaining a high fidelity prediction with an accuracy exceeding 99%. The impact of the chosen architecture on the model accuracy and CPU usage was also highlighted. The proposed framework can be used to predict the thermal field when simulating multi-layer deposition.

Optimization of Direct Energy Deposition of 304L Stainless Steel through Laser Process Parameters

Optimization of Direct Energy Deposition of 304L Stainless Steel through Laser Process Parameters

Numerical investigation on heat transfer of melt pool and clad generation in directed energy deposition of stainless steel

Due to its prevailing advantages such as rapid production of complex metallic parts, efficient cladding and repairing the sophisticated components, the directed energy deposition (DED) process has been widely adopted in the additive manufacturing industry recently. Numerous research efforts were placed in the studies of clad formation and the influences of process parameters during the DED process. As the flow dynamics of melt pool plays a significant role in the heat transfer characteristics as well as the mass transfer mechanism, further understanding of the metallic flow evolutions of melt pool becomes practically demanded for the DED process. Therefore, the abilities to model the complex process of heat transfer and accurately predict the mass addition during the DED process are highly desirable. A computational fluid dynamics model is proposed to simulate the melt pool generation and clad formation in this study. The volume of fluid (VOF) method is incorporated to distinguish the interface between the gaseous and metallic cells. Effect of surface active element-sulfur content on melt pool dynamics is evaluated. Investigations of thermal evolutions of melt pool and solidification of the clad are systematically performed. Moreover, accuracy and reliability of the proposed model are implemented through comparisons between simulation results and their corresponding experimental measurements.

Influence of hot isostatic pressing on the properties of 316L stainless steel, Al-Mg-Sc-Zr alloy, titanium and Ti6Al4V cold spray deposits

In this paper an effect of capsule-free hot isostatic pressing (HIP) on the porosity and mechanical properties of different cold spray deposits was experimentally evaluated. The HIP post-treatment significantly decreased (more than 2 times) the porosity of 316L and pure titanium deposits. The densification effect of HIP on the porous Ti6Al4V deposit was significantly less pronounced. Mechanical properties of the deposits after HIP were also improved. In particular, the tensile strength of the stainless steel, pure titanium and Ti6AL4V increased from 30 MPa, 70 MPa and 20 MPa to 211 MPa, 480 MPA and 560 MPa correspondingly. Compressive strength of the Al-Mg-Sc-Zr alloy samples after HIP was also increased ~25%. Improvement of mechanical properties is explained by material diffusion and microstructure changes during HIP cycle.

Laser Directed Energy Deposition of Bulk 316L Stainless Steel

Laser directed energy deposition (DED) of high-density parallelepiped 316L stainless steel specimens has been performed while varying laser power, scanning velocity, powder mass flow rate and layer thickness. An extensive set of analyses has been performed into the resulting dimensions, surface roughness, microstructure, hardness, tensile strength, build substrate temperature, deposition efficiency, production time and specific energy input with the aim of differentiating variations in component performance and process efficiency for components that have already been optimized in terms of density. Average layer height is found to be an approximately linear function of the powder line mass flow rate, while samples with the same powder feed per unit length have similar microstructures despite differences in scanning velocity and total powder feed rate. The hardness of specimens is generally higher than bulk 316L stainless steel, while yield and ultimate tensile strength are similar or higher than the bulk material but with dependence on the load direction. Deposition efficiency is strongly influenced by the scanning strategy and laser off time, with values as low as 42% with a “raster” scanning strategy and up to 84% with a “snake” scanning strategy. The large number of analyses performed on high-density components provides important new insight into factors affecting build quality and process efficiency that go beyond simply maximizing density, suggesting that significant further improvements can be obtained. Such knowledge is exploitable for application-specific process optimization and implementation of open and closed-loop height control strategies for production of bulk 316L stainless steel components.

Cold spray deposition of 304L stainless steel to mitigate chloride-induced stress corrosion cracking in canisters for used nuclear fuel storage

A feasibility study of the cold spray deposition process of 304L stainless steel on 304 stainless steel substrates as a mitigation method for chloride-induced stress corrosion cracking (CISCC) has been investigated under various substrate conditions. The study is aimed at the application of this technology to mitigate CISCC that may potentially occur in or near the fusion welded regions of stainless steel canisters in Dry Cask Storage System (DCSS) for used nuclear fuels. Spherical gas-atomized 304L stainless steel powder, in the size range of 25 μm–44 μm, was used as feedstock powder for the cold spray process. The powder was deposited on 304 substrates with various surface conditions: as-polished, oxidized, cold-rolled, and plates with prototypical CISCC. The effects of cold spray parameters on quality of cold spray coatings were investigated. Thickness, porosity, and phases in the as-deposited materials were evaluated using scanning electron microscopy (SEM) and X-ray diffraction (XRD), and correlated with microhardness and adhesion strength measured via micro-indentation and ASTM C633 pulling test, respectively. XRD analysis of the coatings was also conducted to examine the effects of cold spray condition on residual stress state in the coating. Detailed cross-sectional examination of coating/substrate interfaces was performed with transmission electron microscopy (TEM) equipped with energy dispersive spectroscopy (EDS). Dense and continuous coatings with good adhesion strength, high hardness, and high degree of compressive stress were produced for the various substrate conditions by adjusting cold spray parameters. The results demonstrate that a cold sprayed stainless steel coating is a viable option to provide a physical barrier against CISCC in fusion weld regions of stainless steel in corrosive chloride salt-bearing environments.

Selection of Process Parameters for Near-Net Shape Deposition in Wire Arc Additive Manufacturing by Genetic Algorithm

In wire arc additive manufacturing (WAAM), deposition of multiple beads in multiple layers is required for fabricating any component. This article presents ways to optimize the selection of parameters for near-net shape deposition to minimize void and excess material in WAAM by GA. Initially single-bead geometry model was developed utilizing response surface methodology and experiments were conducted utilizing Box-Behenken design of experiments for deposition of 304L stainless steel in WAAM. Bead geometry parameters, i.e., bead width, bead height and bead cross-sectional area, etc., were expressed in terms of the process parameters like voltage, wire feed rate, torch speed and gas flow rate. Optimal processing conditions and deposition planning were determined utilizing a GA to minimize void and maximize material yield. The proposed approach was validated through deposition of three shapes, i.e., slice, wall and block. The three shapes fabricated with optimal parameters were found to have minimum void and maximum material yield. It has been revealed that optimal bead sizes and degree of overlapping are different for fabricating different geometries. Mechanical testing and metallurgical characterization of the deposited materials exhibited comparable properties of the deposited material to that of the base material. Moreover, double wire feed deposition was seen to deliver higher deposition rate and superior mechanical properties of the deposited material compared to the single wire feed deposition.