Laser Metal Deposition Shaping Research Articles

The effect of laser energy density on microstructural evolution and mechanical properties of laser clad 316L stainless steel for repair

Laser energy density can affect the powder forming state in laser metal deposition (LMD), ultimately altering the evolution pattern and mechanical properties of formed structures. This work uses high-speed LMD technology to repair the 316L austenitic stainless steel plate with a trapezoidal groove. By analyzing the microstructural evolution and mechanical properties of repaired specimens under different laser energy densities, we revealed the effects of density on LMD shaping. Results indicated that the microstructure in the repair zone developed alternately in forms of columnar, branch, and isometric crystals in the opposite direction of heat flow. Increased laser energy density could enlarge crystal grains in formed structures. Meanwhile, γ-Fe tended to transform into δ phase that was distributed among crystal grains in γ-Fe. When the density reached its peak, the number of the δ phase plummeted. The average microhardness of specimens first increased and then decreased in the repair zone, with its distribution fluctuating repeatedly, and the values in this zone were 1.5–1.7 times that of the substrate. The mechanical properties of repaired specimens were superior to those of the rolled steel substrate at room temperature, and these specimens' ultimate tensile strength (UTS) and elongation at this temperature were positively correlated with the laser energy density. There were dimples and typical cleavage patterns around the fracture of tensile specimens, indicating that the fracture mechanism at room temperature was of quasi-cleavage type. As the density rose, the UTS first increased and then reduced in a tensile test conducted at 500 °C, during which the strain kept growing. Simultaneously, numerous dimples with congregate micropores but without typical cleavage patterns were observed in the fracture, which revealed that all specimens exhibited ductile fracture at high temperatures.

Microstructure and Microhardness of Laser Metal Deposition Shaping K465/Stellite-6 Laminated Material

K465 superalloy with high titanium and aluminum contents was easy to crack during laser metal deposition. In this study, the crack-free sample of K465/Stellite-6 laminated material was formed by laser metal deposition shaping to control the cracking behaviour in laser metal deposition of K465 superalloy. The microstructure differences between the K465 superalloy with cracking and the laminated material were discussed. The microstructure and intermetallic phases were analyzed through scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). The results showed that the microstructure of K465/Stellite-6 laminated material samples consisted of continuous dendrites with a similar structure size in different alloy deposition layers, and the second dendrite arm spacing was finer compared with laser metal deposition shaping K465. The intermetallic phases in the different alloy deposition layers varied, and the volume fraction of carbides in K465 deposition layer of the laminated material was higher than only K465 deposition under the fluid flow effect. In addition, the composition and microhardness distribution of laminated materials variation occurred along the deposition direction.

Effects of substrate preheating on the thin-wall part built by laser metal deposition shaping

Laser metal deposition shaping (LMDS) is a state-of-the-art technology that combines rapid prototyping and laser processing. There are many factors affecting the quality, precision, microstructure and performance of the LMDS-deposited parts. Among these factors, substrate preheating is a significant one because it can change the heat history of the LMDS process. Preheating is generally adopted to reduce the residual stresses and the risk of thermal distortion and cracking. However, it changes the heat transfer conditions and affects the final microstructure and properties. In this work a numerical simulation model was established to analyze the heat transfer characteristics between deposited material and substrate, the influence rules of substrate preheating on the thermal behavior during LMDS, and the distribution characters of temperature and stress field. And then, the experimental methods were used to evaluate the effects of substrate preheating on the surface quality, microstructure, composition, hardness distribution, and mechanical properties of as-built thin-wall parts. The experimental results primarily agree with the theoretical analysis and numerical model, which indicates that in terms of the varied thermo-mechanical coupled field, the investigated microstructure and properties of formed components depend considerably on the initial temperature of the substrate, so the LMDS process can be effectively adjusted and controlled by means of substrate preheating.

Effect of Process Parameters on Residual Stress Distribution during Direct Laser Metal Deposition Shaping

In order to improve the quality of sample and decrease the stress during process, it is important to study the residual stress distribution during direct laser metal deposition (LMDS) process. In this paper, according to the “element life and death” technique of finite element method ,with APDL, we simulated the effects of Laser power, scanning velocity, substrate preheat temperature and powder addition speed in top layer of samples residual stress distribution during whole LMDS process are studied. The residual stress distribution under different process parameters is researched in detail. Using the same process parameters, the simulation results show good agreement with the features of sample which fabricated by LMDS.

Characterization of stainless steel parts by Laser Metal Deposition Shaping

Laser Metal Deposition Shaping (LMDS) is a new rapid manufacturing technology, which can build fully-dense metal components directly from the information transferred from a computer file by depositing metal powders layer by layer with neither mould nor tool. Typically, performed with stainless steel (SS) 316 powder, the orthogonal experiments combining with the ideal overlapping model were applied to ascertain the optimal processing parameters. Then the characteristics of microstructure, composition and phase of as-deposited cladding layers were analyzed through Scanning Electron Microscope (SEM) and X-ray diffraction (XRD), as well as relative model. Furthermore, the cooling rate and the solidification velocity during LMDS were evaluated based on empirical method. With the optimal parameters, some parts were fabricated without obvious defects, and then the mechanical properties of them were tested. Finally, the influencing regularities of critical parameters on microstructure and properties were concluded by comparison. The results prove that the microstructure of SS 316 deposits is composed of the slender dendrites growing epitaxially from the substrate, the mechanical properties are favorable and anisotropic, and the composition is uniform. Besides, the microstructure morphology and the mechanical properties are affected by the varied processing parameters at different degrees. Among them, the scanning speed shows the most remarkable effects on microstructure morphology, characteristic microscale, mechanical properties, as well as geometric shape of as-deposited parts.

Numerical Simulation of the Thermal Behavior during Laser Metal Deposition Shaping Technology

Laser Metal Deposition Shaping (LMDS) is an emerging manufacturing technique that ensures significant reduction of process time between initial design and final components. The fabrication of fully dense parts with appropriate properties using the LMDS process requires an in-depth understanding of the entire thermal behavior of the process. In this paper, the thermal behavior during LMDS was studied, both numerically and experimentally. Temperature distribution and gradient in the fabricated part were obtained by finite element method (FEM) simulation. The numerical results are in good agreement with the experimental observations. The numerical method contributes to the comprehension and control of the thermal behavior, and may be used to optimize process parameters and predict the thermal response of LMDS fabricated components.

Evaluation of the Stability and Precision of Powder Delivery during Laser Additive Manufacturing

The fabrication of metal parts is the backbone of the modern manufacturing industry. Laser forming is combination of five common technologies: lasers, rapid prototyping (RP), computer-aided design (CAD), computer-aided manufacturing (CAM), and powder metallurgy. The resulting process creates part by focusing an industrial laser beam on the surface of processing work piece to create a molten pool of metal. A small stream of powdered alloy is then injected into the molten pool to build up the part gradually. By moving the laser beam back and forth and tracing out a pattern determined by a CAD, the solid metal part is fabricated line by line, one layer at a time. By this method, a material having a very fine microstructure due to rapid solidification process can be produced. In the present work, a type of direct laser deposition process, called Laser Metal Deposition Shaping (LMDS), has been employed and developed to fabricate metal parts. In the LMDS process, the powder delivery system is an important component to perform the powder transport from powder storage box to powder nozzle, which supplies the raw material for the as-deposited metal parts. Consequently, the stability and precision of powder delivery during LMDS is essential to achieve the metal parts with high quality, so it is critical to evaluate the main factors closely related to the stability and precision of powder delivery. The shielding gas flow and the powder feeding rate were ascertained through experimental measure and formula calculation. The results prove that the suitable shielding gas flow and powder feeding rate can promote the stability and precision of powder delivery, which is the basis for the fabrication of as-deposited metal parts with flying colors.

Effects of Processing Parameters on Powder Utilization Ratio during Laser Metal Deposition Shaping

The Laser Metal Deposition Shaping (LMDS) process involves injecting metallic powder into a molten pool created by a high power industrial laser. As the laser traverses across the substrate in a layer-by-layer fashion, a fully dense metal is left in its path. A few processing parameters involved with the LMDS include the laser power, traverse speed, powder feeding rate, and gas flow rate, etc, which affect many factors of LMDS technology. Among them, the powder utilization ratio is an important one because it directly determines the build rate and build height per layer. Due to some objective reasons, the powder utilization ratio is far less than 100%. In order to ensure the stability of LMDS technology, it is necessary to investigate the match between powder utilization ratio and build rate and forming efficiency, and grasp the influence rules of processing parameters on powder utilization ratio. Accordingly, the related experiments were performed with the varied laser power, scanning speed and powder feeding rate. The results prove that the powder utilization ratio is a varied value, and affected by the processing parameters. Consequently, the relative ideal parameter match should be chosen in accordance with the specific circumstances during the LMDS technology, thus ensuring the better powder utilization ratio and promoting the forming efficiency and economic benefit.

Influences of Processing Parameters on Dilution Ratio of Laser Cladding Layer during Laser Metal Deposition Shaping

Laser Metal Deposition Shaping (LMDS) is a state-of-the-art technology that combines rapid prototyping and laser processing. There are many factors affecting the quality, precision, microstructure and performance of LMDS-deposited parts. Among them, dilution ratio is a significant one since it is not only an important index to judge the laser cladding quality, but directly affects the interlayer bonding strength and performance quality of as-formed metal parts. Thus, the substantial LMDS experiments were performed to conclude the influence of processing parameters on dilution ratio of laser cladding layer. The results indicate that the influence degree of scanning speed is most significant, while that of laser power is relatively slight. In order to ensure the perfect forming quality and strong metallurgical bonding, it is necessary to choose suitable dilution ratio to accomplish the LMDS process.

Effect of Scanning Methods on Residual Stress Distribution during Direct Laser Metal Deposition Shaping

In order to improve that quality of sample and decrease the stress during process, it is of great importance to study the residual stress distribution during direct laser metal deposition (LMDS) process. In this paper, according to the “element life and death” technique of finite element method ,with APDL, we simulated the effects of long edge reciprocating scanning method, short edge reciprocating scanning method and direction orthogonal changing in different layers reciprocating scanning method to residual stress distribution during whole LMDS process are studied. The residual stress distribution under different scanning methods is researched in detail. Using the same process parameters, the simulation result show good agreement with the features of sample which fabricated by LMDS

扫描方式对激光直接成形过程中热力耦合场与残余应力分布的影响

扫描方式对激光直接成形过程中热力耦合场与残余应力分布的影响

Numerical Simulation of Temperature Field of Direct Laser Metal Deposition Shaping Process of Titanium Alloys

In order to control the thermal stress of forming process, based on “element birth and death” technology of finite element method, a numerical simulation of three-dimensional temperature field and stress field during multi-track & multi-layer laser metal deposition shaping(LMDS) process is developed with ANSYS parametric design language (APDL). The dynamic variances of temperature field and stress field of forming process are calculated with the energy compensation of interaction between molten pool-powder and laser-powder. The temperature field, temperature gradient, thermal stress field and distribution of residual stress are obtained. The results indicate that although the nodes on different layers are activated at different time, their temperature variations are similar. The temperature gradients of samples are larger near the molten pool area and mainly along z-direction. Finally, it’s verified that the analysis results are consistent with actual situation by the experiments with same process parameters.

Laser Metal Deposition Shaping System for Direct Fabrication of Parts

The fabrication of metal parts is the backbone of the modern manufacturing industry. Laser forming is the combination of five common technologies: lasers, rapid prototyping (RP), computer-aided design (CAD), computer-aided manufacturing (CAM), and powder metallurgy. The resulting process creates part by focusing an industrial laser beam on the surface of processing workpiece to create a molten pool of metal. A small stream of powdered alloy is then injected into the molten pool to build up the part gradually. By moving the laser beam back and forth and tracing out a pattern determined by a CAD, the solid metal part is fabricated line by line, one layer at a time. By this method, a material having a very fine microstructure due to rapid solidification process can be produced. In the present work, a type of direct laser deposition process, called Laser Metal Deposition Shaping (LMDS), has been employed and developed to fabricate metal parts. The LMDS apparatus consists of four primary components: energy supply system, motion control system, powder delivery system, and computer control system. These components have their specified functions, but work in association with each other.

Control Module of Laser Metal Deposition Shaping System

The Laser Metal Deposition Shaping (LMDS) is a state-of-the-art technology which correlates the Rapid Prototyping and Manufacturing (RP&M) and laser processing. During this process, a certain alloy is fused onto the surface of a substrate. Laser deposition devices, namely powder feeder, CNC worktable, and laser shutter, are integrated to automatically make any cladding profile possible. Material is deposited by scanning the laser across a surface while injecting metallic powders into the molten pool at the laser focus. The metal part is then fabricated layer by layer. The LMDS system consists of four primary components: energy supply module, motion control module, powder delivery module, and computer control module. These modules of LMDS system individually perform the specified functions, but coordinate with each other. One of them, the control module plays an important role in causing the LMDS system automatic and intelligent. The control module can be divided into hardware and software components. The hardware structure mainly includes industrial computer, motors, and motion control card, which build the overall framework, and are driven by software structure. The software structure, namely the system application program with GUI, can instruct every module of LMDS system to finish the motion cooperatively adjust the processing parameters freely, and fulfill the LMDS technology automatically and intelligently. The hardware and software structures work in harmony with each other, thus flexibly controlling the LMDS system.

Configuration and Adjustment of Substrate Preheating Device

Laser Metal Deposition Shaping (LMDS) is a state-of-the-art technology that combines rapid prototyping and laser processing. There are many factors affecting the quality, precision, microstructure and performance of LMDS-deposited parts. Among them, substrate preheating is a significant one since it can change the heat history of the LMDS process. Preheating is often used to reduce the residual stresses and the risk of thermal distortion and cracking. In this work a set of substrate preheating device for LMDS system employed with dual-channel control, namely intelligent PID temperature control and realtime serial-port temperature acquisition and feedback control, is designed and developed. In addition, the heating up rule gained through the relative experiment is introduced to reduce the overshoot amplitude of substrate preheating device by cascade control method. The results show that not only can this substrate preheating device realize the dual-channel control and continuous adjustment of substrate preheating temperature, but also can realtime collect and record the substrate temperature.

Data Processing of Laser Metal Deposition Shaping Technology: Part II. Contour Scanning Path Research

This document proposes a kind of contour path offset algorithm of Laser Metal Deposition Shaping (LMDS) technology. The contour path offset is determined by both laser spot radius and post-processing allowance. Besides, the offset direction can be judged through applying the cutter offset principle of NC machining. Then the formed intersecting points of a series of equidistant lines can be connected circularly to solve the contour path offset line. Since this algorithm fully considered the near-net shaping condition of LMDS and the combined effects of laser spot radius, the precision of as-formed metal parts could be further improved.

Data Processing of Laser Metal Deposition Shaping Technology: Part I. Layered Algorithm Research

Based on the characteristics of Laser Metal Deposition Shaping (LMDS) technology, the related data processing algorithm was investigated in detail. This paper provides a kind of layered processing algorithm for STL file based on the limited topological information. At first, the triangle facets intersected with certain layered plane in the STL file were found out in disordered state, and then were read into the memory; secondly, the topological information was built to the found triangle facets; next, the intersecting lines between the layered plane and the triangle facets were figured out, so the contour lines on the layered plane were generated according to the founded topological sorting. Over and over, this process was finished until all layered contours were worked out. This algorithm could process the large STL files, save the memory expense, and improve the computing speed.

Composition and Microstructure of Laser Cladding of 316 Stainless Steel

Laser Metal Deposition Shaping (LMDS) is a Rapid Manufacturing (RM) process that can be classified under the area of layered manufacturing techniques, where parts are built in layers. Parts of any complexity can be built directly from the 3D CAD model without much human intervention and requires minimum post-processing. In fact, LMDS technique can be recognized as multilayer laser cladding. Accordingly, it is necessary to perform the elementary laser cladding experiments with common metal powder so as to better understand the LMDS process. Then the characteristics of microstructure, composition and phase of as-deposited clads were analyzed through SEM and XRD, as well as relative model. The results prove that the microstructure of 316 stainless steel deposits is composed of the slender dendrites growing epitaxially from the substrate, and the composition is uniform without obvious segregation. Besides, it can be deduced from XRD diagram that the microstructure is composed of mono-phase γ.

Microstructure and Properties of Thin-Wall Part by Laser Metal Deposition Shaping

Laser additive direct deposition of metals is a new rapid manufacturing technology, which combines with computer aided design, laser cladding and rapid prototyping. The advanced technology can build fully-dense metal components directly from CAD files without a mould or tool. With this technology, a promising rapid manufacturing system called “Laser Metal Deposition Shaping (LMDS)” is being constructed and developed. Through the LMDS technology, fully-dense and near-net shaped metallic parts can be directly obtained through melting coaxially fed powder with a laser. Typically, the microstructure and mechanical properties of the as-formed thin-wall part were tested and analyzed synthetically. The results show that there are different grain categories distributing in certain position of thin-wall part, and the mechanical properties represent as anisotropy.

Numerical Simulation of Temperature Field and Stress Field of Direct Laser Metal Deposition Shaping Process of Titanium Alloys

In order to control the thermal stress of forming process,based on "element birth and death" technology of finite element method,a numerical simulation of three-dimensional temperature field and stress field during multi-track and multi-layer laser metal deposition shaping(LMDS) process is developed with ANSYS parametric design language(APDL).The dynamic variances of temperature field and stress field of forming process are calculated with the energy compensation of interaction between molten pool-powder and laser-powder.The temperature field,temperature gradient,thermal stress field and distribution of residual stress are obtained.The results indicate that although the nodes on different layers are activated at different time,their temperature variations are similar.The temperature gradients of samples are larger near the molten pool area and mainly along z-direction.The transient thermal stress has concentrated in large temperature gradient area,which is in accordance with thermal stress forming theory.Through analyzing the residual stress of the as-formed samples in different directions,distribution rule of residual stress is achieved from temperature gradient perspective.And formation mechanism of residual stress is verified with an indirect way.Finally,it's verified that the analysis results are consistent with actual situation by the experiments with same process parameters.