Laser Direct Deposition of Metal Matrix Diamond Composite

Diamond, the hardest natural material known to humankind, is an ideal candidate for cutting and wear application, however implementing diamond in tooling is not without its challenges. Consequentially, these challenges cause diamond tools to be very expensive. Additionally, current diamond tools do not capitalize on the full potential of diamond material properties. The most significant challenge associated with using diamond in tooling is trying to avoid diamond graphitization at high temperatures during the tool manufacturing process. Finding an alternative way to fabricate improved performance and better priced diamond tools will revolutionize tooling industry. Laser direct deposition of diamond particles has the potential for production of higher performance and lower priced diamond tools. This is largely due to the low interaction time and high cooling rates involved with the deposition process. The process zone is exposed to high temperatures for a very short time and is followed by very high cooling rates. This research investigates the deposition of a metal matrix diamond composite on mild steel, addressing the associated issues and challenges with the deposition process. These issues include the decomposition of diamond, diamond particle wetting, chemical bonding of the diamond particles and the matrix, diamond retention capacity of the matrix, clad-substrate bonding, porosity and micro cracks, and diamond particles distribution in the deposited clad. A transient three dimensional temperature dependent finite element model for pre-placed laser cladding is developed in order to develop insight into the process window. The model is developed by ANSYS © finite element software. The governing equations, boundary conditions and assumptions used in the model are discussed. Additionally the effects of the direct laser deposition process parameters on the melt pool temperature, cooling rate, and exposure time to high temperatures are investigated. The influence of process parameters on the increase of the melt pool temperature during deposition of a clad is also studied. An experimental study on deposition of ((Cu80Sn20)90Ti10)75+25 diamond weight percent via pre-placed laser cladding on a mild steel substrate is presented. The effects of using both a continuous wave and pulsed laser deposition are explored. The effects of process parameters on diamond decomposition, clad-substrate bonding, porosity and micro-cracks, wetting and chemical bonding of diamond particles, diamond particles distribution in the deposited clad, and the effect of dilution are investigated. The experimental results are in agreement with the trends predicted by the modeling. Both the modeling and pre-placed laser deposition experiments provide an estimation of the process window for blown powder laser deposition. Extensive sets of experiments are conducted for the deposition by blown powder laser cladding. Cross sectional analysis is performed on both transverse and longitudinal cut-planes of the deposited clads. This is performed using Scanning Electron Microscope, Energy-Dispersive X-ray spectroscopy, and nano-indentation. Deposited diamond particles are characterized by Raman spectroscopy and a trend demonstrating reduced diamond graphitization is detected. A study of deposited diamond particles reveals that an interfacial titanium carbide layer is formed between diamond particles and the matrix. This layer is only formed when iron is not present at the vicinity of the diamond particles. In the presence of iron, it forms the interfacial layer which is surrounded by a titanium rich layer. This observation indicates that iron has higher affinity to react with diamond as opposed to titanium. It is also observed that the iron reaction with diamond results in diamond degradation and graphitization. A thermal analysis study of reaction of each elements of the matrix Reaction of each elements of the matrix (titanium, copper, and tin) and iron with diamond up to the temperature of 1300°C is conducted using Differential Scanning Calorimetry. This study reveals that mechanism of reaction between titanium and diamond is completely different from that of iron and diamond. It is found that reaction of titanium with diamond initiates when titanium transforms from α to β at 880-900°C. The formation of a titanium carbide layer starts at this temperature by diffusing titanium into the surface of diamond particles which results in nucleation of titanium carbide on the diamond surface. As this layer grows, the diffusion of titanium into the surface of the diamond becomes more difficult, thus the growth of the titanium carbide layer slows down until stops completely at a certain thickness. Graphitization of diamond does not occur. When the thickness of the titanium carbide layer reaches its maximum, it becomes brittle in a way that can be separated from the diamond particle. The reaction of iron-diamond initiates before transformation of ferrite to austenite begins (below 900°C). It starts via diffusion of detached carbon atoms from the diamond into the iron particles and continues until the melting of the solution at around 1150°C. The melting causes higher levels of diamond decomposition and higher solution of carbon in iron. This results in extreme degradation of diamond particles. Although the solubility of carbon in iron is limited, detaching of carbon atoms from diamond continues past the solubility limit, resulting in super saturation of iron matrix and presence of large graphite inclusions in the iron matrix. On the other hand, iron diffuses into the decomposed diamond particles accelerating the decomposition. This results in a significant carbon atoms detachment from the diamond particles. As a result, high fraction of the remaining carbon atoms decompose to graphite or react with the iron. In fact, not only does the iron react with the diamond, but it also acts as a catalyzer for diamond graphitization. The DSC study reveals that copper and tin neither react nor wet the diamond particles. It is found that diamond-laser interaction before reaching the melt-pool plays an important role in diamond graphitization. Therefore, the effect of process parameters on the diamond-laser interaction are studied and optimized to obtain the minimum laser-diamond interaction as well as minimum temperature rise in diamond particles before joining the molten pool. Experimental analysis indicates that the temperature rise of diamond particles passing the laser beam is only partially responsible for diamond graphitization. Diamond reaction with oxygen at the elevated temperatures due to laser interaction is determined to be the major element of diamond graphitization. A specialized nozzle is designed to provide better protection of the powder stream against the penetration of oxygen. The nozzle design accomplishes this by providing an annular inert gas stream which surrounds the powder stream. Employing this specialized nozzle in conjunction with optimized deposition process parameters significantly reduces diamond graphitization. Optimum process parameters which can minimize the dilution are determined by experimental analysis. Using these parameters, diamond degradation in the deposition is drastically reduced. In an attempt to further eliminate the dilution of deposition by iron, as well as decrease the effective input energy into the process zone an intermediate layer with the composition of Cu-20Sn weight percent is added. The intermediate layer has a thickness of approximately 0.5 mm and is deposited directly on the mild steel substrate. Deposition of the metal matrix-diamond particles on the intermediate layer results in elimination of dilution and almost no graphitization. An interfacial titanium carbide layer with a thickness in the range of 150-350 nm is observed. The substrates with and without the intermediate layer are pre-heated to 700°C prior to performing the deposition. The pre-heating decreases the required effective input energy by approximately 16.7 to 17.5 percent for the substrate without intermediate layer and around 15 percent for the substrate with intermediate layer. The preheating has no effect on presence of iron in the deposition on the substrates with the intermediate layer, since without pre-heating, the presence of iron in the deposit is already negligible; however it reduces the dilution in the deposition on the substrates when no intermediate layer is present. Effects of process parameters on dilution, clad substrate bonding, and aspect ratio are studied. Using experimental analysis, an equation is developed to calculate the aspect ratio from the process parameters. Furthermore, for minimizing dilution, obtaining strong clad-substrate bonding, and obtaining acceptable aspect ratio a set of inequalities (constraints) to be satisfied by process parameters is determined. From these constraints the process window is obtained. In summary, through theoretical and experimental studies, this research develops a practical and reliable metal matrix diamond composite fabrication technology using laser deposition technique. The new fabrication technology can be used in developing high performance tools and highly wear resistance surfaces

Image-Based Feature Tracking Algorithms for Real-Time Clad Height Detection in Laser Cladding

In laser cladding, a material, usually in the form of powder, is deposited on a substrate. Powder particles are intermingled with inert gas and fed by a powder feeder system on the substrate. Laser is employed to melt the additive material and a small layer of surface of the substrate simultaneously. While the powder is being deposited, the laser melts the powder particles and the melted powder particles join the melt pool on the substrate beneath the laser beam. Generating relative motion between the laser focal point and the substrate will result in moving melt pool on the substrate. This will lead to addition of a desired material to the substrate with desired thickness and good bonding as well as minimum dilution. In addition, by producing clads beside and on the top of each other a functional component can be built in a layer by layer fashion. Despite many advantages of laser cladding, it is highly sensitive to internal and external disturbances. This makes a closed-loop control system for laser cladding inevitable. Utilizing a closed-loop control system in laser cladding makes the system insensitive to external and internal disturbances. Having a closed-loop control system for laser cladding would contribute to substantial improvement in clad quality and cost reduction. Feedback sensor is an essential part in a closed-loop control system. Among different parameters that can be used as feedback signals in a closed-loop control of laser cladding, melt pool geometry and in particular clad height is of great importance specifically for the purpose of rapid prototyping. This thesis presents novel algorithms for real-time detection of clad height in laser cladding. This is accomplished by the following: Tackling the issues pertinent to image acquisition in the presence of harsh and intensive light is scrutinized. Important parameters of digital cameras related to selection of proper type of CCD cameras in order to overcome the existent harsh condition are presented. Also, the existent light in laser cladding arisen from different sources is analyzed and based upon that proper bandpass filters and neutral filters are selected. All these lead to capture relatively sharp and clear images of the melt pool. Capturing good quality pictures potentially would provide valuable information about the process. This information could include, but is not limited to, melt pool geometry (i.e., melt pool height, width, melt pool profile, and wet angle), angle of solidification, melt pool temperature, and melt pool temperature distribution. Furthermore, the issues regarding path dependency of the melt pool image are addressed by using a trinocular cameras configuration. By utilizing this, always two cameras monitor the front end of the melt pool regardless of the direction of the clad. Image analysis of the grabbed images is also discussed. Image thresholding is one of the most formidable tasks in image processing and this difficulty is intensified due to characteristics of the grabbed images of the melt pool (e.g., surrounding hazy area around the melt pool). Applying hard partitioning thresholding method did not lead to detec- tion of the melt pool accurately. As a result, fuzzy thresholding by minimizing of the measure of fuzziness is developed and its performance is investigated. The effect of three important membership functions, triangular, Gaussian, and generalized Bell on the performance of the thresholding method is investigated. Also, Image thresholding by utilizing fuzzy c-means clustering is developed. Applying the developed thresholding methods show promising results. Among the developed thresholding methods, fuzzy thresholding with minimizing the measure of fuzziness with Gaussian membership function is selected for the implementation in the algorithm. Finally, Image feature tracking module is presented. The detected borders of the melt pool images are transformed from image plane to the world plane by using a perspective transformation. Four features of the elliptical features of the projected melt pool borders are selected. These four features along with the angle of tangential path vector with respect to the corresponding right hand side camera's axis are fed into an Elman recurrent neural network. The proposed algorithms and the trained neural network are utilized in the process resulting in acceptable detection of the clad height in deposition of straight clads for a specific direction. It is concluded that the system can detect the clad height with about ±0.15 mm maximum error

Direct laser deposited titanium with controlled porosity for bone tissue engineering

The potential of engineered porous materials for aiding the osseointegration of metallic dental and orthopedic implants is well established. A range of techniques, such as powder metallurgy, plasma-spraying and titanium fiber meshing have been tried to produce the implant structures with control on porosity and other pertinent characteristics. In the present work, direct laser deposition of pure titanium powder (particle size: -100/+63 microns) was carried out to form porous structure. A 1 kW fiber laser, integrated with lateral powder feed nozzle and workstation, was used at different primary process parameters (laser power, powder mass flow rate, transverse speed, hatch parameter) to fabricate a number of multi-layer structures. The relevant structural parameters of these fabricated structures were studied as a function of the process parameters using various techniques. The result of these studies are presented