No description available.

No description available.

No description available.

Laser powder bed fusion (L-PBF) refers to an additive manufacturing (AM) process in which a high-intensity laser source melts powders in a layer-by-layer manner to fabricate parts based on a computer-aided design (CAD) model without almost any geometrical limitations. The development of the L-PBF process has provided an outstanding opportunity to manufacture unique parts which are practically impossible to be produced by conventional manufacturing methods. The L-PBF process also does not require intricate build tools and assembly processes. However, quality issues such as non-uniform microstructure or mechanical properties, porosities, and surface roughness deteriorate the quality of the parts fabricated by the L-PBF process. Therefore, the reliability and the repeatability of the process are required to be addressed. This study deals with improving the quality of the part fabricated by the L-PBF process and making the process more reliable and repeatable. The control approach was employed to elevate the quality of the final part from three different aspects. First, making the microstructure and microhardness of the part uniform through a control approach was investigated. Three controllers, namely, proportional (P), adaptive P, and quasi sliding mode, were developed to control the melt pool temperature for the Inconel 625 superalloy. An analytical-experimental model was presented to evaluate the performance of controllers via simulation. A monitoring system consisting of a two-color pyrometer was utilized off-axially to monitor the melt pool temperature for use by the controllers as a feedback signal. The results indicated that the control approach led to microhardness and microstructure uniformity, resulting from the reduced variation in the primary dendrite arm spacing compared to the case with constant process parameters. Second, the control approach was utilized to produce optimum parts instead of using the energy density criterion. Temperature domains corresponding to the most common porosities, namely, lack of fusion (LOF), lack of penetration (LOP), and keyhole, were determined in a range of process parameters using a thermal imaging system. A safe zone was introduced by defining a lower and an upper limit based on the critical temperatures causing transitions from LOP to defect-free and from defect-free to keyhole zones, respectively. A proportional-integral-derivative (PID) controller was used to maintain the melt pool temperature within the safe zone during the L-PBF process for Inconel 625 and avoid the formation of porosities, regardless of the initial condition selected and the scanning speed employed. In all cases, a short settling time in the order of the printing time for a few layers was required to reach the steady-state condition at which defect-free parts could be obtained. Finally, minimizing the top surface roughness of the parts manufactured by the L-PBF process by deploying a Feedforward plus Feedback control system was targeted in this study. The most common factors affecting the surface quality, namely, balling, lack of inter-track overlap, overlapping curvature of laser scan tracks, and spatters, were investigated through a monitoring system consisting of a high-speed camera, a zooming lens, and a short pass filter. The desired melt pool width and the critical value for the level of spatters were determined using the imaging system and subsequent image processing. An experimental model was developed, and the control system was designed accordingly. Both simulations and experimental results showed excellent transient performance of the control system to reach the desired melt pool width only after printing a few layers. Also, the control system was evaluated at different scanning speeds and with different geometries. The results obtained from this study indicated that controlling the geometry of the melt pool can mitigate significant defects occurring during the process and minimize the top surface roughness. / Thesis / Doctor of Philosophy (PhD)

Two direct consolidation methods usually used for advanced ceramics have been combined in this project in order to develop a novel fabrication route for traditional ceramics. Specifically the method used is based on the Additive Manufacturing extrusion process using direct writing of high solid loading ceramic pastes and then freeze-casting to solidify the deposited material. This novel fabrication method, for which a patent has been granted, has been christened “Direct Writing Freeze-Casting” (DWFC). Although the DWFC process is the subject of investigation by other researchers for a range of different applications, including the production of medical implants with alumina, the research presented in this thesis focuses on its use in the manufacture of white wares, giftware, and applied arts and crafts in general. This new system will provide designers, potters, artists, craft makers and manufacturers with a flexible and automated way of manufacturing porcelain objects. One of the major challenges to be overcome to exploit the DWFC process is the development of suitable slurry material formulations. Initial trials demonstrated that it is not possible to use conventional clay based porcelain materials with a platelet shaped microstructure which inhibits freeze casting. In this thesis the development and characterisation of non plastic porcelain slurry, based on substitution of kaolin (clay) with a calcined clay material (molochite), which can be processed using this new method is presented. The new non plastic porcelain formulation, which has a high solid load of 75.47% wt., has been subjected to detailed analysis to assess its suitability at each stage of the process; extrusion, freeze-casting (solidification) and firing.

No description available.

Since their discovery Carbon Nanotubes (CNTs) have attracted much interest from many fields of the scientific community owing to their range of unique and impressive properties. Measurements of the mechanical properties of these nanoscale molecules have shown strengths up to five times greater than that of steel at only a quarter of the density. Consequently many have attempted to unlock these remarkable properties by creating nano-composite structures where CNTs effectively reinforce materials with little increase in density. Unfortunately the tendency of CNTs to form agglomerations when allowed to disperse in fluid suspensions has made this process non trivial, and led to difficulties in achieving effective reinforcement when simply mixing CNTs into a matrix material. As a result it has become clear that new approaches to composite construction will be required if effective composite reinforcement using CNTs is to be achieved. Recent advances in CNT synthesis using Chemical Vapour Deposition (CVD) where tall forests of these nanoparticles are grown from the vapour phase have begun to solve the agglomeration problem. These forests are produced in aligned and dispersed arrays, and wetting of these structures with polymer matrices has demonstrated improvements in modulus of several hundred percent. These improvements arise as the CNTs retain both the dispersion and alignment of the forest when incorporated into the matrix thus overcoming the difficulties observed using traditional manufacture methods. New complications arise when attempting to extend these promising results to larger scale composite components owing to the typically millimetre size of CVD grown vertically aligned CNT (VACNT) forests. From these results it follows that to create large composite parts it will be required to incorporate many individually CVD grown VACNT forests into a single composite structure. Strategies to achieve such a composite are being developed, with a range of ideas extending from knowledge gained from the emerging technology of additive manufacture (AM) described as ‘...the process of joining materials to make objects from 3D model data, usually layer upon layer....’. Indeed it is desirable to reinforce materials used in AM processes and the nano scale diameter of CNTs makes them the perfect choice owing to their high aspect ratios at the micron scale. In this thesis investigations are conducted into the feasibility of manufacturing CNT composite structures using CVD grown forests and AM techniques. These investigations include measurement of the anisotropic mechanical properties of composite samples, and studies of the wetting interactions that occur between CNT forests and polymer materials. Composite samples are constructed and tested mechanically in the transverse orientation and results compared to traditional fibre composite reinforcement models in order to understand the material properties that can be expected if such an AM process is achieved. Results show greater mechanical improvements in transverse modulus than expected, and these results are attributed to the wavy nature of individual CNTs within forest structures providing multi directional reinforcement to the matrix material. Further studies are conducted to investigate the flow of molten thermoplastic materials into CNT forest structures under capillary driven flow. Thermoplastics were allowed to flow into VACNT forests before being cooled and inspected using micro x-ray computed topography (μ-CT) to gain an understanding of the wetting mechanism. Results from μ-CT scans show that the polymer flows into the structure in peaks of similar radius. Finally dynamic investigations were conducted into the fast capillary driven flow of a low viscosity thermoset resin into VACNT forests using a high speed camera. Results are fitted to traditional models for dynamic capillary driven flow in porous media and an effective radius and porosity is calculated for VACNT forests. Experimental values illustrate that these nanoscale structures still fit to traditional flow models of fluids where the height of capillary rise is proportional to the square of the elapsed time. These results provide a further step in understanding methods of incorporating many VACNT structures into polymeric matrices to achieve large scale effective polymer VACNT composite materials.

Recent development of additive manufacturing technologies has led to lack of information on the base materials being used. A need arises to know the mechanical behaviors of these base materials so that it can be linked with macroscopic mechanical behaviors of 3D network structures manufactured from the 3D printer. The main objectives of my research are to characterize properties of a material for an additive manufacturing method (commonly referred to as 3D printing). Also, to model viscoelastic properties of Procast material that is obtained from 3D printer. For this purpose, a 3D CAD model is made using ProE and 3D printed using Projet HD3500. Series of uniaxial tensile tests, creep tests, and dynamic mechanical analysis are carried out to obtained viscoelastic behavior of Procast. Test data is fitted using various linear and nonlinear viscoelastic models. Validation of model is also carried out using tensile test data and frequency sweep data. Various other mechanical characterization have also been carried out in order to find density, melting temperature, glass transition temperature, and strain rate dependent elastic modulus of Procast material. It can be concluded that melting temperature of Procast material is around 337°C, the elastic modulus is around 0.7-0.8 GPa, and yield stress is around 16-19 MPa.

The main purpose of this thesis is to see if it is possible to use additive manufacturing(AM) for field repair and maintenance of the assault rifle AK5C, by finding a suitableadditive manufacturing process and make a functional evaluation of the additivemanufactured components: hammer axis, gas cylinder and the magazine follower. TheSwedish Defense Materiel Administration (FMV) is an administration that supplies theSwedish armed forces with materiel. Therefore, it is in their interest to investigate if itis possible to use AM for field repair and maintenance in example Mali or Afghanistan.Based on a survey, and the fact that the components are purelystructural Powder Bed Fusion AM was selected. The hammer axis was made in amargining steel MS1, the gas cylinder in the nickel-alloy called Inconel 718 and themagazine follower was made in a polymer called nylon 12.The functional evaluation of the components took place at Saab in Östersund, whereeach component was placed in the rifle. The rifle was fired 1000 rounds with thehammer axis, 1000 rounds with the gas cylinder and 500 rounds with the magazinefollower. The functional test for the components was successful, there were also nomajor changes in dimensions and weight except for the hammer axis. The diameterfor the hammer axis went from 5,00 mm before the functional test to 4,98 mm afterthe functional test. The microscope images showed thatabrasion had occurred, not only for the hammer axis but also for the magazinefollower, due to friction. Firing speed was also measured and it should be over 600rounds/min, if everything works properly. The hammer axis had a firing speed of 639rounds/min which is good while the gas cylinder only had a firing speed of 595rounds/min. This was because the inner diameter of the cylinder was too big, causinggas leaking and pressure drop inside the gas cylinder.In conclusion, additive manufacturing does allow for fabrication of functional spareparts – at least these evaluated here.

In many practical situations when analyzing a dependence of one or more explanatory variables on a response variable it is essential to assume that the relationship of interest obeys certain shape constraints, such as monotonicity or monotonicity and convexity/concavity. In this thesis a new approach to shape preserving smoothing within generalized additive models has been developed. In contrast with previous quadratic programming based methods, the project develops intermediate rank penalized smoothers with shape constrained restrictions based on re-parameterized B-splines and penalties based on the P-spline ideas of Eilers and Marx (1996). Smoothing under monotonicity constraints and monotonicity together with convexity/concavity for univariate smooths; and smoothing of bivariate functions with monotonicity restrictions on both covariates and on only one of them are considered. The proposed shape constrained smoothing has been incorporated into generalized additive models with a mixture of unconstrained and shape restricted smooth terms (mono-GAM). A fitting procedure for mono-GAM is developed. Since a major challenge of any flexible regression method is its implementation in a computationally efficient and stable manner, issues such as convergence, rank deficiency of the working model matrix, initialization, and others have been thoroughly dealt with. A question about the limiting posterior distribution of the model parameters is solved, which allows us to construct Bayesian confidence intervals of the mono-GAM smooth terms by means of the delta method. The performance of these confidence intervals is examined by assessing realized coverage probabilities using simulation studies. The proposed modelling approach has been implemented in an R package monogam. The model setup is the same as in mgcv(gam) with the addition of shape constrained smooths. In order to be consistent with the unconstrained GAM, the package provides key functions similar to those associated with mgcv(gam). Performance and timing comparisons of mono-GAM with other alternative methods has been undertaken. The simulation studies show that the new method has practical advantages over the alternatives considered. Applications of mono-GAM to various data sets are presented which demonstrate its ability to model many practical situations.