Repair of metallic components using hybrid manufacturing

Many high-performance metal parts users extend the service of these damaged parts by employing repair technology. Hybrid manufacturing, which includes additive manufacturing (AM) and subtractive manufacturing, provides greater build capability, better accuracy, and surface finish for component repair. However, most repair processes still rely on manual operations, which are not satisfactory in terms of time, cost, reliability, and accuracy. This dissertation aims to improve the application of hybrid manufacturing for repairing metallic components by addressing the following three research topics. The first research topic is to investigate and develop an efficient best-fit and shape adaption algorithm for automating 3D models\u27 the alignment and defect reconstruction. A multi-feature fitting algorithm and cross-section comparison method are developed. The second research topic is to develop a smooth toolpath generation method for laser metal deposition to improve the deposition quality for metallic component fabrication and repair. Smooth connections or transitions in toolpath planning are achieved to provide a constant feedrate and controllable deposition idle time for each single deposition pass. The third research topic is to develop an automated repair process could efficiently obtain the spatial information of a worn component for defect detection, alignment, and 3D scanning with the integration of stereo vision and laser displacement sensor. This dissertation investigated and developed key technologies to improve the efficiency, repair quality, precision, and automation for the repair of metallic components using hybrid manufacturing. Moreover, the research results of this dissertation can benefit a wide range of industries, such as additive manufacturing, manufacturing and measurement automation, and part inspection --Abstract, page iv

A novel process chain for the automated repair of leading edges in aircraft engines

Due to impacts and constant stress, the leading edges of aircraft engine blades often lose their shape, while the other parts of the blade are still functional. This results in unnecessary performance losses. Currently, there is no method for a fast and effective repair process as the initial shape of the blade cannot be restored. This paper presents an automated re-contouring process chain for leading edges without prior material application. Thus, it is a sustainable approach to extend the lifespan until an energy-consuming welding process can be performed. It consists of an in-machine scanning process to obtain information about the worn shape, a subsequent target model generation based on the worn shape, and an automated process planning. The process chain is evaluated using a universal, leading edge workpiece. The results show that the target requirements for shape and smoothness are fulfilled

Remanufacturing of precision metal components using additive manufacturing technology

Critical metallic components such as jet engine turbine blades and casting die/mold may be damaged after servicing for a period at harsh working environments such as elevated temperature and pressure, impact with foreign objects, wear, corrosion, and fatigue. Additive manufacturing has a promising application for the refurbishment of such high-costly parts by depositing materials at the damaged zone to restore the nominal geometry. However, several issues such as pre-processing of worn parts to assure the repairability, reconstructing the repair volume to generate a repair tool path for material deposition, and inspection of repaired parts are challenging. The current research aims to address crucial issues associated with component repair based on three research topics. The first topic is focusing on the development of pre-repair processing strategies which includes pre-repair machining to guarantee the damaged parts are ready for material deposition and pre-repair heat-treatment to restore the nominal mechanical properties. For this purpose, some damaged parts with varied defects were processed based on the proposed strategies. The second topic presents algorithms for obtaining the repair volume on damaged parts by comparing the damaged 3D models with the nominal models. Titanium compressor blades and die/mold were used as case studies to illustrate the damage detection and reconstructing algorithms. The third topic is the evaluation of repaired components through material inspection and mechanical testing to make sure the repair is successful. The current research contributes to metallic component remanufacturing by providing knowledge to solve key issues coupled with repair. Moreover, the research results could benefit a wide range of industries, such as aerospace, automotive, biomedical, and die casting --Abstract, page iv

A Hybrid Process Integrating Reverse Engineering, Pre-Repair Processing, Additive Manufacturing, and Material Testing for Component Remanufacturing

Metallic components can gain defects such as dents, cracks, wear, heat checks, deformation, etc., that need to be repaired before reinserting into service for extending the lifespan of these parts. In this study, a hybrid process was developed to integrate reverse engineering, pre-repair processing, additive manufacturing, and material testing for the purpose of part remanufacturing. Worn components with varied defects were scanned using a 3D scanner to recreate the three-dimensional models. Pre-repair processing methods which include pre-repair machining and heat-treatment were introduced. Strategies for pre-repair machining of defects including surface impact damage, surface superficial damage and cracking were presented. Pre-repair heat-treatment procedure for H13 tool steel which was widely used in die/mold application was introduced. Repair volume reconstruction methodology was developed to regain the missing geometry on worn parts. The repair volume provides a geometry that should be restored in the additive manufacturing process. A damaged component was repaired using the directed energy deposition process to rebuild the worn geometry. The repaired part was inspected in microstructure and mechanical aspects to evaluate the repair. The hybrid process solved key issues associated with repair, providing a solution for automated metallic component remanufacturing

Fully automated tool path planning for turbine blade repair

The recontouring process of aircraft engine parts like turbine blades is a manual or in best-case semi-automated process due to high individuality of the workpiece. This leads to in-process scrap because of low process stability and high process times. An automation of process planning reduces both. This paper introduces a method for a fully automated and individual tool path planning using 3D-scan data. Geometric parameters of the degenerated blade were considered to find best-suitable target geometry in a robust way. For turbine blade repair, the process stability is increased while meeting the dimensional tolerances required for the international aviation certifications

Adaptive geometry transformation and repair methodology for hybrid manufacturing

With hybrid manufacturing maturing into a commercial scale, industries are pushing to integrate and fully utilize this new technology in their production facilities. Using the capability to interleave additive and subtractive manufacturing, these systems provide an opportunity to perform component repair through additive material deposition and resurfacing via machining. This is particularly attractive to industries which utilize complex, often freeform, components which require a large capital investment, such as the aerospace and mold and die industries. However, in service these components may experience unique distortions or wear, and therefore each require a unique repair strategy. This work seeks to create an adaptive transformation method for part geometry, which can adapt the process to match the needs of an individual component within the context of a commercial hybrid manufacturing system using currently available on machine inspection technology; greatly improving the efficiency of repair processes. To accomplish this, a new methodology for the adaptation of a nominal CAD geometry to a component is presented which combines data registration and reverse engineering strategies for aero engine components. The accuracy of this deformation method is first examined, then simulations are completed to explore the potential efficiency gains in both the additive and subtractive phases of a hybrid repair process.M.S

Geometrical Error Analysis and Correction in Robotic Grinding

The use of robots in industrial applications has been widespread in the manufacturing tasks such as welding, finishing, polishing and grinding. Most robotic grinding focus on the surface finish rather than accuracy and precision. Therefore, it is important to advance the technology of robotic machining so that more practical and competitive systems can be developed for components that have accuracy and precision requirement. This thesis focuses on improving the level of accuracy in robotic grinding which is a significant challenge in robotic applications because of the kinematic accuracy of the robot movement which is much more complex than normal CNC machine tools. Therefore, aiming to improve the robot accuracy, this work provides a novel method to define the geometrical error by using the cutting tool as a probe whilst using Acoustic Emission monitoring to modify robot commands and to detect surfaces of the workpiece. The work also includes an applicable mathematical model for compensating machining errors in relation to its geometrical position as well as applying an optimum grinding method to motivate the need of eliminating the residual error when performing abrasive grinding using the robot. The work has demonstrated an improved machining precision level from 50µm to 30µm which is controlled by considering the process influential variables, such as depth of cut, wheel speed, feed speed, dressing condition and system time constant. The recorded data and associated error reduction provide a significant evidence to support the viability of implementing a robotic system for various grinding applications, combining more quality and critical surface finishing practices, and an increased focus on the size and form of generated components. This method could provide more flexibility to help designers and manufacturers to control the final accuracy for machining a product using a robot system

Process planning for the subtractive rapid manufacturing of heterogeneous materials: Applications for automated bone implant manufacturing

This research presents a subtractive rapid manufacturing process for heterogeneous materials, in particular for custom shaped bone implants. Natural bone implants are widely used in the treatment of severe fractures or in tumor removal. In order for the human body to accept the bone implant material and heal properly, it is essential that the bone implant should be both mechanically and biologically compatible. Currently, the challenge of having correctly shaped natural bone implants created from an appropriate material is met through hand-shaping done by a surgeon. CNC-RP is a rapid machining method and software that can realize a fully automated Subtractive Rapid Prototyping (RP) process, using a 3-axis milling machine with a 4th axis indexer for multiple setup orientations. It is capable of creating accurate bone implants from different clinically relevant material including natural bone. However, there are major challenges that need to be overcome in order to implement automated shape machining of natural bones. They are summarized as follows: (1) Unlike homogeneous source materials for which a part can be machined from any arbitrary location within the original stock, for the case of donor bones, the site and orientation of implant harvest need to consider the nature of the heterogeneous internal bony architecture. (2) For the engineered materials, the source machining stock is in the convenient form of geometrically regular shapes such as cylinders or rectangular blocks and the entities of sacrificial supports can connect the part to the remaining stock material. However, irregularly-shaped bones and the heterogeneity of bone make the design of a fixture system for machining much more complicated. In this dissertation, two major areas of research are presented to overcome these challenges and enable automated process planning for a new rapid manufacturing technique for natural bone implants. Firstly, a new method for representing heterogeneous materials using nested STL shells is proposed. The nested shells model is called the Matryoshka mode, based in particular on the density distribution of human bone. The Matryoshka model is generated via an iterative process of thresholding the Hounsfield Unit (HU) data from a computed tomography (CT) scan, thereby delineating regions of progressively increasing bone density. Then a harvesting algorithm is developed to determine a suitable location to generate the bone implant from within the donor bone is presented. In this harvesting algorithm, a density score and similarity score are calculated to evaluate the overall effectiveness of that harvest site. In the second research area, an automated fixturing system is proposed for securing the bone implant during the machining process. The proposed method uses a variant of sacrificial supports (stainless surgical screws) to drill into appropriate locations and orientations through the free-form shaped donor bone, terminating at proper locations inside the solid part model of the implant. This automated fixturing system has been applied to machine several bone implants from surrogate bones to 3D printed Matryoshka models. Finally, the algorithms that are developed for setup planning are implemented in a CAD/CAM software add-on called CNC-RPbio . The results of this research could lead to a clinically relevant rapid machining process for custom shaped bone implants, which could create unique implants at the touch of a button. The implication of such high accuracy implants is that patients could benefit from more accurate reconstructions of trauma sites, with better fixation stability; leading to potentially shorter surgeries, less revisions, shorter recovery times and less likelihood of post-traumatic osteoarthritis, to name a few

Numerical and experimental study of mechanical properties for Laser Metal Deposition (LMD) process part

Laser Metal Deposition (LMD), also called as, Laser Engineered Net Shaping (LENS), Directed Energy Deposition (DED), is a typical Additive Manufacturing (AM) technology, is used for advanced free-form fabrication. It creates parts by directly melting materials and depositing them on the workpiece layer by layer. In this process, the metal powder or fiber is melted within the melting pool by laser beam or electron beam and quickly solidifies to the deposited layer. LMD technology shows great advantages over traditional manufacturing on complex structure fabrication, including high building rates, easy material replacement and reduced material waste. These merits make the wide application of this technology in industry, such as new components fabrication and parts repairing manufacturing, coatings, rapid prototyping, tooling, repair, etc. The proposed project is to investigate the key parameters to improve the mechanical properties of different fabricate parts in LMD manufacturing by combined approach of experimental analysis and FEA simulation method. Therefore, several sets of experiments will be designed to reveal the processing parameters on properties of deposited components in the method of LMD process. The microstructure, Vickers hardness, phase identification, tensile properties of LMD parts are measured to investigate the fabricated qualities. The features of thermal stress and deformation involved in the DMD process were predicted by the FEA model. This work helps to fully study the thermal analysis to analyze the temperature profile, cooling rate and temperature gradients on microstructure and residual stress, which further influences the engineered mechanical properties of build parts --Abstract, page iv

Aseptic Machining of Live Bendable Osteochondral Allografts for Articular Surface Remodeling

Young patients diagnosed with post-traumatic osteoarthritis (PTOA) face significant hurdles to restoring pain-free joint function. While surgical interventions exist for replacing damaged cartilage, few are able to offer complete replacement of the articular surface with a bearing material that maintains the longevity and mechanical properties of native articular cartilage necessary to prevent the need for costly and painful revision procedures. Osteochondral allograft technology has begun to address this need by allowing surgeons to resurface constrained small to medium articular defects with live tissue-bank-sourced cartilage tissue explants. A primary limitation surgeons face when choosing osteochondral allotransplantation to treat large articular surface deficits is the scarcity of high-quality live explant tissue with sufficient congruence to fully restore the biomechanical function in the affected joint. This dissertation asserts that augmentation of native tissues donated to tissue banks is a promising strategy for providing more physiologically appropriate tissue replacements for patients with PTOA, providing significant symptomatic relief and allowing young patients to delay or prevent invasive total joint arthroplasty treatments.This dissertation aims to improve treatment modalities for this patient population by developing a surgical technique that enables adaptive reshaping of the articular surface of donor osteochondral tissue explants. The driving hypothesis of this dissertation is that osteochondral allografts that conform better to the opposing articular surface result in better clinical outcomes than those with lesser congruence with the native joint. The corollary hypothesis is that better conformity may be achieved by providing some measure of bending flexibility to the allograft, using streamlined tissue processing procedures to cut grooves in the bony substrate. To address these needs, we first developed, implemented, and validated the technology for milling grooves on the back of large human and canine osteochondral allografts. This resulted in the development of a process for milling grooves in patellar osteochondral samples using a computer-numerically controlled 3-axis milling machine. Sample-specific spatial information was captured within machining fixtures to generate machining paths. The curvature of human and canine osteochondral allografts was captured using a laser scanning system to fit B-Spline surfaces and generate articular curvature maps for the modified allografts. We hypothesize that due to the surface modification enabled by the bending method, bendable osteochondral allografts may provide better curvature matching for patella transplants in the patellofemoral joint. We used a cadaveric knee joint model to investigate patellofemoral joint congruence for unbent and bendable osteochondral allografts at various flexion angles. Shell and bendable allografts were machined from donor human patellae and inserted into the patellofemoral joint space of five knee joints, creating 25 femur-patella osteochondral allograft pairings. Patellofemoral joints with either shell or bendable allografts were loaded at 15-degree increments from 15 to 90 degrees flexion, and the resultant patellofemoral joint contact area was measured and compared against the native patellofemoral contact areas. On average, no significant difference in contact area was found between native patellofemoral joints and OCAs or BOCAs, indicating that both types of allografts restored native congruence. This result aligned with prior computational models of the behavior of bendable and shell allografts in the patellofemoral joint. This finding suggests that future investigations of the benefits of BOCA for allografting other joints could be initiated using computational methods, as the results of the current study suggest that the computational predictions may remain valid under the right set of conditions. Clinical studies of outcomes of osteochondral tissue transplantation indicate that maintenance of donor chondrocyte viability is crucial for the long-term success of the transplanted tissue. In order to assure that CNC machined allografts maintained appropriate chondrocyte viability and tissue sterility, we created a sterile environment for CNC milling of fresh canine patellar osteochondral allografts and quantified allograft chondrocyte viability for up to two weeks post-milling. Following machining and extended culture, bending of the allografts produced neither fracture of the samples nor resulted in loss of chondrocyte viability when compared to non-grooved controls. Therefore, these results provide basic scientific support for the clinical use of bendable osteochondral allografts. Having developed a method of bendable allografts and verifying the tissue viability and sterility, in addition to simulating joint contact in the cadaveric model, we ran a study to assess the performance of bendable osteochondral allografts and shell allografts in the contralateral stifle joints of purpose-bred dogs. This animal model was used to measure the clinical outcomes of bendable osteochondral allografts transplantation following in-vivo loading. Functional clinical outcomes were collected, including force mat kinematics, lameness scoring, range of motion, and pain scoring. At the termination of the study, allograft tissue and synovial fluid from the joint were recovered to assess the sterility, chondrocyte viability, chondrocyte morphology, and bony integration of the allograft. The allografts showed no signs of infection or rejection, and the CNC-machined shell allografts performed well in the joint. Unfortunately, the grooves machined for the bendable allograft patellae were more appropriate in width for the human patella. The removal of excess bony tissue destabilized the bendable allografts and led to fractures and fissures in the tissue. Based on the fissuring and fragmentation mode of failure noted in the canine BOCAs, the size and number of the machined grooves must be optimized for preclinical testing so the potential advantages of bendable OCAs can be realized without compromising their integrity and osteointegration during healing. Bulk mechanical properties and failure thresholds dependent on the width of allograft grooves must be established to reduce the risk of post-transplantation failure. Ongoing work aims to establish safe geometrically-based machining criteria and determine load-to-failure thresholds for osteochondral allografts to improve tissue integrity and functional viability post-transplantation. This aim will be addressed by loading canine bendable allografts with variable groove widths to assess the threshold for mechanical failure against simulated femoral trochlea. The aim of this study is to define allograft bulk mechanical properties and failure thresholds for producing bendable osteochondral allografts. The final chapter of this dissertation aims to assess the impact of sustained mechanical loading on the fluid exchange between the interfibrillar and extrafibrillar space in native articular cartilage, as the fluid load support in articular cartilage is crucial to the maintenance of the low coefficient of friction within the tissue. In our study, we developed a technique to measure water extruded from the interfibrillar space in articular cartilage by applying static compression to unconfined tissue. Preliminary results indicate that the loading and pressurization of the articular tissue can potentially make previously trapped interfibrillar water content more accessibl