Design of an adaptive force and stiffness controlled compliant device for robotic polishing

Polishing is a repetitive task done in an unhealthy environment. Often more than half of the manufacturing time is required to polish a die. The manual polishing process is a tedious work actively rely on a skilled human worker. Industrial Robot has replaced the human in performing these tasks. For robotic polishing to control the polishing force, an active compliant device is used. Due to the compressibility of air, a pneumatic system is preferred as the actuator of the device. The force of the actuator is controlled by regulating air pressure in both chambers of the cylinder. However, to improve productivity, a constant polishing force alone is not sufficient, the stiffness is also considered. The current work involved a new adaptive approach to model and control of the force and stiffness of an active compliant device. The device can adaptively control the compliance and force in real time compensating the gravitational effect due to the mass, gravity, and orientation of the tool. The designed single axis controller consists of a dual acting pneumatic cylinder attached to the end effector of an industrial robot. The effectiveness of the force and stiffness controlled polishing system was proved through experiments --Abstract, page iii

Characterization of high cycle fatigue and laser-aided machining and polishing of additively manufactured materials

“Additive manufacturing (AM) and laser-aided machining and polishing (LAMP) of materials are emerging manufacturing processes both for research and industrial sectors. The AM process can manufacture near-net-shape parts with complex geometries. Meanwhile, the LAMP process integrated with an AM system offers a high processing rate, minimum heat-affected zone, and easily adjustable process parameters during machining and polishing. In mechanical properties characterization of AM metals and alloys, fatigue is a vitally important test method to understand the behavior of materials in cycling loading and unloading circumstances since most mechanical failures of structures are due to fatigue. To characterize AM metal fatigue behavior, it is also crucial to understand and analyze how the fabrication process parameters, build orientations, and defect formations affect the ability of materials to resist fatigue failure. This research aims to study the needed fundamental knowledge for a high-speed fatigue testing method with miniature specimens and investigate the effect of build process parameters on the high cycle fatigue performance of AM materials. In this study, the implementation of miniature specimens with increased surface area and uniform stress distribution within gauges captures a large population of surface and subsurface defects, reduces the stress gradient effect, maintains symmetric loading, minimizes material and test equipment costs, and decreases sample preparation and test time. The acquired knowledge from this study helps understand the influence of defects on the fatigue behavior of AM materials and determine the high fatigue strength yielding process parameters. Since the fatigue strength of materials can be improved by machining and polishing part surfaces, the objectives of this research also include developing a multilaser LAMP process and investigating the effect of different process parameters on part surface quality improvements. The research results lead to new knowledge that could benefit a wide range of manufacturing industries”--Abstract, page iv

A Review on Metallic Alloys Fabrication using Elemental Powder Blends by Laser Powder Directed Energy Deposition Process

The laser powder directed energy deposition process is a metal additive manufacturing technique, which can fabricate metal parts with high geometric and material flexibility. The unique feature of in-situ powder feeding makes it possible to customize the elemental composition using elemental powder mixture during the fabrication process. Thus, it can be potentially applied to synthesize industrial alloys with low cost, modify alloys with different powder mixtures, and design novel alloys with location-dependent properties using elemental powder blends as feedstocks. This paper provides an overview of using a laser powder directed energy deposition method to fabricate various types of alloys by feeding elemental powder blends. At first, the advantage of laser powder directed energy deposition in manufacturing metal alloys is described in detail. Then, the state-of-the-art research and development in alloys fabricated by laser powder directed energy deposition through a mix of elemental powders in multiple categories is reviewed. Finally, critical technical challenges, mainly in composition control are discussed for future development

Fabricating TiNiCu Ternary Shape Memory Alloy by Directed Energy Deposition via Elemental Metal Powders

In this paper, a TiNiCu shape memory alloy single-wall structure was fabricated by the directed energy deposition technique with a mixture of elemental Ti, Ni, and Cu powders following the atomic percentage of Ti50Ni45Cu5 to fully utilize the material flexibility of the additive manufacturing process to develop ternary shape memory alloys. The chemical composition, phase, and material properties at multiple locations along the build direction were studied, using scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, Vickers hardness test-ing, tensile testing, and differential scanning calorimetry. The location-dependent compositions of martensitic TiNi and austenitic TiNi phases, mechanical properties, and functional properties were investigated in detail. Variations were found in atomic compositions of Ti, Ni, and Cu elements along the build direction due to the complex interaction between elemental powders and laser pro-cessing. Good correlations were present among the chemical composition, phase constituent, hard-ness, and feature of phase transformation temperatures at various locations. The ultimate tensile strength of the as-deposited TiNiCu alloy is comparable with the previously reported additively manufactured TiNi binary alloys. By adding Cu, a much lower thermal hysteresis was achieved, which shows good feasibility of fabricating ternary TiNiCu shape memory alloys, using elemental powders in the directed energy deposition to adjust the thermal hysteresis

High Cycle Fatigue Performance of LPBF 304L Stainless Steel at Nominal and Optimized Parameters

In additive manufacturing, the variation of the fabrication process parameters influences the mechanical properties of a material such as tensile strength, impact toughness, hardness, fatigue strength, and so forth, but fatigue testing of metals fabricated with all different sets of process parameters is a very expensive and time-consuming process. Therefore, the nominal process parameters by means of minimum energy input were first identified for a dense part and then the optimized process parameters were determined based on the tensile and impact toughness test results obtained for 304L stainless steel deposited in laser powder bed fusion (LPBF) process. Later, the high cycle fatigue performance was investigated for the material built with these two sets of parameters at horizontal, vertical, and inclined orientation. In this paper, displacement controlled fully reversed (R = 1) bending type fatigue tests at different levels of displacement amplitude were performed on Krouse type miniature specimens. The test results were compared and analyzed by applying the control signal monitoring (CSM) method. The analysis shows that specimen built-in horizontal direction for optimized parameters demonstrates the highest fatigue strength while the vertical specimen built with nominal parameters exhibits the lowest strength

A Displacement Controlled Fatigue Test Method for Additively Manufactured Materials

A novel adaptive displacement-controlled test setup was developed for fatigue testing on mini specimens. In property characterization of additive manufacturing materials, mini specimens are preferred due to the specimen preparation, and manufacturing cost but mini specimens demonstrate higher fatigue strength than standard specimens due to the lower probability of material defects resulting in fatigue. In this study, a dual gauge section Krouse type mini specimen was designed to conduct fatigue tests on additively manufactured materials. The large surface area of the specimen with a constant stress distribution and increased control volume as the gauge section may capture all different types of surface and microstructural defects of the material. A fully reversed bending (R = -1) fatigue test was performed on simply supported specimens. In the displacement-controlled mechanism, the variation in the control signal during the test due to the stiffness variation of the specimen provides a unique insight into identifying the nucleation and propagation phase. The fatigue performance of the wrought 304 and additively manufactured 304L stainless steel was compared applying a control signal monitoring (CSM) method. The test results and analyses validate the design of the specimen and the effective implementation of the test bench in fatigue testing of additively manufactured materials

A Novel Laser-Aided Machining and Polishing Process for Additive Manufacturing Materials with Multiple Endmill Emulating Scan Patterns

In additive manufacturing (AM), the surface roughness of the deposited parts remains significantly higher than the admissible range for most applications. Additionally, the surface topography of AM parts exhibits waviness profiles between tracks and layers. Therefore, post-processing is indispensable to improve surface quality. Laser-aided machining and polishing can be effective surface improvement processes that can be used due to their availability as the primary energy sources in many metal AM processes. While the initial roughness and waviness of the surface of most AM parts are very high, to achieve dimensional accuracy and minimize roughness, a high input energy density is required during machining and polishing processes although such high energy density may induce process defects and escalate the phenomenon of wavelength asperities. In this paper, we propose a systematic approach to eliminate waviness and reduce surface roughness with the combination of laser-aided machining, macro-polishing, and micro-polishing processes. While machining reduces the initial waviness, low energy density during polishing can minimize this further. The average roughness (Ra = 1.11 µm) achieved in this study with optimized process parameters for both machining and polishing demonstrates a greater than 97% reduction in roughness when compared to the as-built part