Process planning for robotic wire ARC additive manufacturing

Robotic Wire Arc Additive Manufacturing (WAAM) refers to a class of additive manufacturing processes that builds parts from 3D CAD models by joining materials layerupon- layer, as opposed to conventional subtractive manufacturing technologies. Over the past half century, a significant amount of work has been done to develop the capability to produce parts from weld deposits through the additive approach. However, a fully automated CAD-topart additive manufacturing (AM) system that incorporates an arc welding process has yet to be developed. The missing link is an automated process planning methodology that can generate robotic welding paths directly from CAD models based on various process models. The development of such a highly integrated process planning method for WAAM is the focus of this thesis

On the Performance of NOMA with Hybrid ARQ

In this paper, we investigate the outage performance of hybrid automatic repeat request with chase combining (HARQ-CC) assisted downlink non-orthogonal multiple access (NOMA) systems. A closed-form expression of the individual outage probability and the diversity gain are obtained firstly. Based on the developed analytical outage probability, a tradeoff between the minimum number of retransmissions and the transmit power allocation coefficient is then provided for a given target rate. The provided simulation results demonstrate the accuracy of the developed analytical results. Moreover, it is shown that NOMA combined with the HARQ-CC can achieve a significant advantage when only average channel state information is known at the transmitter. Particularly, the performance of the user with less transmit power in NOMA systems can be efficiently improved by utilizing HARQ-CC

Disclosure of Corporate Social Responsibility Practices

This study aimed to depict the disclosure of corporate social responsibility (CSR) practices of commercial banks in Bangladesh. The sample included annual reports for the year 2018 of twenty-eight commercial banks out of thirty commercial banks listed on the Dhaka Stock Exchange (DSE) as of June 30, 2019. The data were analyzed using the content analysis technique. The findings indicate that commercial banks have made CSR contributions to eight sectors and disclosed CSR information through thirteen sections of the annual report covering a mixture of four tools. Moreover, although most of the commercial banks have disclosed some quantitative data, the aggregate amount of qualitative and mixed types of CSR disclosure is higher than that of purely quantitative ones. Additionally, all commercial banks have utilized ‘other expense' section for CSR expenditures in the body of ‘financial statements', but most of the commercial banks have ignored ‘corporate social responsibility' sub-head and preferred ‘Donation' or ‘Subscription and Donation' sub-heads in the ‘notes to financial statements'. The overall finding indicates that the CSR disclosure issue in Bangladesh has not received sufficient attention from the commercial banks. This study, therefore, recommends that CSR reporting should be formalized and regulated to enhance stakeholders' confidence in an entity's CSR practice

A multi-bead overlapping model for robotic wire and arc additive manufacturing (WAAM)

Wire and arc additive manufacturing (WAAM) is a promising alternative to traditional subtractive manufacturing for fabricating large aerospace components that feature high buy-to-fly ratio. Since the WAAM process builds up a part with complex geometry through the deposition of weld beads on a layer-by-layer basis, it is important to model the geometry of a single weld bead as well as the multi-bead overlapping process in order to achieve high surface quality and dimensional accuracy of the fabricated parts. This study firstly builds models for a single weld bead through various curve fitting methods. The experimental results show that both parabola and cosine functions accurately represent the bead profile. The overlapping principle is then detailed to model the geometry of multiple beads overlapping together. The tangent overlapping model (TOM) is established and the concept of the critical centre distance for stable multi-bead overlapping processes is presented. The proposed TOM is shown to provide a much better approximation to the experimental measurements when compared with the traditional flat-top overlapping model (FOM). This is critical in process planning to achieve better geometry accuracy and material efficiency in additive manufacturing

A practical path planning methodology for wire and arc additive manufacturing of thin-walled structures

This paper presents a novel methodology to generate deposition paths for wire and arc additive manufacturing (WAAM). The medial axis transformation (MAT), which represents the skeleton of a given geometry, is firstly extracted to understand the geometry. Then a deposition path that is based on the MAT is efficiently generated. The resulting MAT-based path is able to entirely fill any given cross-sectional geometry without gaps. With the variation of step-over distance, material efficiency alters accordingly for both solid and thin-walled structures. It is found that thin-walled structures are more sensitive to step-over distance in terms of material efficiency. The optimal step-over distance corresponding to the maximum material efficiency can be achieved for various geometries, allowing the optimization of the deposition parameters. Five case studies of complex models including solid and thin-walled structures are used to test the developed methodology. Experimental comparison between the proposed MAT-based path patterns and the traditional contour path patterns demonstrate significant improved performance in terms of gap-free cross-sections. The proposed path planning strategy is shown to be particularly beneficial for WAAM of thin-walled structures

A tool-path generation strategy for wire and arc additive manufacturing

This paper presents an algorithm to automatically generate optimal tool-paths for the wire and arc additive manufacturing (WAAM) process for a large class of geometries. The algorithm firstly decomposes 2D geometries into a set of convex polygons based on a divide-and-conquer strategy. Then, for each convex polygon, an optimal scan direction is identified and a continuous tool-path is generated using a combination of zigzag and contour pattern strategies. Finally, all individual sub-paths are connected to form a closed curve. This tool-path generation strategy fulfils the design requirements of WAAM, including simple implementation, a minimized number of starting-stopping points, and high surface accuracy. Compared with the existing hybrid method, the proposed path planning strategy shows better surface accuracy through experiments on a general 3D component

Drag-Reducing Flows in Laminar- Turbulent Transition Region

This study makes an attempt to investigate Newtonian/non-Newtonian pipe flows in a laminar-turbulent transition region, which is an extraordinarily complicated process and is not fully understood. The key characteristic of this region is its intermittent nature, i.e., the flow alternates in time between being laminar or turbulent in a certain range of Reynolds numbers. The physical nature of this intermittent flow can be aptly described with the aid of the intermittency factor c, which is defined as that fraction of time during which the flow at a given position remains turbulent. Spriggs postulated that a weighting factor can be used to calculate the friction factor, applying its values in laminar and turbulent states. Based on these, a model is developed to empirically express the mean velocity and Reynolds shear stress in the transition region. It is found that the intermittency factor can be used as a weighting factor for calculating the flow structures in the transition region. Good agreements can be achieved between the calculations and experimental data available in the literature, indicating that the present model is acceptable to express the flow characteristics in the transition region

Advanced Design for Additive Manufacturing: 3D Slicing and 2D Path Planning

Commercial 3D printers have been increasingly implemented in a variety of fields due to their quick production, simplicity of use, and cheap manufacturing. Software installed in these machines allows automatic production of components from computer-aided design (CAD) models with minimal human intervention. However, there are fewer options provided, with a limited range of materials, limited path patterns, and layer thicknesses. For fabricating metal functional parts, such as laser-based, electron beam-based, and arc-welding-based additive manufacturing (AM) machines, usually more careful process design requires in order to obtain components with the desired mechanical and material properties. Therefore, advanced design for additive manufacturing, particularly slicing and path planning, is necessary for AM experts. This chapter introduces recent achievements in slicing and path planning for AM process