Hybrid additive and subtractive machine tools – Research and industrial developments
Introduction
The use of additively manufactured metal components in tight-tolerance and critical applications is limited by the attainable accuracy, uniformity of materials properties, and surface quality. Prevailing quality issues in additive manufacture relate to part resolution due to the smallest built-element, part density, partially bonded particulate and residual stresses. Until such a time as a step-change in build-material or energy delivery methods is made, it will not be possible to improve part tolerances without a significant increase in cost-to-build-rate ratio. This means that obtaining the resolution required to achieve conforming part in tight tolerance applications is currently not feasible. As such, additively manufactured metal parts almost always require post-processing to improve part quality characteristics and relieve residual stresses.
One possible solution to overcome these limitations is to ‘hybridise’ two, or more, processes to create a heightened capability. At the present time, workstations for hybrid additive and subtractive processing (termed ‘WHASPS’ by the authors) are emerging on the machine tool market. These machines combine an additive manufacturing process, with a conventional subtractive process, such as CNC machining. WHASPs are creating significant opportunities in the design and manufacture of finished parts, and also in the reincarnation and remanufacture of high-value components [1]. The ability to both add and subtract material helps to address geometrical challenges, such as internal and overhanging features, and parts with a high ‘buy-to-fly’ ratio [2]. These advantages help to reduce material wastage, and excessive consumption of tooling.
There are already review papers in the field of hybrid additive and subtractive manufacturing. Wang et al. [3] discuss the repair of parts via laser-based additive manufacturing processes. This deals predominantly with welding-based processes and gives a general discussion on the necessary components for an integrated system. Similarly, reviews have been undertaken relating to hybrid manufacturing processes [4], [5]; however, these do not go into detail about specific configurations, themes and challenges in HASPs. Lorenz et al. [6] have recently published a review of hybrid manufacturing processes and machine tools that incorporate directed energy deposition (DED) processes. This review is highly focused and does offer coverage of alternative additive manufacturing processes. In terms of process planning and manufacturing strategies, Simhambhatla and Karunakaran [7] introduce strategies to manufacture undercut and internal geometries using HASPs, and Kulkarni et al. [8] have reviewed process planning in layered manufacturing. In recent history this area has drawn significant attention in academia and industry, including several commercialised systems. As such, this review aims to update and extend previous works, covering manufacturing process exploitation, machine configuration and design principles. Finally, future challenges and opportunities in WHASPs are identified, concluding with a future vision of this area.
Section snippets
Additive manufacturing of metal components and its limitations
The current additive manufacturing process landscape comprises eight process families, as defined by the “Standard Terminology for Additive Manufacturing Technologies,” which is part of the ASTM F2792-12A standard series [9] (see Fig. 1). In addition to those detailed in this standard, ‘cold spraying’ has been added, which refers to an additive process that propels powdered material at a substrate at a sufficiently high velocity to cause adhesion and material build-up [10]. In metal additive
An Introduction to WHASPs
This research gives an insight into the technological and process developments that have furthered field of WHASPs. Most, if not all, WHASPs exhibit key modules, arranged into suitable configurations. The general architecture of a WHASP is described in Fig. 2. As will be seen throughout this review, the definition of a new WHASP almost always begins with a target motion platform e.g. an existing machine tool. This platform is typically optimised in its layout for either additive or subtractive
Hybrid additive and subtractive manufacturing processes – research
Based on the architecture detailed in Fig. 2, academic research relating to WHASPs may be broken down into the constituent layers, namely: the hardware, controller and software layers. The proceeding sections report on the literature from the perspective of each of these layers.
Hybrid additive and subtractive manufacturing processes – industrial perspective
Since 2003, additively manufactured part production has increased from 3.9% to 34.7% of all product and service revenues [12]. With specific reference to metal AM processes the future market size and growth rate are expected to exceed polymeric-based AM [101]. Furthermore, focus lies in customised and reconfigurable manufacturing, the application of layered and other freeform manufacturing techniques to fabricate intermediate and end-use products, and near-net shaping that reduces the need for
Observations, emerging trends and future perspectives
As a result of the literature survey undertaken in this research, a number of key observations have been made, emerging trends identified and future perspectives forecasted. These are grouped into subsections addressing machining platforms and their structural elements, control systems and process planning software, metrology and the further integration of additive and subtractive processes. A final subsection then outlines the future vision for this research area.
Research challenges and future vision
This research scrutinises emerging trends and technologies and current research challenges to form what the authors believe to be the future of WHASP research. These trends and challenges have been categorised and structured to form a roadmap for future lines of enquiry regarding research and development. This roadmap is presented in Fig. 13, and selected themes are expanded upon in the proceeding subsections.
Conclusions
The design of hybrid additive and subtractive processes has been an active research theme since the late 1990s; however, the transition from research into the commercial arena has been gradual. Research has shown that HASPs may be used to manufacture geometrically and compositionally complex parts, which were previously considered too time consuming or even impossible. With the exception of some early-adopters, the number of commercial WHASPs has increased significantly since the late 2000s.
Acknowledgements
The authors are pleased to acknowledge the support of Innovate UK for their support in Project FALCON (Finishing of Additive Layered Components on a Novel Platform - 102183), and the Engineering and Physical Sciences Research Council for their support in Project DHarMa (Design for Hybrid Manufacture - EP/N005910/1).
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