High Energy Fluid Jet Machining (HEFJet-Mach): From scientific and technological advances to niche industrial applications
Introduction
With the development on harder (e.g. ceramics)/structurally heterogeneous (e.g. composites)/heat sensitive (e.g. shape memory alloys)/higher strength at elevated temperatures (e.g. aerospace alloys) materials there has been an increased demand on production scientists to identify/advance appropriate machining methods able to cope with such variety of engineering scenarios. Moreover, considering that these advanced materials are used on wide range of high-value/precision/miniature products (from medical to aerospace industries), the identification of a “perfect and universal” cutting process to encompass these demands is difficult, if not impossible; thus, groups of conventional/non-conventional machining methods, with defined application portfolios, have been established.
In this context, a niche set of abrasive technologies that use High Energy Fluid Jets (HEFJet) as “universal cutting tool” was developed and has evolved, over the years, as capable processing method. HEFJet uses free (i.e. unrestrained) jet plumes of water, air, oils to remove and/or deform the target material either by themselves or in association with abrasive particles entrained/pre-mixed within the jet. The pressurised fluids escaping through small diameter orifices result in kinetic energy of the plain/abrasive jets that are further utilised as processing tools, i.e. High Energy Fluid Jet Machining (HEFJet-Mach).
Considering the variety of methods and setups, HEFJet-Mach, can be categorised based on the following key characteristics:
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Jet energy: this is usually related to the pressure pumps (p) utilised in the machine systems:
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low pressures p < 10 MPa, characterised by low jet energies, find their engineering use in surface polishing.
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high pressures (30 MPa < p < 700 MPa), characterised by high jet energies and used for through/controlled depth cutting.
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Constitutive phases of the jet: plain fluid jets: 2 phase (liquid–air); abrasive fluid jets: 3 phase (liquid–solid–air).
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Type of the fluids: the most common being: air only (blasting) [2], [3], [4]; water (with possible additives); oil.
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Nature of solids injected into jets: hard-abrasives (garnet, SiC, Al2O3, and even diamond); soft-abrasives (ice, plastics).
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Method of generating the abrasive jets:
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slurry jets where the abrasives are pre-mixed with the fluid before pressurising [44], [89], [90], [145].
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entrainment of solid particles within the jet by use of mixing chambers where the abrasives are drawn from external containers due to pressure difference [107], [172].
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With these initial clarifications and starting from a critical evaluation of key advantages, challenges and niche application remit (Section 2) of the HEFJet-Mach technology, the paper introduces the technology enablers (Section 3), e.g. machine systems/setups, that allow the highlights of main advancements in related hardware and process approaches to be made; this establishes the base to review the latest advancements in modelling and simulation (Section 4) aspects on key constitutive elements (from stream formation to jet footprint) HEFJet system to further allow commenting on the quality aspects on processed parts (Section 5) and associated methods for monitoring and control (Section 6) of the process. Then, the paper discusses the applications (Section 7) on which HEFJet technology makes its mark, flags-up the particular issues on health and safety and standardisation (Section 8) that ends by commenting on the future directions of the technology and conclusions (Section 9).
Section snippets
HEFJet-Mach – advantages
HEFJet-Mach offers a set of key advantages over the other competitive machining methods [76].
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Enables machining of difficult-to-cut materials (e.g. Ni/Ti alloys, ceramics). In contrast with the conventional (e.g. milling)/non-conventional (e.g. EDM) cutting techniques, HEFJet processing can cut any material regardless of their properties.
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Involves very low specific cutting forces (<10 N/mm2) at acceptable material removal rates (10–50 mm3/s). With these unique attributes, HEFJet-Mach has the
Technology enablers for HEFJet-Mach
The commercial market for HEFJet-Mach systems seems to be dominated by entrainment abrasive waterjet (Fig. 1a); as such, in the following sections of the paper more attention will be given to the developments of these systems. Alternatively, the slurry jets (Fig. 1b) systems, that pass pre-mixed pressurised abrasive/water slurry through a nozzle to form a cutting jet, are employed; however, as abrasive slurries cannot be directly pumped, a pressure vessel containing a rubber bladder filled with
Advances in the analytical/numerical/empirical modelling aspects of HEFJet-Mach
The HEFJet-Mach process is a combination of various physical phenomena that have been studied in the literature either separately (single-physics models) or simultaneously (multi-physics models). As shown in Fig. 17, the physical aspects involved are:
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1 or 2 phase fluid dynamics for jet formation at the level of orifice.
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2 or 3 phase fluid dynamics in the mixing chamber.
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2 or 3 phase fluid dynamics at the jet plume.
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Kinetic energy transfer at the jet footprint.
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Curvature influence of the surface
Quality of parts processed by HEFJet-Mach and methods to overcome potential drawbacks
Despite the distinctive advantages of HEFJet-Mach over other competitively processing methods, there are some limitations and challenges related to quality of parts that are of great concern for manufacturers and end-users. As the material removal mechanism plays an important role in the contribution of the surface formed, the investigation about wear mechanisms and erosion modelling of particles have been traditionally derived, e.g. Finnie's ‘micro-cutting’ model [55] and Bitter's
Water flow rate and orifice monitoring
By interpreting both pump power and water pressure signals in an abrasive waterjet machining system, Annoni et al. [8] have reported on the estimation of water flow rate (via pumping frequency) and further on the performance (discharge efficiencies on power and water flow rate) and the condition (breakage, partial clogging) of the orifices of the waterjet heads by observing the shape of power signals as shown in Fig. 30.
Abrasive flow rate monitoring
In reference to the supervision of the abrasive mass flow to enable the
Applications of HEFJet-Mach
Due to its special properties HEFJet-Mach found its way into a wide range of state-of-the-art and niche applications in various industries as well as consequential machining purposes. HEFJet-Mach methods can be structured in different levels of jet penetration on the target surfaces.
Health and safety
High pressure HEFJet machining requires special safety measures not only on the construction of the system but especially in operation [154]. Working with pressures exceeding of thousands of bars, the HEFJet systems need to utilise high pressure certified piping/fittings/valves. As for the high-pressure HEFJet-Mach operation, any direct human contact with the direct/secondary (e.g. reflected) jets must be avoided as it can result in catastrophic injuries. Additionally, the noise level on these
Conclusions
Machining by use of high energy fluid jets represents a niche technology of unique capabilities difficult to match by other non-conventional processing methods. Not only any material can be machined at negligible cutting forces and virtually no heat affected zones but the recent developments in key hardware components of the HEFJet machines allow multi-axis motions of micro/macro/fan jets and thus putting this set of technologies at the forefront of high-value manufacturing industries.
Based on
Acknowledgements
The authors would like to acknowledge the support and useful suggestions given by the CIRP STC G members to make this paper evolve from an incipient idea to final submission.
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