Low cost integration of additive and subtractive processes for hybrid layered manufacturing

Abstract

While CNC machining (subtractive method) is the only option when it comes to high quality components, it demands greater human intervention to generate the CNC programs, making it a slow and costly route. On the other hand, Rapid Prototyping (additive method) is able to convert the design into the physical objects without any human intervention. But its total automation comes with compromises in the qualities of geometry and material. A hybrid layered manufacturing process presented here combines the best features of both these approaches. In this process the near-net shape of the object is first built using weld-deposition; the near-net shape is then finish machined subsequently. Time and cost saving of this process can be attributed to reduction in NC programming effort and elimination of rough machining. It is envisioned as a low cost retrofitment to any existing CNC machine for making metallic objects without disturbing its original functionalities. Near-net shape building and finish machining happening at the same station is the unique feature of this process. A customized software generates the NC program for near-net shape building. The intricate details of integrating arc welding unit with a CNC milling machine are presented in this paper.

Introduction

Rapid prototyping (RP), also referred to as layered manufacturing (LM) or 3D printing, has drawn a significant research interest owing to decreasing product development times and increasing complexity of the components. LM offers total automation in converting the virtual models into physical ones. This is archived by slicing the 3D geometric model into layers and realizing each layer at a time.

As LM was introduced as a design visualization tool, the early focus of the research was on the physical realisation of the shape rather than its functionality. Thus most of the existing LM processes produce objects using resins and other non-metals, limiting their applications. In comparison, metallic prototypes have a larger domain of applications [1]. Thus, efforts have been going on for extending LM for manufacture of metallic objects. Several techniques like laser-engineered net shaping (LENS), 3D welding, direct metal deposition (DMD), shape deposition manufacturing (SDM), laser based additive manufacturing (LBAM), 3D micro-welding (3DMW) and electron beam melting capable of producing metallic prototypes have been developed by different research groups [2], [3], [4], [5], [6], [7], [8]. Fig. 1 summarizes various existing methods for LM of metallic objects. When the metallic object is obtained in a layer by layer manner, it is known as direct process. If a casting process is used in conjunction with the consumable pattern manufactured in a layer by layer manner, it is known as indirect process. Quick cast patterns of SLA and polystyrene patterns of SLS are popularly used in indirect processes.

Based on the method of deposition, direct processes can be further classified into laminated tooling, powder-bed and deposition technologies (Fig. 1). In laminated tooling, the tool shape is achieved by usually cutting and stacking metal sheets together. It is a relatively fast and simple method to make metal tools directly for injection molding. In powder-bed technology, a layer of metallic powder is first spread and the required regions of the layer are sintered selectively. On the other hand when the metal is deposited only in the required regions of the layer, it is known as deposition technology. While powder-bed technology has the advantage of an inherent support mechanism, it generates only a porous structure. On the contrary the deposition technology cannot produce overhanging features but has better structural integrity. Furthermore, with the exception of 3DP, power-bed methods are not as amenable to produce functionally gradient materials (FGMs) as deposition methods are.

Direct LM processes generally use arc, laser or electron beam as the energy source for metal deposition. Table 1 lists various existing technologies in each category. Fig. 2 shows some metallic objects realized using different LM processes. Components manufactured through LENS, EBM and Arc welding have an accuracy of 0.4, 0.1 and 0.2–0.5 mm, respectively [6], [8]. Thus it can be inferred that all of them produce only rough surfaces and cannot be directly used for high precision applications like tooling where the accuracy required is of the order of 1 μm.

The low accuracy of the LM components is due to splitting of the component into slices. Automation is attained in LM by compromising on quality. Rapid prototypes are inferior in geometric and material quality to machined parts. CNC machining, the subtractive manufacturing strategy, on the other hand, can handle any material and offers high accuracy and surface finish demanded in the tooling applications. But it requires substantial human intervention for path planning and is limited in features that can be realised. hybrid layered manufacturing (HLM) processes, which combine the advantages of both additive and subtractive manufacturing, have been developed by many researchers [18], [20], [23]. However, as the creation of the near-net shape and its finish machining occur at different stations, it limits the foolproof implementation of computer aided process planning (CAPP). It also becomes expensive due to the use of two sets of motion controls. Considerable amount of time is also wasted in setting and positioning of the job on LM and CNC machines separately. IIT Bombay’s arc hybrid layered manufacturing (ArcHLM) presented here overcomes these limitations by achieving both near-net metal deposition and finish machining on the same CNC machine.

ArcHLM employs arc welding for deposition as it has the added advantages of higher deposition rates, lower costs and safer operation. Deposition rate of laser or electron beam (EB) is of the order of 2–10 g/min, whereas deposition rates of 50–100 g/min have been reported in arc-based LM [6], [17]. Furthermore, it also has the potential to control the size, flux, velocity, trajectory and thermal states of the droplet and the substrate thermal state precisely, which are critical to the geometric accuracy and metallurgical properties of the deposited part [24].

The following are some of the significant advantages of this process:

• Total automation across LM phase of building near-net shape and CNC phase of finish machining is possible.

• It uses arc weld-deposition, which is economical, faster and safer than competing laser and EB based processes.

• It can be retrofitted to any existing CNC machine as an optional feature.

• The retrofitment does not require any proprietary information from the machine builder. Therefore this integration is independent of the make of the machine.

• As the weld-deposition torch is mounted on the same spindle head no additional sets of axes and controller are needed.

The details of this novel integration method are discussed in the subsequent sections.