FEM Simulations for the Optimization of the Inlet Gate System in Rapid Investment Casting Process for the Realization of Heat Exchangers

28 Nov.,2022

 

Cast Aluminium Gate

With the evolution of manufacturing methods, new design choices become possible due to the state of the art advance of several engineering components production processes. Additive manufacturing (AM) is a new technology developed in the latest years and now available for functional metallic parts production.

Despite initially applied for visualization prototypes, its application increased with the exploitation of advanced metallic materials such as stainless steel,5,6,19,25,47,58 titanium alloys,41,54 aluminium,29,44 and nickel-based alloys.18 Within 10 years (2003–2013), the direct part production passed from 3.9 to 34.7% of the AM total product and service revenue.10 Directed energy deposition (DED) and powder bed fusion (PBF) are the principal processes exploited to fabricate metallic parts with AM. The former provides for in situ delivery of metal through powder or wires in addition to a focused energy source. In the latter, a powder bed is selectively melted to produce a 3D layer by layer structure. Usually, an electron or laser beam is exploited as the thermal source. However, selective laser melting (SLM) for PBF allows high design freedom and production of 3D objects with demanding geometries.34,40,49,50

Wire and arc additive manufacturing (WAAM) is a DED-AM process fed by a metallic wire which is relatively simple and low cost compared to other AM methods.12 However, it involves severe residual stresses because of the high deposition rate and heat inputs.14 Also, the high energy and wide heat-affected zone produced by the WAAM afflict the metal deposition in the micro-scale level, concerning the powder-based methods.20 However, besides few studies,39,46,56,57 a thorough understanding of the thermal history along with mechanical and surface properties as a function of the process conditions lacks is lacking.36

The application of AM for heat transfer devices is of great interest allowing miniaturization and performance improvement. The latter may be achieved with the realization of complex designs, capillarity structures, channel arrays, and more. However, many challenges in engineering AM processes must still be faced to achieve workpieces free of defects and high-quality features to match the tight standards required for engineering applications.45

Common AM component defects include discontinuities, porosity, highly textured columnar grains, complex phases, and compositional variations.11 However, most of the features inherited from the manufacturing process can be reduced by post heat treatment like hot isostatic pressing (HIP).21,28,31 Nevertheless, porosity production is still a major problem during AM of aluminium alloys.8,9,33,52 In WAAM, the metal suffers a complex thermal process and anisotropies within the microstructure and, therefore, in the components properties.27,58

Proper management of the process parameters, e.g., temperature gradient, solidification rate, alloy composition, thermal cycles and high cooling rate, hinder the anisotropies.24,59 The SLM process parameters and building strategy also have a broad influence on the structure of the part and, thus, on the mechanical properties. Hence, they must be specifically selected for a given combination of material, geometry and size. In general, a rapid cooling rate produces high residual stresses and fine microstructure . In the powder bed condition, this leads to the creation of porosity. Also, the orientation of the components during the SLM processes exert an influence on grain growth and surface texture.43,48 Properties such as density and surface roughness are fundamental as they noticeably affect the heat transfer. Generally, solidified grains present textured large columnar grains tough with equiaxed grains close to the melt pool surface.1,23,30,55,63

The morphology transition within the solidification phase is governed by the ratio of thermal gradient G on the kinetics of mass transfer R rule. Several grains configuration can be achieved through the process parameter managements, from equiaxed dendritic to planar. These concerns are valid all the more reason when considering thin-walled components.60 The smaller the 3D structure thickness, the higher is the defects and morphology influence on their properties.

Aluminium flat thin pipes find several applications in different sectors such as automotive, aeronautics, conditioning, and heat exchange applications. Up to now, a typical production process of hollowed flat aluminium pipes is continuous extrusion. However, several limitations like expansive production costs and billet pre-treatment requirements afflict this manufacturing process. Also, the welded pipes produced cannot be exploited for high-pressure liquid containment.61 Instead, flow forming (FF) is a forming process exploited to produce seamless thin-wall tubes from thermally treated preformed tubes prepared by extrusion or forging processes.13,35,37 A challenging multiple parameters setting is required for the FF process, which being a dynamic process requires demanding management. Thus, the variables are often set based on engineers’ experience and practice. However, the flow-formed pipes can respect the higher standards in specific strength, geometrical tolerance and surface roughness.42

A lately proposed rapid investment casting (RIC) overtake both the limitation of AM and casting. It provides for the 3D printing of meltable models made by foundry resin exploited for a casting process. It has already been proved how this methodology is valuable for metal foams and reticular structure production with outstanding results.2,4,62 It is mainly due to the peculiar conformation of the metallic cellular structures, which offer a wide exchange surface.7 Indeed, metal foams have already been applied in heat exchange prototypal devices.3,22,32,38 In heat exchange devices, this is important as such the contact between these surfaces and flat heat pipes.

Through RIC, it is possible to integrate metallic reticular structure branches within the tube wall bulk in the fundable model. The devices produced by foundry processes will feature a seamless connection between heat pipes and heat exchange surface that improve the heat dissipation performance. The main concerns regard the thickness of the flat tubes achievable with the foundry processes as even modest defects on the thin-walled structure would compromise the device’s functionality.

The finite element method (FEM) help to prevent flaw that may arise during the metal casting.15,16,17,26,51,53 Huang et al.26 optimized a casting process through numerical simulations of the gating system for precision 17-4 PH stainless steel rotor used for hemodialysis machines.

Both flow turbulence and inadequate supply and internal shrinkage can be reduced through appropriate numerical investigation. Fang et al.17 investigated the preliminary design in investment casting of an exhaust manifold using a numerical analysis with FEM investigation. The results of numerical simulations highlighted the eventuality to optimize and redesign the parameters involved during the casting process. Also, the results obtained were checked and confirmed by the actual production and testing of the exhaust manifold.

The study aims at defining the main foundry parameters to achieve an aluminium radiator free of defects, with flat heat pipes seamlessly connected to a cellular structure. CAD models of the device were produced, considering four different pipes thickness and length. Also, a numerical simulation was performed to design the casting process and avoid defects. The CAD patterns were manufactured through 3D printing and cast with the RIC processes. Numerical investigation and experimental work led to the definition of a range of process parameter values for which the process is feasible, which depends on the geometry of the flat heat pipes.