Development and Studies of Open-Pore Metal Foam for Diverse Applications

Abstract

This thesis presents a method for producing open-cell metal foams by vibration-assisted casting (VAC) and its studies for diverse engineering applications. Metal foams have a wide range of applications due to their several beneficial properties such as high specific strength, low relative density, high effective thermal conductivity, and permeable porous structure with a high surface area to volume ratio. However, their applications are currently limited due to the complexity and high manufacturing cost. Traditional techniques involve a complex production setup that includes an inert environment and high-pressure compressed gases to accomplish molten metal infiltration into the preform to process metal foam. As a result, we have developed a method to simplify and minimize manufacturing costs by eliminating the usage of inert environments and high pressure compressed gases. The VAC method carefully controls the pouring temperature and cooling rate with the additional use of a low-frequency vibrator for infiltration of molten metal into the preform. The VAC method facilitates the design and manufactures several metal foams with high specific surface area with pore diameters ranging from 100 μm to 10 mm. Additionally, the VAC method produces various types of metal foam, homogeneous, heterogeneous, functionally graded, and composite metal foams for diverse engineering applications. Furthermore, the VAC method also enables the production of compact integral foam heat exchangers (HXs) or heat sinks in single-step casting. Controlling the cooling rate (preheating temperature) allows the metal foam to be fused bonded to a metal substrate. Thus, it eliminates many intermediate processes that are unavoidable in conventional metal foam HXs, such as machining, use of epoxy glue, brazing, press-fitting, or welding, making it easily recyclable. Fusion bonding between the metal foam and substrate reduces thermal contact resistance (RTCR) and improves the thermal performance of foam HX. The current technology facilities development of various metal foam heat exchanger designs such as integral foam heat sink (HS), foam-plate-fin HS, and foam-pin-fin HS. The following major findings are obtained from the contribution chapters that are enumerated below:

(i) The effect of foam height, pore diameter, number of foam fins, orientation, and bonding methods on thermal performance are examined. The heat transfer rate per unit mass of foam-fin heat sink is approximately double that of commercially available fin heat sinks. (ii) The thermal contact resistance (RTCR) of fused-bonded foam heat sinks is approximately 19 times lower than an epoxy-glued foam heat sink, resulting in ∼30% higher Nusselt number (Nu) of the fused-bonded foam-fin heat sink than the epoxy-glued foam-fin heat sink. (iii) Two types of foam heat exchangers (HXs): single-sided fused bonded foam tube heat exchanger (SFMF) and double-sided fused bonded foam tube heat exchanger (DFMF) with spherical and ovoid shape pores are developed. (iv) Increment in foam height has a significant influence in reducing the thermal resistance (Rth). For the same pumping power (W), the DFMF heat exchanger provides up to ∼25% lower Rth than the fin heat exchanger (Fin HX) and SFMF heat exchanger. (v) Composite metal foam (CMF) is an efficient energy absorber as compared to foam since it provides a larger stress plateau region while undergoing deformation. Moreover, the damping capacity of CMF is ∼7.5 times higher than aluminium foam and synthetic elastomer, and ∼3.5 folds higher than solid metal due to the high loss modulus of CMF. This study concludes that the composite metal foams are efficient energy absorbers and vibration dampers. Furthermore, an integral foam-fin heat sink with minimal thermal contact resistance is an excellent solution for thermal management in high-powered electronics with the benefits of being lightweight, compact, and easily recyclable.