Bending Mechanism Analysis for Laser Forming of Metal Foam

Abstract

To date, the industrial production of metal foam components has remained challenging, since few methods exist to manufacture metal foam into the shapes required in engineering applications. Laser forming is currently the only method with a high geometrical flexibility that is able to shape arbitrarily sized parts. What prevents the industrial implementation of the method, however, is that no detailed experimental analysis has been done of the metal foam strain response during laser forming, and hence the existing numerical models have been insufficiently validated. Moreover, current understanding of the laser forming process is poor, and it has been assumed, without experimental proof, that the temperature gradient mechanism (TGM) from sheet metal forming is the governing mechanism for metal foam. In this study, these issues were addressed by using digital image correlation (DIC) to obtain in-process and post-process strain data that was then used to validate a numerical model. Additionally, metal foam laser forming was compared with metal foam 4-point bending and sheet metal laser forming to explain why metal foam can be bent despite its high bending stiffness, and to evaluate whether TGM is valid for metal foam. The strain measurements revealed that tensile stretching is only a small contributor to foam bending, with the major contributor being compression-induced shortening. Unlike in sheet metal laser forming, this shortening is achieved through cell wall bending, as opposed to plastic compressive strains. Based on this important difference with traditional TGM, a modified temperature gradient mechanism (MTGM) was proposed.

Advances in laser forming of metal foam: mechanism, prediction and comparison

“…When transitioning from solid metal sheets to metal foam, the previously discussed bending mechanisms are either not valid or require modification. The BM, for instance, cannot occur in metal foams, since buckling requires large compressive stresses that would crush the fragile foam structure (Bucher et al, 2017). Moreover, metal foams generally have a large sheet thickness as well as a high geometry-induced stiffness that render buckling impossible.…”

“…Several cell walls were bent during the laser scans (shown by white arrows), indicating that the foam cannot withstand high compressive stresses. Source: Bucher et al 2017Secondly, thin cell walls undergo some melting, regardless of how low the laser power is chosen (Bucher et al, 2017). Melting initiates at the first scan and continues until either the entire cell wall is melted, or the cell wall thickness increases.…”

“…The moment of area (about z = 0) of a metal foam with 89% porosity is 43.4 times higher than the area moment of a solid with the same cross-sectional area Note: Hence, foams have a high geometry-induced stiffness to bending deformation. Source: Bucher et al (2017) Finally, the TGM does not explain why metal foam can bend despite its high bending stiffness and poor tensile formability. The TGM predicts that compression-induced shortening occurs near the top surface.…”

“…Due to these unique properties, metal foam excels in many applications such as crash absorbers in cars, as well as structural components in solar power plants and aerospace equipment (Banhart, 2001). Laser forming studies have been performed with different types of metal foams, such as open-cell foams (Quadrini et al, 2010), closed-cell foams (Zhang et al, 2015;Bucher et al, 2016Bucher et al, , 2017, and closed-cell foams with protective skins (Guglielmotti et al, 2009), all reporting that laser forming is capable of bending metal foams up to 45° and beyond. An example for closed-cell aluminium foam is shown in Figure 1.…”

“…Near-net-shape processes, such as powder metallurgy processes (Claar et al, 2000) or 3D printing (Kennedy, 2012), are no valid alternatives either, since the former requires moulds and is limited to small parts with large production volumes, whereas the latter is inherently slow and limited to very small part sizes. Hence, laser forming has a great potential, since it allows for the manufacture of metal foam parts with complex curvatures and arbitrary sizes without tool wear (Bucher et al, 2016(Bucher et al, , 2017. Within the last few years, a lot of progress has been made towards a better understanding of laser forming of metal foams.…”