To date, metal foam products have rarely made it past the prototype stage. The reason is that 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. However, the process is still poorly understood when used on metal foam, and many issues regarding the foam's mechanical response have not yet been addressed. In this study, the mechanical behavior of metal foam during laser forming was characterized by measuring its strain response via digital image correlation (DIC). The resulting data were used to verify whether the temperature gradient mechanism (TGM), well established in solid sheet metal forming, is valid for metal foam, as has always been assumed without experimental proof. Additionally, the behavior of metal foam at large bending angles was studied, and the impact of laser-induced imperfections on its mechanical performance was investigated. The mechanical response was numerically simulated using models with different levels of geometrical approximation. It was shown that bending is primarily caused by compression-induced shortening, achieved via cell crushing near the laser irradiated surface. Since this mechanism differs from the traditional TGM, where bending is caused by plastic compressive strains near the laser irradiated surface, a modified temperature gradient mechanism (MTGM) was proposed. The densification occurring in MTGM locally alters the material properties of the metal foam, limiting the maximum achievable bending angle, without significantly impacting its mechanical performance.
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.
A technique is developed for measuring accurately the strains in a cylindrical compression specimen both on the cylindrical surface and on the flat ends. It is found that by using polytetrafluorethylene (p.t.f.e.) sheets as lubricants, a bollard-shaped specimen is produced, and that a suitable amount of this lubricant produces deformations very nearly the same as those in the ideal compression test. The deviations from the deformations in the ideal compression test are represented by the strain paths of particles on the surfaces of the specimen. The stress-strain relationship based on the middle section of the specimen with optimum amount of lubricant is considered to be the standard, and comparison is made with corresponding stress-strain relationships in barrel- and bollard-shaped specimens.
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