Essay: Metal foams

Metal Foams are a cellular structure comprising of a solid metal containing a large capacity portion of gas filled pores. The solid metal structure is most commonly made from aluminium, titanium or tantalum. The gas filled pores can come in two forms, closed-cell foam and open-cell foam. Closed-cell foams have a distinct honeycomb effect. They can be defined when the gas-filled pores are sealed, and are commonly referred to as metal ‘foams’, figure 3.2. Open-cell foams have interconnected cellular network struts and are more commonly referred to as porous metals, figure 3.1. The initial gas filled pores that form the foam are a three-dimensional, well-structured collection of similar sized pores in which each pore has a maximum volume for the minimal surface area.

An aluminium foam structure progressively increases/straightens to a plastic strain of around 0.5% caused by cyclic ratcheting whilst subjected to tension-tension loading with an associated non-zero stress. At this point a single macroscopic fatigue crack begins to develop at the weakest section of the cellular structure and continues to spread across the section with insignificant plastic deformation. When the aluminium foam structure is subjected to compression-compression fatigue the behaviour is reversed. Once the initial compression phase is over, large plastic strains up to 0.6, begin to develop and the foam structure behaves in a quasi-ductile manner. (ASHBY, et al., 2000) Essentially the aluminium foam will progressively bend under compressive loading, with an associated non-zero stress. This compressive stress leads to a high macroscopic ductility. The underlying mechanism is believed to be an amalgamation of scattered cracking of the cell walls and cyclic ratcheting under non-zero mean stress. These mechanisms result in progressive crushing of the foam cells. Three different types of deformation patterns develop due to the crushing of the foam cells. (McCULLOUGH, FLECK, & ASHBY, 1999).

‘ Type 1 – Uniform strain amasses throughout the foam, with no evidence of crush band development.

‘ Type 2 – Crushed bands form at random sections causing strain to accumulate. The first band starts to develop at the weakest sections.

‘ Type 3 – Single crush band develops and widens with an increasing number of fatigue cycles.

When aluminium foam is exposed to a repeated compressive load it will experience impact fatigue and will primarily yield elastically. Depending on the size of the test piece, at approximately 4 to 6% of strain, the aluminium foam structure will start to collapse and buckle continuously at a relatively constant stress. The constant collapse of the aluminium foam will progress to approximately 50 to 70% of strain, depending on the initial relative density of the foam. At the point in which the foam has entered a constant collapse stage the stress-stain curve will start to rise as the compressed foam enters the ‘densification’ phase. At this stage a densification strain can be defined, this in turn reflects the transition from the foam structural cell wall collapse mechanism to the solid phase compression of the majority of the cell walls.

The densification phase essentially is the point in the stress-strain curve in which the foam changes from the elastic to plastic deformation phase and defines the ‘crush strength’ of the foam. (ERG Corporation, 2011) The S-N curve for compression-compression loading is shown in figure 3.3. The fatigue life of the aluminium foam under compressive loading equals 107 cycles when the stress amplitude is 0.85, compared to a stress amplitude of 0.5 under tension-tension loading. The scattered test results are common in closed cell aluminium alloy foams, which are caused due to imperfections in the cellular microstructure. When the aluminium foam is under compression-compression fatigue loading with a maximum stress amplitude of 0.8, progressive shortening is evident, as shown in figure 3.4.

Aluminium metal foams have the following properties:
‘ Very high porosity
‘ High Compressional strengths
‘ Good energy absorption during deformation
‘ High strength
‘ Extremely lightweight material as 75-95% of its volume consists of open space

Metal foams can be used for:
‘ Car body structures
‘ Impact energy absorption components
‘ Motorway sound insulation
‘ Heat exchangers

1.2 Crystals

The structural integrity single crystal and polycrystalline magnesium oxide, MgO, are effected under repeated point contact loading. A single crystal has a cubic structure, etch pits well and is not too hard. Polycrystalline magnesium oxide has a high melting point and good resistance to attack by metals, fluxes and superconductor compounds.

To evaluate the impact surface fatigue behaviour a soft impresser technique has been developed. This procedure involves pressing a softer metallic cone against the harder flat surfaces of the single crystal and polycrystalline magnesium oxide. The benefit of using surface impact fatigue testing are that the plastic deformation of the metallic cone during initial loading cycle results in good alignment of the two contacting surfaces and ensures a uniform pressure distribution (GUILLOU, HENSHALL, & HOOPER, 1993).

The point contact fatigue testing on the single crystal MgO shows a relatively high amount of surface scratches, after only 10 cycles (figure 3.5a). Cracks are initiated at the edge of the contact zone after approximately 2,000 cycles (figure 3.5b). The cracking quickly spreads around the edge of the contact zone, which is followed by multiple cracking in the radial direction. As the number of cycle’s increases, the substantial cracking worsens. After 5,000 cycles, deep cracking becomes apparent) and the cracks occurred largely remain crystallographically oriented (figure 3.5c). The fatigue cracking was most likely caused due to dislocation interactions. (MAERKY, HENSHALL, & GUILLOU, 1999).
The dislocation emissions and twin bands occurring in the single crystal MgO at the crack tips are caused due to stress concentration. These play major roles on the mechanism of fatigue crack growth. The plastic deformation at the cracked zone has dual effects on fatigue crack growth. It can either improve the fatigue crack growth or increase the resistance to fatigue crack growth. (TANG, KIM, & HORESTEMEYER, 2010).

The polycrystalline magnesium oxide proved to have a greater resistance to impact surface fatigue cracking and induced deformation, compared to the single crystal. The main observations from point contact fatigue testing were the formation of a very degree of plastic ‘sinking-in’ in the contact zone, and some limited cracking at the edge of the contact zone. The introduction of a vacuum grease between the contact surfaces efficiently constrains metal to ceramic transfer for otherwise equal test conditions. Surface steps arising from localised plastic deformation occurs in the grain structure towards the edge of the contact zone, most likely due to slip of the contact surfaces. Surface cracking and grain spalling become apparent around the edge of the contact zone (figure 3.6b) after around 106 cycles. The cracking is primarily intergranular, with cracks linking the pre-existing pores (figure 3.6a). (MAERKY, HENSHALL, & GUILLOU, 1999).

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