Diffusion in aluminum/mangesium composites

Investigating and proving bonding processes

As part of a special research project examining “high strength aluminum-based lightweight materials for safety components”, the Chemnitz-based Fraunhofer Institute for Machine Tools and Forming Technology (IWU) has been looking at the production of aluminum-magnesium composites using incremental reforming processes. Supporting it on this project is the Steinbeis Transfer Center for Industrial Surface Engineering, which is responsible for producing oxide-free surface specimens, developing layer systems (mediator layers), and evaluating test samples using optical and electron microscopes.

In the automotive industry, aluminum/magnesium composite materials have every potential to open up new areas of application. For years, one of the car companies’ biggest aims has been to slash exhaust emissions while at the same time reduce fuel consumption. One highly effective way of achieving this is to drastically reduce vehicle weight by using alternative materials with similar properties but a lower mass. For example, magnesium has a higher strength-to-weight ratio than steel. Also, it is more rigid than aluminum and is excellent for machining. As a result, magnesium compounds are particularly well suited to lightweight constructions. The only problem until now has been the relatively low levels of corrosion stability, meaning that their use in automotive construction remained fairly limited.

Of course, one way to protect materials from the onslaught of corrosion is to use corrosion-resistant metallic layering. The advantage of this approach over varnish or plastic layers lies in the higher mechanical resistance. Two or multi-layer compounds, which, depending on requirements, can be several millimeters thick, can only be made using mechanical processes. The best known machine-based processes are roll or explosion cladding, but there is also extrusion, or extrusion molding. Compared to layering techniques in which the intended change in material properties or functional characteristics is restricted to the area near the surface, these approaches make it possible to produce semi-finished cross sections with optimized structures. According to recent investigations into compound extrusion, metallic bonding is possible between aluminum (face-centered cubic crystal system) and magnesium (hexagonal closepacked system), despite the different lattice structures. Under certain conditions, it was known that a distinct diffusion zone could be created between a wrought magnesium alloy (AZ31) and a standard aluminum alloy (6060). The microstructure of the resulting join was examined using a standard optical microscope. Nearly all samples contained fissures and a porous area near the boundary. How these came about was not examined further. A firmly-bonded, sufficiently strong join can only be expected if there is free flow of matter beyond phase boundaries between the two materials. As well as the chemical requirements, the following factors are also decisive for the initiation of a solid state reaction: “near-ideal” contact between the reactants, a high degree of disorder in the lattice structures, and the addition of energy (“activation energy”). As such, for this bonding to occur it is necessary to control the parameters which allow maximum enlargement of the boundary area under high standard stress levels. Another decisive aspect is the temperature at which the reforming occurs. As well as dictating the flow properties of each bonding element, it provides the energy needed to activate the flow of matter.

We know from literature that the chemical requirements for diffusion in the solid state are fulfilled in the case of an Al/Mg pairing. Generally, it is only possible for matter to move beyond phase boundaries if the system involves single-line mixed crystals. Apart from this fundamental prerequisite, the diffusion process is influenced by a string of external factors, which must be quantified to be able to control the bonding mechanism as desired. Progress is made when process-specific parameters fields can be successfully replaced by differentiated quantitative models. The biggest advantage over the commonly used process-oriented view is that the knowledge gained can be applied generally. Such approaches require a differentiated description of the local factors in the boundary layer using measurable parameters. The relevant variables for the bonding process were identified based on its phenomenological description in the literature. A pilot experiment was used to verify the parameter field determined in this way.

The best way to do this appeared to be to an analogy experiment based on lateral extrusion experimentation, allowing for a wide range of relevant parameters. Samples of the material combinations to be investigated (diameter size 20 by 45) were put through the die in a hot state. The surface areas were compressed onto each other under high pressure, and during the whole extrusion process, they had to stay within the separation layers of the die. Deviations can arise due to major differences in flow stress or differences in the tribological properties of the materials being investigated. To keep samples at the right temperature, the die had heat-controlled matrices. The applied extrusion pressure, which depended on the molding direction, was tracked with an integrated force-displacement measurement device. The functional interdependence between influencing factors directly affecting the formation of the compound and parameters defined as part of the experiment could then only partly be ascertained through analysis. The temperature and stress distribution in the boundary area under defined marginal conditions were pinpointed using a finite element method simulation.

Metallographical sections traversing the join area between the two materials were cut from the extruded samples. In certain areas, even under optical examination at the highest possible magnification (1000x) no joint could be seen in the zone between each substrate. Using an electron microscope at 4000 times magnification, a join measuring around 5 μm could be seen between the two materials. The parts had been completely joined in this area due to the compression. To prove that diffusion had occurred, energy- dispersive X-ray spectroscopy (EDX point analysis) was used along the line of separation between the two materials. The center of the zone was set as zero. The measurement was started in the magnesium around 30 μm from the center, and ended in the aluminum around 17 μm from the center of the zone. This made it possible to prove that the border layer – which was about 8 μm thick – contained both magnesium and aluminum. Another observation which made sense was that compared to the magnesium, the diffusion layer in the aluminum was thicker, due to its lower atomic mass. Further experiments are now planned to examine the influence of the main process parameters on the quality of the compound – such as the surface preparation of the bonding materials, and the pressure, temperature and speed of extrusion.  

Contact

Prof. Dr.-Ing. habil. Rudolf Förster
Steinbeis Transfer Center Industrial Surface Engineering (Eibbau-Walddorf)
stz1090@stw.de

Dr.-Ing. Roland Glaß
Dipl.-Ing. (FH) Mike Popp
Dipl.-Ing. (FH) Christopher John

Fraunhofer IWU

Dipl.-Phys. Dietmar Kitta
TECHNO-COAT Oberflächentechnik GmbH

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