Introduction
In recent years, there is an increasing trend towards the development of hybrid structural components. The advantage of such load-adapted components lies in particular in the combination of advantageous mechanical, physical or chemical properties of various materials in one component. The potential application of hybrid structure component are stringer profiles in the fuselage of airplanes or profiles in chassis of trains and cars which use high strength, lightweight material without compromising of corrosion resistance. An example of hybrid steering tie rod composites of AA7075 core and AA6060 shell was demonstrated in [
1].
One possibility for the production of hybrid components is the co-extrusion of billets made of different materials. The billets used consist of a core and a sleeve material, which are manufactured independently of each other and are interlocked for extrusion [
2]. Scientific investigations show, that the realization of a sufficiently composite quality represents a particular challenge for the compound cold extrusion process. In investigations by [
3], although cold-welded bimetals could be manufactured from the material combinations titanium/aluminum and titanium/austenitic steel. This was only possible by the preparation of the joints with special coatings. Research by [
4] showed, that the tensile strengths of the joints behave in proportion to the surface enlargement. Cold welding is understood to be the formation of adhesive bonds that occur when metals come into contact, when the oxide layers are torn open under high surface pressure and magnification [
5]. In [
6], the influence of various factors on the quality of cold welding is discussed. Therein, the avoidance of fats and the presence of oxide-layer-free surfaces are mentioned as a prerequisite. An important aspect is the enlargement of the surfaces in order to break up the oxide layers. In this case, the required surface enlargement becomes lower with increasing temperature.
With regard to the temperature, [
7] investigated the co-extrusion of the lightweight materials aluminum and magnesium in the indirect hot extrusion process at different temperatures (380 ° C – 420 ° C) and die angles. EN AW-6060 in various wall thicknesses was chosen as sleeve material of the billets. The core consisted of AZ31 magnesium, which was separated from the sleeve material by a single or double layer of zinc or titanium foil in some experiments. The tests show, that under the given conditions it is not possible to achieve sufficient bond strength without an intermediate film. In addition, it should be noted that higher temperatures and extrusion ratios as well as thicker aluminum sleeves increase the bonding strength for the co-extrusion of aluminum and magnesium materials.
The extrusion billets that prepared by interlock initially has no bonding. In the work of [
8], two different types of block preparations are investigated. In variant 1, the titanium core is cast around with molten aluminum. In variant 2, the production was carried out by inserting the titanium core into a drilled aluminum bolt. The tests carried out by direct extrusion show that, despite the occurrence of core fractures, it is fundamentally possible to produce aluminum-titanium composite profiles. Irrespective of the type of block preparation, these profiles have integral bonds with intermetallic phases within the boundary layer. Deeper microstructural analyzes of the boundary layer were performed by [
9]. By mechanical testing of tensile specimens taken from the profiles, differences regarding the tensile strength as a function of the block preparation could be detected. Samples extracted from the cast block (variant 1) reached a tensile strength of about 40 MPa. The breakage of the sample took place in the aluminum. The composite zone remained undamaged. In contrast, samples, in which the titanium core was inserted, the aluminum block (variant 2) had significantly lower strengths.
Production of composite billets by casting technique allows the formation of a cohesive metallic bond. Different mechanisms are responsible for the formation of bonds at the interface between the participating composite partners. In particular, melting, crystallization and diffusion processes as well as dissolution and precipitation processes with the formation of mixed crystals and/or intermetallic phases are of importance [
10]. Fundamental experimental and numerical investigations on the boundary layer formation and characterization of compound casting of different aluminum alloys on a laboratory scale are described in [
11].
Regarding the technological implementation of compound casting, some discontinuous and continuous processes for the production of multi-component castings have been developed in the past. In discontinuous compound casting procedures, which are described in detail for example in [
12‐
14], a fundamental distinction is made as to whether the composite casting is formed by casting various melts into a mold or by pouring a melt onto a solid in the mold cavity. An example of a continuous composite casting process is the technology known as Novelis Fusion™, which is used industrially for the production of composite cast ingots made of different aluminum alloys [
15].
For the assessment of the interfacial bonding in material composites produced by compound casting as well as co-extrusion, various methods of material testing are used to allow a qualitative or quantitative characterization of the structure and the mechanical properties. To determine the mechanical properties of composites, it is useful to measure the microhardness to record the effects of diffusion processes and microstructural transformations at the interface [
11]. In addition, there are several destructive testing methods to determine the bond strength, which is often considered as a relevant parameter for assessing the composite quality.
Metallurgical bonding was achieved by means of compound casting of the equal alloy systems AA7075/6060 [
16]. The formation of solid solutions at the interface provides a high potential in terms of bonding strength and load-bearing capacity. Graded material properties are confined to a small transition zone between the layers [
17]. Brittle intermetallic layers could not be determined. Nevertheless, in static compound casting, the unequal thermal conditions over the height of the cast product induce the weakening of the bonding in greater distance from the casting gate.
In this article, a hot extrusion process following the compound casting was conducted to homogenize the interfacial bonding throughout the as-cast billets. The evolution of interfacial bonding during the process chain of compound casting and co-extrusion of AA7075/6060 bilayer billets are studied. The boundary layer properties are evaluated by metallography, chemical composition analysis as well as mechanical testing.
Conclusions
The evolution of the interfacial bonding of AA7075/6060 bilayer billets throughout the process chain of discontinuous compound casting and hot extrusion was investigated. The metallography, chemical composition analysis as well as mechanical tests were conducted to depict the evolution of the bonding properties. The conclusion was drawn as follows:
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Metallurgical bonding can be achieved by using appropriate casting conditions. Remelting and recrystallization are important mechanisms to form a cohesion with solid solutions at the interface. A small diffusion zone and a shear bonding strength up to 93.8 MPa are reached.
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Due to the non-uniform thermal conditions during static compound casting inhomogeneous bonding along the casting direction and non-symmetrical bonding in cross-sectional plain present in discontinuous compound casting process.
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The hot extrusion process significantly homogenizes the interfacial bonding in the above-mentioned aspects. The high pressure and temperature improve the mechanical properties of the core AA7075 material, as the voids during discontinuous compound casting process are eliminated. The shear bonding strength increases from maximum of 93.8 MPa to 145.9 MPa, which is close to the monolithic AA7075 material.
The future work is planned concerning the influence of extrusion parameters, e.g. temperature, extrusion ratio on the evolution of the interface bonding.
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