Andreas Reeb

Characterization of a hybrid Al2O3–aluminum matrix composite manufactured via composite extrusion

The development of new metal matrix composites for lightweight applications is aiming for an increase in specific strength and stiffness compared to conventional light metal alloys. The composite extrusion process is a promising manufacturing method for continuously reinforced light metal profiles. Especially the reinforcement with ceramic fibers leads to an increase in the specific strength and stiffness. For these investigations a hybrid composite is produced by using an Al2O3-fiber/AlMg0.6 composite wire which is embedded in an EN AW-6082 aluminum matrix. It is shown that the mechanical properties of the composite exceed those of the unreinforced matrix material. An explicit investigation of the deformation and damage behavior of this composite is given by optical strain analysis and in situ tensile tests in an X-ray micro computed tomograph (µ-CT). It was observed that during tensile loading multiple fracture of the composite wire occurs while exceeding the strain limit of the non-embedded composite wire. It could be shown that fracture of the composite wire is accompanied by strain localization and therefore strain hardening occurs in vicinity of the internal fracture, which leads to multiple necking of the specimen. The µ-CT analysis reveals the intrinsic damage mechanisms and shows the beginning of ceramic fiber fracture which showed evidence for a local load distribution between the fibers resulting in a planar fracture of the composite wire. The multiple fracture of the wire allows for an interface shear strength analysis and indicates a good bonding of the composite wire.

Preparation and Etching Methods for a Light-Optical Microscopy Analysis of Extrusion Seams and Microstructural Contrasting for Aluminum Alloys EN AW-6063 and EN AW-6082

Abstract To begin with, a visualization of the seam location as well as of the microstructure of the entire profile cross-section is necessary in order to evaluate the stability of extrusion processes and the resulting relationships between process, microstructure, and properties of extrusion profiles characterized by an extrusion seam. The investigations aim at determining appropriate preparation and etching methods for a light-optical microscope analysis of the longitudinal as well as the transversal extrusion seam in the alloys EN AW-6063 and EN AW-6082 and – in the best case – a simultaneous microstructural contrasting. For this purpose, different chemical etching techniques have been applied. A reliable uncovering of the seam was possible for both alloys using a HF solution. Etching with Wecks reagent after NaOH pre-etching turned out to be a promising method for microstructural contrasting of the EN AW-6063 alloy. The transversal extrusion seam could not be rendered visible by means of conventional etching methods. In this context, extrusion of both alloys (EN AW-6082 and EN AW-6063) applying the billet-to-billet extrusion method in line with visioplastic analyses allowed for an indirect representation of the extrusion seam.

Numerical Approach for the Evaluation of Seam Welding Criteria in Extrusion Processes

The accurate simulation and the optimization of extrusion processes can be a helpful technique to ensure producibility of complex aluminum profiles, for example for the automobile industry. Currently, the die designing is based on expert’s knowledge and cost-intensive prototyping. The paper deals with numerical investigations based on finite element simulations as well as experimental investigations of an industrial extrusion process. A newly developed method for longitudinal seam weld prediction is applied to analyze the position of the longitudinal welding line and the welding quality.

Influence of heat treatment on microstructure and mechanical properties of the interface in an EN AW-6082/1.4310 composite extrusion

Abstract The presented paper deals with a unidirectional steel wire reinforced aluminum matrix composite manufactured by composite extrusion. The main objective of this work was to determine the effect of heat treatment, and the influence of long solution annealing times on the composites interface regarding microstructural changes and the resulting interface strength. For evaluation of the microstructure high resolution transmission electron microscope (TEM) investigations accompanied with electron dispersive X-ray spectroscopy (EDX) were performed. It could be shown that diffusion from the steel wire into the aluminum matrix occurs and that the diffusion paths as well as particle formation is influenced by the preceded heat treatment. Diffusion paths in the range of 40–150 nm could be observed for Al, Fe, Cr and Ni. After annealing times over 5 h an extensive growth of an intermetallic reaction layer was found. The mechanical properties of the interface were determined by push-out-tests and tensile tests radial to the interface, which provided the debonding shear strength σdeb and for the latter experiment the interfacial radial strength σIR. It has become apparent that debonding shear strength is highly influenced by matrix properties. In radial tensile tests the failure is predominantly controlled by the chemical bond of the interface. It was shown that interface strength of specimen with small reaction zones of about 3 μm were beneficial for the mechanical behavior in both loading conditions. Longer annealing times showed a drastic decrease of interface shear strength. It was concluded from EDX measurements and in comparison with literature that the reaction zone is dominated by the growth of Al5Fe2 (η-phase).

Composite peening - A novel processing technology for graded reinforced aluminium matrix composites

Recently, the attention paid to Metal Matrix Composites (MMCs) has increased markedly. In particular, particle-reinforced MMCs are outstanding due to superior specific properties and their wear resistance. In order to further improve material utilization, recent investigations with local reinforcements in highly stressed component sections, the so-called Functionally Graded Metal Matrix Composites (FGMMC), are concerned. The production of such FGMMC was realized with composite peening - a modified process on the basis of micro shot peening. Due to this solid-phase process, ceramic particles can be introduced into regions close to the boundary layer. As preliminary studies on tin show, ceramic particles can be introduced close to the specimen surface even at room temperature. By varying process parameters, in particular by increasing the temperature, the penetration depth of the particles can be significantly increased. In case of aluminium as base material, an input of particles into the surface could be observed at a process temperature of 150 °C. The combination of aluminium with reinforced ceramic particles makes this process interesting for lightweight, wear-resistant and cyclically highly stressed structural components. Using composite peening to produce FGMMCs is a novel, economic approach.

Interface Characterization of Hybrid Composite Extrusions

Through the development of new metal matrix composites, the specific strength and stiffness can be increased above the level of conventional light metal alloys and increase their potential for lightweight applications. The composite extrusion process is a promising manufacturing method for reinforced light metal extrusions. Particularly, the reinforcement with ceramic fibers can increase both the specific strength and stiffness which are essential for lightweight purposes. To exploit the full potential of the reinforcement, the interface of this MMC has to be optimized regarding the load transfer between matrix and fiber and therefore has to offer a strong bonding. In this contribution a hybrid composite is produced by using an Al2O3-fiber/AlMg0.2 composite wire which is embedded in an EN AW-6082 extrusion profile. Both the characterization of the interface and determination of the influence of processing and heat treatment are presented. For that purpose, the composites are characterized qualitatively by metallographic analysis and quantitatively by micro push-out testing of the ceramic fibers prior and after composite extrusion. To investigate the influence of additional heat treatment the state as fabricated, which equals a T4 state of the matrix material, as well as a T6 state with additional solution annealing and artificial ageing are compared. It was found that the extrusion process has a beneficial influence on the microstructure and the mechanical interface properties and therefore confirms applicability of composite extrusion for manufacturing of alumina reinforced profiles. The heat treatment however showed no significant influence on the embedded composite wire and its interface properties.

The Influence of Stress and Heat on the Transformation Behaviour of NiTi for Actuator Applications in Extruded Aluminium Matrix Composites

The integration of functions in lightweight structures features great potential for future applications in diagnosis and control. The combination of shape memory wires or ribbons made of NiTi embedded in aluminium and manufactured by composite extrusion offers the possibility to produce a composite actuator material in a single production step. The extrusion process allows a wide range of shapes and provides higher versatility than actuators made of bi-metals. The transformation temperature of NiTi varies depending on the composition of the alloy, between -100 °C and 100 °C. However, NiTi can also transform stress-induced. In the designated application, a force is applied via the interface onto the matrix material to deform it. Due to the resulting stress, the transformation temperature rises to temperatures higher than those of the unloaded material. Furthermore the production of composite extrusions leads to a significant heat input on the shape memory alloys followed by another increase of the transformation temperature.Therefore it is essential to reproduce the heat treatment and the stress-induced transformation to predict the transformation temperature in the resulting composite influenced by the interface. For that purpose, the wire gets annealed in a furnace with different durations at a temperature similar to that of the bar extrusion process. After this, the transformation temperatures can be observed at various stresses to evaluate their applicability for aluminium composite actuators.