Papers by Author: Martin Wenzelburger

Abstract: The demand for lightweight structures in the automotive and aerospace industry increases permanently, and the importance of lightweight design principles is also increasing in other industrial branches, aiming towards improved energy efficiency and sustainability. Light metals are promising candidates to realize security relevant lightweight components because of their high specific strength; and amongst them, aluminum alloys are the most interesting materials due to their high plasticity and strain to failure, good processability, passivation in oxygen containing atmosphere, and low cost. However, for many applications, their stiffness as well as strength and fatigue behavior at elevated temperature are insufficient. Metal matrix composite (MMC) formation by integration of reinforcements in the form of continuous or discontinuous (short) fibers can yield a high increase in the alloys’ specific mechanical properties at room temperature and at elevated temperature. The integration of fibers with conventional manufacturing techniques like squeeze casting, hot pressing or diffusion bonding leads to restrictions in the component’s geometry. Moreover, these techniques result in elevated process costs mainly caused by long cycle times and the need of additional protective fiber coatings. In the present paper, an alternative method for the manufacturing of aluminum matrix composites is described, combining thermal spraying and semisolid forming (thixoforging) technologies for the formation of fiber prepregs and subsequent forming with simultaneous densification. Therefore, prepregs with the matrix alloy as a thick surface coating on the reinforcement fibers are manufactured in a fast, automated coating process, while reheating, densification and shaping are performed in a separate process, allowing an optimization of both processes towards cycle times and resulting material properties. Continuous fiber and short fiber reinforced aluminum matrix composites are manufactured using woven or parallel arranged continuous fibers, or short fibers as a fleece or fiber paper material. For the coating process, twin-wire electric arc spraying is applied as a well established, cost efficient thermal spray technology. The coating process is optimized towards microstructure of the matrix alloy prior to semisolid forming, which requires a globular alloy microstructure, and reduced fiber damage during the high-temperature liquid melt deposition. The thermally sprayed fine-grained matrix material enables semisolid forming at liquid contents of 40-60 vol% of the alloy, with short flow paths, reduced mechanical loads and short cycle times. Thus, limited fiber damage and residual stresses will occur, leading to good mechanical material properties. A production line for industrial-scale coating of fiber fabric coils in a continuous process is introduced in order to provide prepregs of various fiber-reinforcement materials and fiber architectures; moreover, a winding equipment for simultaneous fiber winding and coating is presented that enables local reinforcement for components with adapted, tailored composite material design.

Abstract: Processing of light-metals in semi-solid state offers some advantages regarding process temperatures, handling of the material, but also the resulting micro-structure of the final component. Reinforcement of light-metal components with fibers or particles can be applied in order to increase elastic modulus and yield strength of the material as well as its wear resistance. But, the manufacturing of metal matrix composites by thixoforging requires the supply of raw material that shows thixotropic behavior at processing temperature and that contains a definite volume fraction of well distributed reinforcement phase. In this work, an arc wire spray process is applied for the manufacturing of semi-finished parts by coating of long-fiber fabrics and by deposition of ceramic particle containing billets. The process technique is described and the suitability of thermally sprayed matrix material for semi-solid processing is verified.

Abstract: Advanced continuous fiber reinforced light metal matrix composites (MMC) are manufactured using thermally sprayed prepregs and subsequent semi-solid forming. The prepregs consist of fiber woven fabrics, which will serve as reinforcement, and a thick metal alloy coating on top of the fabrics as the final matrix material. The coated fabrics are trimmed to the shape of the component and laminated in packages of 40 to 80 prepregs to form a component. The prepregs are reheated into the semi-solid state of the alloy and solidified to a dense, complex shaped MMC with no residual porosity by semi-solid forging (thixoforging). The deposited alloy infiltrates the fabrics with 40 % vol. to 60 % vol. liquid fraction. The fine-grained structure of the thermally sprayed metal coatings allows the required formation of globular grains during remelting and improves the impregnation behavior. The semi-solid forming and simultaneous infiltration in short cycle times offer the possibility to realize complex near net-shape geometries and make additional fiber coating not obligatory. A pilot plant was developed to provide the MMC production with prepregs consisting of various reinforcement fiber materials and a wide range of metal matrix alloys capable for thixoforging densification.