Andreas Lohmüller

Effect of solidification microstructure and Ca additions on creep strength of magnesium alloy AZ91 processed by Thixomolding

Abstract The compressive creep behaviour of AZ91 processed by Thixomolding with varying solid phase content (2–45%) was investigated. It can be shown that with increasing solid phase content the creep rate at 150°C is reduced by a factor of 3–10. The results can be rationalised in terms of a grain size effect. The solid phase (primary α-Mg phase solidified prior to mould filling) is relatively coarse (40 μm), i. e. the average grain size increases significantly when the material is processed in the semi-solid regime. Grain boundary sliding which is known to contribute significantly to creep deformation depends on the grain size. The creep rate of high pressure die cast material is found to be similar to thixomolded material. The high pressure die cast materials although nominally processed in the fully liquid regime, contains pre-solidified coarse primary α-Mg phase as well. The coarse solid phase particles are formed early in the process when the melt cools rapidly in the shot sleeve. AZ91 with Ca-additions up to 5 wt.% were prepared using a newly developed second feeding system that allows the addition of a second sort of granules to the screw in parallel to the main feed. As expected, Ca increases the creep strength very significantly. The reasons for this increase will be discussed.

Reinforced Light Metals for Automotive Applications

Efficiency and dynamic behavior of a vehicle are strongly affected by its weight. Taking into consideration comfort, safety and emissions in modern automobiles, lightweight design is more of a challenge than ever in automotive engineering. Materials development plays an important role against this background, since significant weight decrease is made possible through the substitution of high density materials and more precise adjustment of material parameters to the functional requirements of components. Reinforced light metals, therefore, offer a promising approach due to their high strength to weight ratio. The paper gives an overview on matrix and reinforcement structures suited for the high volume output of the automotive industry. Further analytical and numerical approaches to describe the strengthening effects and the good mechanical characteristics of these composite materials are presented. The potential of reinforced light metals is shown by means of compression samples cast in laboratory scale.

Predicting defects and material quality in castings by means of computational determination of flow length values

Abstract A new criterion for the optimisation of casting design by numerical simulation is proposed. Based on the determination of flow length values during filling in castings, possible locations of defects can be predicted together with their effect on mechanical properties. This approach is demonstrated for a magnesium casting with special emphasis on comparison with experimental results.

Partikelverstärkung von Magnesiumgussbauteilen

Im Rahmen des von der Bayerischen Forschungsstiftung geforderten Projekts „Metall-verbunde“ konnte bei der BMW Group ein Motoranbauteil aus der Magnesiumlegierung AJ62 mit einer Verstarkung durch Siliziumkarbidpartikel umgesetzt werden. In Kooperation mit den Projektpartnern Neue Materialien Furth GmbH und den Lehrstuhlen „Konstruktionslehre und CAD“ sowie „Metallische Werkstoffe“ der Universitat Bayreuth entstand eine durch gangige Prozesskette von der Mikrostruktursimulation bis zur Bauteilerprobung.

Influence of Ti and Zn on Particle Incorporation of AlB 2 , B and B 4 C Particles in Aluminum Using the Stir Casting Process

The outstanding performance of many aluminum matrix composites (AMCs) regarding specific stiffness makes AMCs attractive materials for lightweight construction. Low density boride compounds promise both an increase in stiffness and decrease in composite density. Therefore for this study AlB2, B and B4C were chosen for composite manufacturing. The composites were fabricated with the stir casting process. To avoid gas entrapment during mixing and ensure nonporous composites, partial vacuum was adapted during particle feeding and stirring. Poor wettability of used particle material in contact with liquid aluminum hindered particle incorporation, but alloying elements such as titanium were shown to affect wettability and particle incorporation for B4C. Zn had no influence on wettability or reactivity and did not improve particle incorporation. In contrast to Zn, Ti improved adhesion and wettability, but particle incorporation was improved exclusively for B4C. Besides alloying Ti, the use of high-shear force mixers improved particle incorporation enabling uniform particle distribution. AMCs with up to 12 vol.% of B4C particles were produced via stir casting without alloying Ti.

Continuous Powder Extrusion for Fabrication of Carbon Fibre Reinforced Aluminium

Short fibre reinforced aluminium was produced using the Temconex® process which is a continuous extrusion using a mixture of metal powder and ceramic short fibre as feedstock. The Temconex® process was derived and further developed from the ConformTM process which uses metal rod rather than powder as feedstock. In the present paper the effect of the prechamber length on the mechanical properties was examined. As material Al99.7 powder with different volume fractions of milled carbon fibres was used. Distribution, orientation and geometry of the embedded fibres were examined using light microscopy. The mechanical properties were determined via tensile testing and resonance frequency analysis. An important increase of the Young’s modulus is observed because of the introduction of fibres. It can be rationalized based on Clyne’s Shear Lag model. Results show that an extension of the prechamber enhances the Young’s modulus and the elongation of fracture due to reduced fibre fracture and better fibre alignment.

Particle reinforcement of cast magnesium components

As part of the “Metallverbunde” project sponsored by the “Bayerische Forschungsstiftung” research foundation, an engine accessories component made from magnesium alloy AJ62 with reinforcement from silicon carbide particles was produced at BMW Group. The integrated procedures described below were developed in cooperation with the project partners “Neue Materialien Furth GmbH” and the departments of “Design Engineering and CAD” and “Metallic Materials” at the University of Bayreuth.