Christian W. Schmidt

Tailored Heat Treated Accumulative Roll Bonded Aluminum Blanks: Microstructure and Mechanical Behavior

Aluminum alloy AA6016 was accumulative roll bonded up to eight cycles and investigated regarding formability by bending tests. Due to the limited bendability of accumulative roll bonding (ARB) processed materials, a tailored laser heat treatment was performed along the bending edge before forming. This tailored laser heat treatment causes a local recrystallization and recovery of the bending samples at the deformation zone, which locally increases ductility and allows higher bending angles achievable with lower forming forces. Between the recrystallized heat treated zone and the unaffected ultrafine-grained (UFG) base material, a gradient in grain size with a bimodal region is formed. This observed microstructural profile is confirmed by local mechanical testing measuring the hardness and strain rate sensitivity by nanoindentation techniques.

Influence of Nanoparticle Reinforcement on the Mechanical Properties of Ultrafine-Grained Aluminium Produced by ARB

Dispersed nanoparticles are introduced from stabilized suspensions during the accumulative roll bonding process in aluminium AA1050A by air gun spraying up to a final volume fraction of 0.1 % after eight cycles. Additional strengthening caused by particle insertion is observed and strongly depends on the suspension medium and stabilizing agent as both influence interfacial bonding of the particles to the matrix. The particle insertion furthermore results in reduced peel strength of the sheets irrespective of particle material and size caused by a reduction of effective metal to metal bonding area during rolling through the presence of the particles.

Properties of Magnesium Strips Produced by Twin-Roll-Casting and Hot Rolling

The combination of Twin-Roll-Casting (TRC) and subsequent rolling constitutes the most promising process chain for producing magnesium strips economically. Fast solidification (10 times faster than continuous casting) combined with partial deformation lead to a fine primary microstructure as well as less shrinking holes, pores, segregations and brittle precipitations which all together has a very positive effect on forming behavior of the initial material and quality of the final product. The paper elaborates on metallurgical processes in consideration of microstructure and texture results and on the obtained mechanical properties of TRC magnesium strips and finished strips. In addition, the influence of twin-roll-cast and rolling conditions on the mechanical properties will be discussed. The investigation has also been expanded to possible heat treatments and their influence. Concluding remarks will be made on results of rolling trail which were carried out on an industrial scale rolling mill, revealing that the production of hot rolled thin sheets of magnesium alloy AZ31 is possible with a very promising combination of strength and ductility.

Experimental and Numerical Investigation of Texture Development during Hot Rolling of Magnesium Alloy AZ31

Due to the deformation mechanisms and the typical basal texture rolled magnesium sheets show a significant asymmetry of flow stress in tension and compression. In order to avoid this undesired behavior it is necessary to achieve non-basal texture during rolling, or at least, to reduce the intensity of the basal texture component. The reduction of the anisotropy caused by the basal texture is very important for subsequent forming processes. This project aims at optimizing the hot rolling process with special consideration of texture effects. The development of the model is carried out in close cooperation with the experimental work on magnesium alloy AZ31 .The experimental results are required for the determination of model parameters and for the verification of the model. Deformation-induced texture is described by the visco-plastic self-consistent (VPSC) model of Lebensohn and Tomé. The combination of deformation and recrystallization texture models is applied to hot compression tests on AZ31, and it is found, that the model describes the observed texture and hardening/softening behavior well. In some cases rotation recrystallization occurs in AZ31 which appears to be a possibility to reduce the undesired basal rolling texture.

Formability of Ultrafine-Grained AA6016 Sheets Processed by Accumulative Roll Bonding

Aluminium alloy AA6016 was accumulative roll bonded up to eight cycles in order to produce an ultrafine-grained microstructure. The formability of these sheets was investigated by means of bending tests. Furthermore the influence of a local laser heat treatment at the bending edge is observed. The strength of the UFG samples is increased by a factor of around two compared to the conventionally grained T4 condition which also results in up to 50 % higher punch forces needed for bending of ARB processed samples. An anisotropic bending behaviour is observed. By applying a tailored laser heat treatment along the bending edge prior to the bending tests a local recrystallization and recovery at the deformation zone of the samples is achieved. Thus, ductility is increased locally whereby bending to an angle of 80° is possible with lower forming forces compared to the non-heat treated specimens. The results are compared to previous studies on mechanical properties and formability investigations of ARB processed AA6016.

Tailored heat treated accumulative roll bonded aluminum blanks: failure under bending stresses

Ultrafine-grained accumulative roll bonded (ARB) sheet metals of aluminum alloys have a high potential for lightweight construction. The mechanical properties can be enhanced regarding strength and ductility by the combination of ARB and a local heat treatment according to the Tailor Heat Treated Blanks technology. The present investigation focuses on the failure behavior of ultrafine-grained ARB blanks. Due to the low formability of these high-strength ARB metals, a detailed understanding of the failure mechanisms is essential. For this purpose, an established approach to determine the different stages of damage of the material for conventional materials is now applied to multilayered aluminum in the as-received and heat-treated condition. In this context, air bending tests are used to qualify and quantify the bending and forming behavior of ARB sheets of AA1050A and AA6016 aluminum alloys.

A 10 Gb/s highly-integrated adaptive pseudo-noise transmitter for biomedical applications

A highly-integrated 211 −1 pseudo-random bit sequence (PRBS) transmitter for biomedical applications is presented. The chip consists of an ultra-wideband synthesizer with an integrated divider to drive a PLL, a linear feedback shift register (LFSR) to generate an M-sequence and a programmable binary divider to enable adaptive subsampling technique in the signal processing path. The circuit is created to be used in a miniaturized portable PRBS based sensor system for biomedical applications. A conceivable application is the measurement of dehydration in a human body. The PRBS generator is capable of generating a bit-rate up to 10Gb/s, correlating to a maximum bandwidth of the generated sequence of 5GHz, which is sufficient for the designated applications. The circuit is manufactured in an 0.35 μm SiGe-Bipolar technology with an ft of 200GHz using 12mm2 chip area.

Determination of changes in NaCl concentration in aqueous solutions using an M-sequence based sensor system

In this paper measurements of changes in NaCl concentration in aqueous solutions are presented. For these measurements a simplified pseudo random binary sequence (PRBS) based sensor system, which is based on the binary noise theorem, is used. The pseudo random noise signal is represented by an M-sequence with a bandwidth of 10GHz. The transfer functions of the sensor with different NaCl concentrations in a fixed tube of a contactless distributed probe are determined and compared to each other. The results show that there is a frequency range with a clear relation between the magnitude of the transfer function and the NaCl concentration. This demonstrates, that it is possible to measure changes in NaCl concentration of aqueous solutions with a PRBS based sensor system.

Feasibility analysis of a novel production method for monolithic integrated MEMS with nanogaps

This paper analyses a novel production technique for the fabrication of microelectromechanical system (MEMS) with a gap in the scale of nanometer. The technology is based on a density-changing layer, which let a gap arise through the shrinkage of this layer. After the description of the technology, the requirements for monolithic integration and CMOS compatibility with respect to this technology are worked out. It has been discovered that there are several challenges to be overcome. Especially to find a material composition which match the requirements is important as well as to set up the process without affecting other CMOS device parameters. But if these challenges are solved the process will be suitable for various applications and enable to open up new markets.

Method for the Depletion of the Basal Texture in Rolled AZ31 Magnesium Sheets

The deformation and hardening mechanisms of magnesium usually lead to a typical basal orientation of crystals during a production of sheets by forming techniques. The basal texture related anisotropic property behavior and especially the further decrease in formability at room temperature is disadvantageous and undesired for subsequent rolling and final forming processes.The objective of this work is to find methods to improve these texture-related properties and the cold forming ability of magnesium sheets. Firstly, rolling at different temperatures and pass reductions, with the goal of weakening the basal texture component in semi-finished products is investigated, based on the advantageous initial texture and microstructure of twin-roll-cast (TRC) magnesium strips. In this context, texture and microstructure development is examined after a particular multi-pass rolling and heat treatment processes. Twin-roll-cast magnesium strips of alloy AZ31, with an initial thickness of 4.5 mm, rolled to a final thickness of 1.2 mm, are used as feedstock.Secondly, a new thermo-mechanical magnesium strip treatment has been developed in order to completely disintegrate the basal texture and intentionally generate only non-basal orientations with high Schmid-factors for the easy-to-activate basal slip systems. This process, which is designed as a final strip treatment, has been investigated regarding its texture change effect on rolled 1.2 mm AZ31 sheets, which also originate from TRC feedstock.It has clearly been found that the developed rolling technology for TRC feedstock leads to a significantly reduced basal texture due to grain boundary rotation and recrystallization at those rotated regions. The application of the separately developed strip treatment effects a complete elimination of the basal texture in a large volume of the sheet. Applying both technologies on magnesium sheets results in a tremendous increase in formability at room temperature as a consequence of the altered texture.