Volker Wesling

Low Heat Joining – Manufacturing and Fatigue Strength of Brazed, Locally Hardened Structures

Low-heat brazing as a joining option for high strength steel sheets under cyclic load is still a challenge for manufacturing. For application arc brazing to locally hardened, high strength steel sheets a material specific manufacturing concept is needed to ensure a quality-related, reliable and reproducible processing. Thus the interaction of brazing parameters, metallurgical properties, types of brazed seams and different arc-processes under cyclic load were investigated. This should extinguish the base for an extended application of arc-brazed joints at cyclic loaded, high strength, property adjusted structures.

Superplasticity of an AZ91 Magnesium Alloy

The superplastic deformation characteristics of the AZ91, the mostly used magnesium alloy, were investigated at various strain rates in the interval from 3x10-5 to 1x10-2 s-1 and temperature of 420 °C. To prepare superplastic alloys thermo-mechanical treatment was used. Cast materials were heat-treated in two stages, after homogenization at 415 °C for 10 h were submitted to the precipitation annealing at temperature in the range of 200-380 °C for 10 h, and deformed by hot extrusion. Microstructure of samples was observed by the light microscope Olympus. Strain rate sensitivity parameter m has been estimated by the abrupt strain rate changes method. The strong strain rate dependence of the m-parameter was found. The highest elongation to failure, 584%, was found for the samples aged at 380 °C. Possible physical mechanisms of the superplastic flow are discussed.

Processing, Superplastic Properties and Friction Stir Welding of Fine-Grained AZ31, AZ91, AE42 and QE22 Magnesium Alloys

The grain refinement after thermo-mechanical treatment (TMT) was investigated in AZ91, AE42, und QE22 magnesium alloys. The optimal over-aging temperature was determined to be 300 °C in the case of AZ91 and AE42 alloys and 350 °C for QE22 alloy. After optimized TMT, the average grain sizes were 13.5 µm (AE42), 11.1 µm (AZ91) and 1.9 µm (QE22). The QE22 alloy exhibited the superior superplastic properties, with maximum elongation to failure 750 % and strain rate sensitivity parameter m=0.73. The Friction Stir Welding showed that the original base material grain structure of the alloys AZ31 and AZ91 replaced by ultrafine grains in the stir zone. The purpose of the present paper is to present the results of the grain refinement in magnesium alloys by thermo mechanical treatment and stir welding.

Systematic Design Approach to the Development of High-Strength, Locally Adapted Structures with the Aid of the Finite Element Design (FED) Method

Design engineers can choose from a large variety of materials in order to fulfill a certain function. In those fields of application with a lower level of complexity, it is often sufficient to manufacture the entire component in a “monolithic” manner from one single material. Concurrent, partly contradictory and, most probably, local requirements that must be fulfilled by a component often make material selection more difficult. As a consequence, it is often necessary to use several different materials with a local and functional orientation, which is a part of the multi-material design strategy. The potential of different materials can be used most effectively if this information is made available to the design engineers as early on in the design process as possible. The aim of the SFB 675 sub-project C7, therefore, is the development of a systematic design approach (Finite Element Design, FED) that focuses on finite component elements. As a result, the potential of the optimization of local properties is taken into consideration, and the interaction between the materials, production processes, and design can all be described.

Investigation of the Process- and System-Induced Activation of Material Reactions during Discontinuous High-Frequency Welding

Besides weldable component geometries for the high-frequency welding process also possible process and system induced activated material reactions during discontinuous high-frequency welding are presented in this paper. Among others such material reactions can be a locally limited thermal influence on the base metal, defined plastic derformations during the upsetting process as well as grain refinement in the weld seam, comparable to thermomechanical treatment during rolling for increasing strength or ductility.

Material Aligned Process Control for the Welding Technology of Locally Hardened Materials

The subproject B5 examines the welding technological processing of locally hardened materials to produce structures and knots by means of high-freqency welding (HFW). The aim of B5 is a defined intervention in process and plant technology to control current voltage, temperature and compressive stress distribution of the entire weld seam. Particularly the effects on locally hardened areas have to be measured and optimized. Also the process specific advantages of HFW (e.g. plastic deformations and the application of an in situ heat treatment) have to be examined and optimized to improve structural strength.

Low Heat Joining – Manufacture and Fatigue of Soldered Locally Strengthened Structures

The advantages of low heat joining techniques, such as low distortion and little influence on the local material properties due to the low introduced amount of heat, shall be made usable for the manufacture of high strength structures by increasing the process reliability. The dependency between the parameters of the joining process, the seam geometry, the type of solder, the load type und the fatigue life especially of soldered structure with local strengthening shall be examined to allow a calculative estimation of the part’s life.

Design Strategies for the Development of High Strength Coupling Elements from Requirement Optimized Composite Materials

New interdisciplinary material and manufacturing technologies make local configuration of material properties possible. The availability of material property information in the early construction process and adequate welding technologies are essential needs to realize a maximum utilization of material. The research objective of two cooperating institutes at the Technical University of Clausthal is to develop a design strategy with related design rules in order to engineer high strength coupling elements of requirement optimized composite materials. Special attention is paid to the usage of future welding technologies and their influence on material properties of locally altered materials. Further objective is an early survey of the consequences caused by the usage of locally altered materials in mixed architecture design projects to the whole production process.