Advanced Materials Research Vol. 22

Abstract: This paper presents the focus of research and the structure of the Collaborative Research Centre SFB 675 “Creation of High-Strength Structures and Joints by setting up local material properties” funded in 2006 by the German Research Foundation (DFG).

Abstract: The industrial application of stainless steels is of high importance because of their high corrosion resistance and forming behaviour. The evolution of martensite during the deep drawing processes leads to an increasing strain hardening of the material. In the collaborative research centre 675 “Erzeugung hochfester metallischer Strukturen und Verbindungen durch gezieltes Einstellen lokaler Eigenschaften” (Creation of high strength metallic structures and joints by setting up scaled local material properties), metal forming processes is being researched. Emphasis on this part of the project is the stress-induced formation of martensite in sheet metal and bulk metal components in metastable austenitic steel. The aim of the investigations is to develop partial structure fields of martensite in sheet metal components in order to construct a lightweight structure. Therefore, components are divided into stretched and non-stretched parts. This leads to a defined buckling of components, for example in case of a crash. Furthermore, the effect of the transformation induced formation of martensite in metastable austenitic steel should be utilised on bulk metal forming components. Thereby special load adapted components with locally optimized properties are producible, like austenitic ductile regions and martensitic high-strength areas.

Abstract: Within subproject A2 processes leading to local Bake Hardening (BH) effects in multiphase steels will be investigated. The characteristic values which integrally describe the Bake Hardening effect depend on the deformation path and the degree of preforming, as well as on temperature and duration of the subsequent heat treatment. The thermal stability of the induced strengthening is also determined, in order to specify the boundary conditions for thermal assembling. Thus, they can be qualified for the assembly of locally strengthened structural components.

Abstract: The subproject A3 deals with the production and manufacturing and the subsequent treatment of partially strengthened, three layered, symmetrical Sandwich Structures. With this type of strengthening, complex material properties can be developed, e.g. high strength with high stiffness, good thermal joining properties at the place of strengthening, as well as an improved strength and strength absorbing ability. The forming behaviour is still widely unknown.

Abstract: The investigation of the basic principles for the production of foamed out sections using magnesium foam for support structures by including the foaming process into the cold forming of sections to produce indiviually locally strengthened components is the subject of this research project. To absorb tensile stress, the metal foam will be strengthened with three-dimensional branched struts of high-tensile materials. The quantification of the influence of locally introduced foaming elements on e.g. stiffness alterations and the influence of the resonance frequency of the total structure will be effected by destructive but particularly also by nondestructive tests.

Abstract: In subproject B1, local physical and geometrical effects which have only been observed so far as side effects in the laser joining process, are to be used purposefully, in order to achieve graded strength properties and to improve the component rigidity significantly. One aim of the work in the first requested period is the investigation of effects of laser-based joining connections on the structure rigidity for simplified sample geometries. A defined local strength increase will first be done on blind seams and later on seams with suitable seam geometries. In the context of SFB 675, laser joining processes are to be developed further so that the final assembly can take place with and other methods to increase strength for semi-finished products without considerably changing the local material characteristics. Beyond that, general rigidity effects of the connections are to be used purposefully for rigidity improvement.

Abstract: High strength aluminium wrought alloys as well as powder metallurgical aluminium alloys are limited regarding massive formability. The formability at room temperature can be significantly affected by superimposing hydrostatic pressure. Depending on the process control, cold forming enables locally induced strain hardening effects, whereby increased hardness or hardness gradients can be regulated. Simultaneously, the necessity of mechanical post processing is reduced by a metal forming fabrication of joint and connection elements at room temperature. By splitting the component in strengthened and not strengthened regions, specially adapted property profiles can be adjusted to the application. Thus, specially load adapted components with locally optimised property profiles e.g. ductile or high strength, brittle areas can be manufactured. A defined buckling or folding of a component in case of a crash can thereby be achieved. In this project innovative tool principles for superimposed cold solid forming will be developed. They will be used to manufacture high strength and complex aluminium structure components with specific adjustment of local strain hardening. A tool technique is to be created in order to generate locally hardened areas within massive structures by metal forming. Furthermore, the task is to determine the procedures limits for superimposed cold massive forming with specifically adjusted strain hardening of aluminium alloys. For the realisation of these aims fundamental research has to be made, by which the coherences between specific process parameters and the increased formability are determined. Furthermore, the cold hardening effects are to be adjusted by cold massive forming with superimposed hydrostatic pressure and displayed with the help of FEA. In the long term, the analysis aims at the development of pressure superimposed forming that is technically utilisable as a near net shape manufacturing process for high complex aluminium structure components with selective adjustment of local properties.

Abstract: Novel manufacturing technologies for high-strength structure components of aluminium allow a local modification of material properties to respond to operational demands. Machining and finishing processes for changing material properties like deep rolling or rubbing are to be combined to a single process step. Intention is the controlled modification of the component’s properties by manipulation of its subsurface. For that purpose the essential understanding of interaction mechanisms of the basic processes turning, deep rolling and rubbing is necessary. Influences of tool geometry as well as process parameters on material properties are investigated. The results are to be extended by parameter studies, done by numerical simulation. Combined processes like “turnrolling” or “turn-rub-rolling” will be developed.

Abstract: 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.