Helmholtz-Zentrum Geesthacht, Monday, 21-May-2012 18:08:42 CEST
http://www.hzg.de/institute/materials_research/structure/materials_mechanics/joining_and_assessment/profile/index.html.en

Profile

Realizing multi-material (hybrid) components and joining new structural materials require innovative design approaches and advanced joining processes such as the laser beam welding technique. In turn, assessment of structure-property relationships of such components can be complex and requires a sophisticated testing facility, supplemented by an updated flaw assessment procedure such as FITNET (Fitness for Service Procedure). Since the welded components usually exhibit strength mis-match, a thorough understanding on the welding process and the materials mechanics in terms of the deformation and the failure behavior is required to improve structural properties as regards fatigue and fracture. So as to provide system solutions for welded lightweight structures, work is directed to following R&D activities:

Process Optimization

  • Laser beam welding for a range of structural materials such as Al-and Mg-alloys and advanced high strength steels (AHSS) using Nd:YAG lasers (2.2 kW and 3.3 kW) to obtain butt-joints, T-joints, laser spot welds and laser riveting
  • Diffusion bonding and brazing for TiAl-alloys and for their dissimilar joints
  • Design of tailored weld joints by the implementation of "Local Engineering” to improve the structural performance. This is being realized, among others, by using novel approaches such as crenellations, selective reinforcements and mechanical tensioning

Local and Global Properties

Systematic investigations are conducted to determine local and global property gradients in the welds by employing following techniques:

  • Microstructural characterization (light-optical, SEM, EDX, EBSD)
  • Micro-mechanical characterization using micro-hardness testing and micro-flat-tensile testing. The latter uses 0.5 mm thick tensile specimens, extracted from very narrow laser welds and heat affected zones, to determine the gradient in the tensile properties
  • Fatigue (S-N) and fatigue crack propagation testing of welds and analysis using a wide-range of specimens under constant and variable amplitude loading
  • Fracture toughness (Charpy-V, Instrumented Charpy-V, KIC, CTOD and J-Integral) of weld and heat affected zones using the standard and in-house procedures at RT and sub-zero test temperatures. The specimens [standard bend and C(T) types] cover a range of section sizes
  • Wide-plate testing of welds under tension up to 400 ton loading capacity
  • Modeling and prediction of weld residual stresses with software tools such as SYSWELD and ABAQUS, and experimental measurements using the neutron and the synchrotron scattering techniques
  • Non-destructive testing of weld defects using X-ray radiography

Weld Performance

We aim to combine joining technology, property tailoring (local engineering) and characterization as basis for the assessment and improvement of the weld performance. Here, the objective is to improve damage tolerance, lifetime and structural safety of the welded lightweight components.

  • Fatigue crack initiation, fatigue crack propagation and residual strength tests on welds using small and large scale specimens to clarify the effect of process parameters, residual stresses and strength mis-match on the structural performance
  • Identification and description of damage evolution in welds under static and cyclic loading, including spectrum loading and in-service temperatures
  • The newly developed European fitness-for-service procedure FITNET (www.eurofitnet.org) is used to predict critical conditions of the welded structures

We are engaged in research and in the development of novel solutions for joining and assessment problems of the national and the European industries in aerospace, transport and energy sectors. Thus, our Department makes a significant contribution to the R&D topic "Mechanics and Joining of Lightweight Materials" of the "Advanced Engineering Materials- AEM" programme of the Helmholtz research area "Key Technologies".

WFM_Profil