Papers by Author: Mustafa Koçak

Abstract: This paper describes microstructure and microtexture development in dissimilar friction welded -TAB-Ti64 joints. The effect of friction welding parameters on microstructure and local properties are examined and discussed. It was found that the intermetallic -TiAl based alloy Ti-47Al-3.5(Mn+Cr+Nb)-0.8(B+Si) (denoted as -TAB) is more sensitive to the applied friction welding parameters used in this study. Furthermore, the bonding between these two alloys was controlled by a diffusion process during a very short process duration. Grain refinement as well microstructure transformation led to local improvement of the friction-welded joints.

Abstract: GKSS is currently investing heavily into new beamlines at DESY in Hamburg, Germany. After the completed installation of the wiggler beamline HARWI II at DORIS III GKSS is now building two new undulator beamlines at the new PETRA III storage ring. The High Energy Materials Science Beamline (HEMS) will allow high resolution diffraction experiments using samples and sample environments with masses up to 1 t, 3DXRD measurements, and high-energy micro-tomography experiments. The Imaging Beamline (IBL) will provide a nano-tomography as well as a micro-tomography station for X-ray energies up to 50 keV. Examples of typical experiments in the field of residual stress analysis, micro-tomography, and high-energy small-angle X-ray scattering will be given.

Abstract: The effect of different welding sequences between a 4.5 mm thick AA 6156 T6 base plate and a 2 mm thick AA 6013 T6 clip – resembling a skin-clip joint of an airframe – using a 3.3 kW Nd:YAG laser is investigated. Under cyclic loading the breakdown of such T-joints happens at one end of the clip, which is due to local residual stress concentrations. Recent measurements indicated that tensile stresses could be lower at the run-in than at the run-out locations. For a deeper investigation of this effect sheets with different welding sequences were produced. One welding sequence was made with two starting points in the centre, and a second with starting points at the clip ends. Temperature measurements were made using thermocouples to verify the heat conditions for a finite element simulation of the welding process, which is used for predictions of the residual stress distribution. Actual values of the residual stress fields were determined by neutron diffraction. The influences of the welding sequence on the measured temperatures and the residual stresses are discussed.

Abstract: Microstructure, hardness and residual stresses of the laser beam overlap welds between AZ31B sheets and AZ31, AZ61 and AZ80 extruded profiles are investigated using microscopy and X-ray diffraction. The result of the investigations reveal that weld microstructure, the size of the HAZ, precipitate density and the maximum compressive residual stress values depend strongly on the Al content of the weld zone of two Mg-alloys.

Abstract: The residual stress distribution in three laser welded T-joint configurations of aerospace aluminium alloys were measured using neutron diffraction and FE-simulation was conducted to compare with the experimental results. A 2 mm thick AA 6013 T6 sheet (as clip) was welded to a 4.5 mm thick and 400 mm wide AA 6156 T4 base plate (as skin of the airframe). In two samples, the thickness of the plate was reduced in some areas after welding to produce so-called “pockets” with the purpose of the weight reduction to resemble the fabrication practise in aircraft industry. The effect of pocketing process, which produced two different geometries around the clip weld on the residual stress evolution was analysed. In the plain sample (without pockets), residual stresses were predicted using the SYSWELD finite element software. The strain measurements on the base plate were performed at three locations; namely, the middle of the weld length (mid-clip), welding start (run-in) and end (run-out) locations. In all welded plates, slightly higher longitudinal tensile residual stresses were detected at the midclip locations, whereas transverse residual stresses were similar for all locations. In the run-out location, higher longitudinal tensile residual stresses were present than in the run-in location, which was the case in our previous results on other samples. The first results of the SYSWELD FE-simulation of the plain sample were compared with experimental results. The comparison has shown particularly good agreement for the transverse stresses. Although the simulation yields higher longitudinal tensile stresses than the experimental results, the stress distributions were very similar.

Abstract: Recently a European community funded thematic network project (participation of 17 countries) FITNET (www.eurofitnet.org) has completed a new and unified engineering assessment procedure (FITNET FFS Procedure) of flaws in metallic structures and welds. This newly developed procedure (under CEN Workshop Agreement WA22) provides assessment rules for flaws or damage due to fracture, fatigue, creep and corrosion to demonstrate the structural integrity of the component. This paper gives an overview of the FITNET Fitness-for-Service (FFS) Procedure and specifically presents the features and basic equations of the Fracture Module. It also presents two brief examples for the validation of the procedure using laser welded specimens.

Abstract: Dissimilar welded components joined with any kind of welding technologies gain an increasing interest due to significant improvements in engineering structures by using new materials (alloys) or new materials combinations. The present paper deals with laser welding of Al5083-H111 and Al6013-T6 and the characterization of the crystallographic texture. Fine grained Al5083 was joined with coarser grained Al6013.Whereas, Al6013 is strongly oriented dominated by a strong cube component and a much lower Goss component, the finer grained Al5083 shows a week deformation texture. The welding seam itself has a moderate cube texture with a minor <100> fiber texture in welding direction. A small texture variation about the texture strength and the volume fraction of the fiber component was observed along the weld. Results were obtained by neutron, hard X-ray and electron diffraction.

Abstract: In this study the distribution and magnitude of residual stresses in a T-joint of aerospace grade aluminium alloy weldment was determined using neutron diffraction. A 2 mm thick AA 6013 sheet (as clip) was laser beam welded to a 6 mm thick AA 6056 base plate (as skin) to resemble the “short distance” welded clip-skin joints of an airframe. The total length of the weld was 120 mm and it was welded using 3.3 kW Nd:YAG laser source and 12% Si containing wire from one side only. No post weld heat treatment was applied after the welding. Start (run-in) and end (run-out) locations of the T-joints are generally considered as high risk areas with respect to solidification cracking and crack initiations under external loadings. It is of interest to investigate the weld residual stress fields at these locations to develop optimum joint design. Therefore, strain measurements have been performed not only in the middle of the weld seam but also at the run-in and run-out locations of these short distance welds. Higher longitudinal tensile residual stresses are detected at the run-out locations than the run-in locations. The measurements in the clip showed that the clip has a longitudinal tensile stress peak away (about 8 mm) from the weld seam.

Abstract: Residual stresses in CO2 laser beam welded AA6056 Al-sheets of 3.2 and 6.0 mm thicknesses were studied using neutron and high-energy X-ray diffraction. The influence of the temper T4 and T6 before welding, the effect of sheet thickness in T6 temper, and the effectiveness of post-weld heat treatments T6 and T78 for the reduction of residual stresses in the sheets welded initially in the T4 temper were examined. It was found that tensile longitudinal stresses were significantly higher when welded in T6 than in T4. With the increase in the sheet thickness from 3.2 to 6.0 mm, the residual stress profile was affected more than the stress level when welded in T6. For 3.2 mm sheet, the post-weld heat treatments T6 and T78 did not lead to a significant reduction in residual stresses when welded in T4.

Abstract: The non-destructive and phase selective determination of residual stresses caused by material processing (such as welding) in polycrystalline samples is usually performed by diffraction methods. In order to obtain information about stress fields at high spatial resolution with conventional methods, for example with micro beam techniques, the sample needs to be scanned in a very time consuming manner. A much faster method is the simultaneous investigation of a larger area using position sensitive diffractometry. This method was used for the analysis of the residual stress distribution in laser beam welded thin (2 mm and 3 mm) magnesium sheets.