Stefan Riekehr

Trends in laser beam welding technology and fracture assessment of weld joints

Abstract Recent advances in the industrial application of laser welding are reviewed, drawing on examples from the aerospace, automotive, shipbuilding, offshore, and white goods industries. Attempts are being made to introduce laser welding procedures into construction industry standards. The impact of the narrow, overmatched weld zone on joint properties and the difficulties this raises for quality assessment are briefly considered.

Influence of the Welding Sequence on Residual Stresses in Laser Welded T-Joints of an Airframe Aluminium Alloy

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.

Process Optimization of Dual-Laser Beam Welding of Advanced Al-Li Alloys Through Hot Cracking Susceptibility Modeling

Abstract Laser welding of advanced Al-Li alloys has been developed to meet the increasing demand for light-weight and high-strength aerospace structures. However, welding of high-strength Al-Li alloys can be problematic due to the tendency for hot cracking. Finding suitable welding parameters and filler material for this combination currently requires extensive and costly trial and error experimentation. The present work describes a novel coupled model to predict hot crack susceptibility (HCS) in Al-Li welds. Such a model can be used to shortcut the weld development process. The coupled model combines finite element process simulation with a two-level HCS model. The finite element process model predicts thermal field data for the subsequent HCS hot cracking prediction. The model can be used to predict the influences of filler wire composition and welding parameters on HCS. The modeling results have been validated by comparing predictions with results from fully instrumented laser welds performed under a range of process parameters and analyzed using high-resolution X-ray tomography to identify weld defects. It is shown that the model is capable of accurately predicting the thermal field around the weld and the trend of HCS as a function of process parameters.

Crashworthiness of Magnesium Sheet Structures

A hollow rectangular profile, as an example of a typical structural component made of magnesium alloy sheets has been built, tested and evaluated in order to assess its behaviour during axial crushing. The profiles were joined from plane sheets of AZ31 and ZE10, respectively, by laser beam welding and were then tested in compression. Numerical simulations have been conducted to understand the complex interplay between hardening characteristics of the materials under investigation, profile cross-section variation and energy absorption. The results from the compression testing of the profiles show that the welds are not the source of damage initiation and failure. The performance of the magnesium profiles in terms of dissipated specific energy is confirmed for small and intermediate displacements to be comparable to that of aluminium profiles. For large displacements, however, the shear-type failure mode of magnesium causes a sharp drop of the crushing force and thus limits the energy absorption. These findings demonstrate the requirement for an alloy and wrought magnesium process development specifically for crash applications which aims at progressive hardening along with high ductility for improving the bending and shear behaviour.

Microstructure and Residual Stresses in Dissimilar Mg-Al-Zn-Alloy Single Overlap Laser Beam Welds

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.

Mechanical Characterization and Fatigue Performance of Laser and Resistance Spot Welds

Mechanical characterization, fatigue performance and failure analysis of laser spot welds and resistance spot welds under tensile-shear loading have been investigated. Similar and dissimilar joints of dual phase advanced high-strength steel (DP780) and deep-drawing steel (DC04) of 2.0 mm thickness for application in the automotive industry were performed. The structural stress concept was used to explain fatigue lives of laser and resistance spot welded joints. The results revealed different failure types with different fatigue behaviour for laser and resistance spot welds under applied cyclic loads at “high load” and “low load” levels. Similar joints of the DP780 show the longest fatigue life at high load levels when compared with similar joints of DC04 and dissimilar joints. However, this difference disappeared when the fatigue test results were considered in terms of the structural stress, which also enabled correlation of the results for RSW and LSW spot welds with different sizes.

Damage Tolerance Analyses of Laser Welded “Skin-Clip” Joints for Aerospace Applications

Future metallic airframes may contain laser beam welded clips (to the skin) between two stringers to achieve lightweight integral airframes. In this study, an investigation involving stress analysis and fatigue testing of skin-clip laser welded T-joints in 6156 T4 aluminium alloy plates was carried out. The thickness of the base plate (skin) was locally reduced between the welded joints by machining to produce “pockets” to obtain weight reduction. The effects of pocket thickness (3.0 and 1.5 mm) and socket width (the width of the full-thickness part between pockets; 11 and 20 mm) as skin-clip joint parameters on stress distributions around the weld region were studied by FE analyses of 400 mm wide plates under tension. Several numerical analyses were performed considering the effect of different locations of the tips of weld toe cracks in the T-joints, and the results obtained were compared with the experimental observations from fatigue tests of such joints in order to describe the fatigue performance of the laser welded skin-clip joints. Pocketed skin-clip joints exhibited weld region protection as a result of a reduction in the stress in the part containing the welded joint. Among the joint configurations analyzed in the absence of weld toe cracking, 3.0 mm pocket thickness with 11 mm socket width (narrowly prepared pocket design) provided almost full protection of the weld toe with little increase in stress on the opposite side of the plate containing the weld detail. However, in the presence of a weld toe crack, a region of stress concentration arose in the pocket itself, in line with the crack tip. This encouraged deviation of the crack path away from the weld toe but, if the socket width was too small, as in the narrowly prepared pocket design, the two stress fields combined and led to an increase in fatigue crack growth rate. In this study, the best fatigue performance was obtained when crack deviation into the base material occurred but remained within the socket area.

Laser Weldability of High-Strength Al-Zn Alloys and Its Improvement by the Use of an Appropriate Filler Material

Heat-treatable Al-Zn alloys are promising candidates for use as structural lightweight materials in automotive and aircraft applications. This is mainly due to their high strength-to-density ratio in comparison to conventionally employed Al alloys. Laser beam welding is an efficient method for producing joints with high weld quality and has been established in the industry for many years. However, it is well known that aluminum alloys with a high Zn content or, more precisely, with a high (Zn + Mg + Cu) content are difficult to fusion weld due to the formation of porosity and hot cracks. The present study concerns the laser weldability of these hard-to-weld Al-Zn alloys. In order to improve weldability, it was first necessary to understand the reasons for weldability problems and to identify crucial influencing factors. Based on this knowledge, it was finally possible to develop an appropriate approach. For this purpose, vanadium was selected as additional filler material. Vanadium exhibits favorable thermophysical properties and, thereby, can improve the weldability of Al-Zn alloys. The effectiveness of the approach was verified by its application to several Al-Zn alloys with differing amounts of (Zn + Mg + Cu).

Laser Weldability of Different Al-Zn Alloys and its Improvement

Weld defects - such as porosity and hot cracking - occur especially during the laser beam welding of high-alloyed Al-Zn alloys. This significantly limits the application range of these promising high-strength alloys. In the present study the laser weldability of different Al-Zn alloys was investigated regarding the used welding parameters and the chemical composition of the alloys. In addition, the novel approach of the Helmholtz-Zentrum Geesthacht for overcoming the weldability problems was applied to the different Al-Zn alloys in order to assess its capability. It was shown that the laser weldability of Al-Zn alloys deteriorates with an increasing amount of Zn, Mg and Cu. The variation of laser beam welding parameters did not lead to any improvement of weldability. Only the use of the new approach resulted in promising welding results even for the high-alloyed Al-Zn alloys.

Fracture mechanical behaviour of laser beam-welded AA2198 butt joints and integral structures

Purpose – Composite materials and metallic structures already compete for the next generation of single-aisle aircraft. Despite the good mechanical properties of composite materials metallic structures offer challenging properties and high cost effectiveness via the automation in manufacturing, especially when metallic structures will be welded. In this domain, metallic aircraft structures will require weight savings of approximately 20 per cent to increase the efficiency and reduce the CO2 emission by the same amount. Laser beam welding of high-strength Al-Li alloy AA2198 represents a promising method of providing a breakthrough response to the challenges of lightweight design in aircraft applications. The key factor for the application of laser-welded AA2198 structures is the availability of reliable data for the assessment of their damage tolerance behaviour. The paper aims to discuss these issues. Design/methodology/approach – In the presented research, the mechanical properties concerning the quasi-static tensile and fracture toughness (R-curve) of laser beam-welded AA2198 butt joints are investigated. In the next step, a systematic analysis to clarify the deformation and fracture behaviour of the laser beam-welded AA2198 four-stringer panels is conducted. Findings – AA2198 offers better resistance against fracture than the well-known AA2024 alloy. It is possible to weld AA2198 with good results, and the welds also exhibit a higher fracture resistance than AA2024 base material (BM). Welded AA2198 four-stringer panels exhibit a residual strength behaviour superior to that of the flat BM panel. Originality/value – The present study is undertaken on the third-generation airframe-quality Al-Li alloy AA2198 with the main emphasis to investigate the mechanical fracture behaviour of AA2198 BMs, laser beam-welded joints and laser beam-welded integral structures. Studies investigating the damage tolerance of welded integral structures of Al-Li alloys are scarce.