Effects of residual stresses on the fatigue behaviour of welded steel structures

Residual stresses due to the welding process in steel structures can significantly affect the fatigue behaviour. Usually, high tensile residual stresses up to the yield strength are conservatively assumed at the weld toes. This conservative assumption can result in misleading fatigue assessments. Areas with compressive residual stresses may be present in complex structures, where the details are less critical than predicted. This is shown in the paper by the example of fillet‐welded stiffener ends, where beneficial compressive residual stresses cause the initiation of fatigue cracks at other locations in less‐strained areas. Another example for the effects of residual stresses concerns the stress initiation and propagation at a structural detail under fully compressive load cycles. Fatigue cracks are possible here due to high tensile residual stress fields. The conclusion is that the welding‐induced residual stresses should be known in advance for a reliable fatigue assessment, which becomes possible to an increasing extent by numerical welding simulation.     

Eigenspannungsmessungen an oberflächenbearbeiteten Proben aus einem Gamma‐Titanaluminidwerkstoff

Residual Stress Measurements on Machined Specimens of a Gamma Titanium Aluminide Material Residual stress measurements by x‐ray diffraction and hole drilling were carried out on specimens made of the commercially available gamma titanium aluminide alloy ABB‐2. The specimens had been ground and milled by three various methods in each case. Evaluation of X‐ray measurements displayed significantly different stress profiles in the surface layers and enabled qualitative ratings of the applied machining techniques. Evaluation of the hole drilling results showed agreements as well as discrepancies with the X‐ray findings. Determination of X‐ray elastic constants indicated similar values as obtained in a previous investigation on a different gamma titanium aluminide alloy.     

Residual stress field determination in welds by means of X‐ray,synchrotron and neutron diffraction

To what extent the welding residual stresses influence fatigue is still unclear and matter of debate. An important reason for this lack of clarity is that the exact determination of residual stress fields in welds is complicated which leads to conservative assumptions about these stresses in the fatigue design codes. The advances in the diffraction analysis of materials offer the opportunity for the full‐field residual strain mapping in welds albeit at the cost of time and technical complexity. In this work residual stress field determination in welded S1100QL specimens by means of the x‐ray, synchrotron and neutron diffraction techniques was undertaken. The results revealed that the maximum values of surface residual stresses are not as frequently assumed, as high as the yield strength in small scale specimens. At the weld toe which could serve as a fatigue crack initiation site, even lower residual stresses than the weld centreline could be present. The in‐depth measurements revealed that the effective part of the residual stress field which could be decisive for the fatigue failure initiation is concentrated at the surface of the weld.     

Ausgewählte Ergebnisse der Untersuchung der Prozesse des Schmelzens und der Erstarrung beim Schmelzschweißen und Schmelzlöten mittels der materiellen Modellierung

In the article a method for the material modelling is described under application of visually transparent materials of the thermal and chemical solidification processes when welding and soldering. All solidification processes carried out independently of the form of the primary crystals, first about the planar one, then cellular and in the end dendritically growth the solidification structure. It is determined by the chemical composition of the weld and soldering good, the weld, soldering, crystallization or solidification speeds and by the temperature gradients at the solidification front particularly. The vacancy concentration is made by the temporary formation of so‐called zipper primary grain boundaries in the solidification structure when welding. Among other things a preferential growth direction of the primary crystals explains itself by the fast growth of so‐called victim crystals which completes the known theories of the preferential growth.     

Verhalten geschweißter Magnesiumlegierungen unter mechanisch‐korrosiver Komplexbeanspruchung

Corrosion and corrosion fatigue of welded magnesium alloys In addition to the prevalent use of magnesium cast alloys a high potential for lightweight constructions is offered by magnesium‐wrought alloys, in particular in the automobile industry. The use of rolled and/or extruded magnesium alloys (profiles and sheet metals) requires suitable and economic join technologies like different welding procedures in order to join semi finished parts. Thus, the realization of lightweight constructions asks for high standards of materials‐ and joining‐technologies. In this context, the mechanical properties as well as the corrosion behaviour of the joints are of large interest. During welding of magnesium alloys, influences concerning the surface, the internal stresses and the microstructure occur. These influences particularly depend on the energy input and thus, on the welding procedure as well as the processing parameters, which all affect the corrosion behaviour of the joints. Sheets of magnesium alloys (AZ31, AZ61, AZ91) were joined with different welding procedures (plasma‐, laser beam‐ and electron‐beam welding in the vacuum and at atmosphere). The corrosion behaviour (with and without cyclic mechanical loading) of the welded joints was investigated by different methods such as corrosion tests, polarisation curves, scanning electron microscopy and metallography. Furthermore, substantial influencing variables on the corrosion behaviour of welded joints of magnesium alloys are pointed out and measures are presented, which contribute to the improvement of the corrosion behaviour.     

Correlation of processing route and heat treatment with the abrasive wear resistance of a plastic mold steel

The processing of polymers necessitates the use of corrosion and wear resistant tool materials being in direct contact with the feedstock material. Corrosion resistant cold work tool steels, the so called plastic mold steels, are successfully applied here, offering both a good wear and corrosion resistance. The lifetime of this tool depends on the applied heat treatment but also the processing route has a distinct effect on the resulting properties. In this work, different powder metallurgical routes like hot isostatic pressing, build‐up welding (plasma transfer arc (PTA)) and thermal spraying (high velocity oxy fuel (HVOF) and atmospheric plasma spraying (APS)) were applied to produce coatings on low‐alloyed construction steel. Coatings are compared in relation to the changes in microstructure and the feasibility of an adequate heat treatment. This paper discusses strategies to maximize wear resistance in dependence of heat treatment and the microstructural changes arising from the processing.     

Elektronenstrahlschweißen an Atmosphäre – von der Entwicklung bis zum industriellen Einsatz

Non vacuum electron beam welding – from development to industrial application Attributable to the increasing degree of standardisation in many fields of industrial manufacturing, the saving of resources and thus the demand for light weight constructions and also the rapid development on the material sector have made joining tasks increasingly complex. This involves the joining method which must meet the metallurgical demands of the base materials to be welded and also the method’s profitability. In this connection, electron beam welding in atmosphere – NV‐EBW ‐ as a joining method is getting more and more important and is, from the side of the industry, becoming increasingly popular. NV‐EBW combines the many, well‐known advantages of electron beam welding in vacuum with the possibility to work under normal ambient pressure. With an equipment efficiency of more than 50 % and very high, achievable welding speeds of up to 60 m/min for aluminium materials, the electron beam in atmosphere is an efficient and profitable tool for welding. Under the direction of Professor U. Dilthey, the ISF has for many years and in close contact with industrial partners carried out research work in the field of the NV‐EBW technology. At that, elementary contribution to the development of rotationally‐symmetrical orifice assemblies and also to the testing and optimization of the method with regard to respective welding tasks has been carried out.     

Effects of type of shielding gas upon local mechanical properties of laser welded joint in heat‐treatable steel

Small additions of oxygen or carbon dioxide to argon shielding at laser beam welding can increase welding speed and productivity and decrease the mechanical properties of welded joints. The effect of the type of active shielding gas mixtures based on argon with additions of oxygen and/or carbon dioxide upon the local mechanical properties of laser welded joints of heat‐treatable steel 25 CrMo 4 was studied. Microshear test method has been used to investigate the local mechanical properties of welded joints, including microshear strength, microshear plasticity and microshear thoughness. The obtained data were statistically processed, and a mathematical modeling of mechanical properties applying the method of response surfaces was carried out. The analysis revealed that the impact of the used shielding gas mixtures upon the local mechanical properties of the joint is not very significant. The results indicate that the microshear test can be used successfully for estimation of the local mechanical properties distribution of laser welded joints.