Laserstrahl‐ und Reibschweißen von Magnesiumlegierungen

Laser Beam and Friction Welding of Magnesium Alloys The need for weight and cost reduction forces the engineer to develop new alloys with good options for design as well as to constantly optimize the industrial processes with regards to their utilizability and efficiency. With regards to these aspects it is of great interest to investigate the weldability of these newly developed alloys. The focus is put on both a high performance process and a special prosess for low heat influence. Furthermore, new methods will be presented which will make it possible to reduce the number of test runs as well as to identify influences and interactions between main process parameters and marks of quality.     

Einfluß von Schweißen und Kaltumformung auf das Kurzzeitschwingfestigkeitsverhalten von austenitischem Stahl

Influence of Welding and Cold Forming on the Low-Cycle-Fatigue Behaviour of Austenitic Steel The low-cycle-fatigue behaviour of the austenitic steel X 10 CrNiTi 189, as received and welded, was investigated after cold forming up to 10 per cent by strain controlled tests. The fatigue life to crack initiation of the unwelded material is 10 times higher than the fatigue life of the butt-welded and unfinished material. The grinding of the butt-welds however leads to an increase of the fatigue life by a factor of 4 in comparison to the unfinished state. In comparison to these results obtained under 1 Hz the reduction of the frequency to 0.1 Hz causes a significant cyclic hardening and decrease of the fatigue life. The cold forming remains in all cases without influence. This hardening is a result of the diffusionless transformation of the austenitic microstructure into ferromagnetic α′-martensite under the lower frequency. The increase of the hardness caused also by the cold forming makes the stainless material more sensitive to corrosion.     

Superplastizität von Magnesiumlegierungen

Superplasticity of Magnesium Based Alloys In despite of the increasing interest of the industry in extremely lightweight materials during the last years an intensive industrial use of those materials due to the their restricted cold-workabilit caused by the hexagonal lattice is still very limited. Considering this limitation a solution is provided by the superplastic forming of magnesium based alloys which in contrast to other types of materials is neither metallurgically developed nor process optimized. A very promissing step is the cost-saving and recycling-favorable production of a very fine grained microstructure through the use of extruded magnesium alloy chips. After a short introduction of the theoretical basics and methods to quantify the superplastic behavior the results of velocity change test with different magnesium alloys will be shown.     

Fügen von Magnesium und Magnesiumlegierungen

Welding of Magnesium and Magnesium alloys Magnesium is mainly connected by screws. In this paper the results of experiments with different welding processes will be presented. The following methods have been applied: TIG, MIG, Nd: YAG‐Laser and CO₂‐Laser welding, electron beam welding and High Power Diode Laser welding.     

Galvanisches Verzinken von Magnesiumlegierungen

Zinc‐Plating of Magnesium Alloys Magnesium alloys are highly susceptible to corrosion that limits their application when exposure to corrosive service conditions is needed. One of the ways to prevent corrosion is to coat the magnesium‐based substrate to avoid a contact with an aggressive environment. Results concerning corrosion behaviour of wrought AZ31 magnesium alloy with electrolytic zinc coatings deposited from different electrolyte solutions are described. Evaluation of corrosion processes in chlorides containing solutions was performed by electrochemical measurements. It was found that thick and dense electrolytic zinc coatings formed on AZ31 significantly improve the corrosion behaviour of magnesium alloy after one hour immersion of zinc coated magnesium alloys in corrosive media. Further increase of immersion time leads to relatively fast decrease of corrosion properties. Electrolytic zinc coatings obtained in consecutive alkaline / acidic process demonstrate an improvement of corrosion resistance of coated AZ31. The time to coating degradation strongly increases.     

Erfahrungen beim Schweißen gasempfindlicher hochreiner Werkstoffe

Experiences in Welding Highly Pure, Gas-Sensitive Materials Highly pure steels and non-ferrous react during processing, particularly welding, with atmospheric gases and change their service properties. Hence special measures are often necessary in forming and inert gas welding. Taking the specific example of a high pure chromium steel (superferritic), welding properties, processing data and selection of suitable inert gas torches are described. The possibility of applying this information to other materials such as titanium is discussed.     

Schweißen mit dem Laserstrahl im Vakuum

Welding with the Laser Beam in Vacuum In the vehicle industry, beam welding methods are successfully applied for the joining of high‐quality parts. Here, the focus is mainly put on drive section components. Especially in component manufacturing of couplings, toothed wheel – shaft joints and drive shafts, electron beam welding using small vacuum cycle chambers is applied. The application of powerful solid‐state lasers results, however, often in negative consequences, such as soiling by process emissions or increased tendency to spatter. The application of laser beam welding in vacuum allows for the reduction of these side effects and, at the same time, for the increase of penetration depth and weld quality. This article introduces first results gained from tests made with workpieces from the vehicle industry.     

Der Einfluß von Temperatur und Zeit beim Schweißen von Baustählen mit Strahlen hoher Intensität

Temperature and Time Dependent Fusion Welding with Beams of High Intensity By a special set-up very high local and time-dependent temperature gradients were measured in the weld zone of beam affected areas. Under certain weld conditions a leading preheat wave does not exist. As to the resulting microstructure in the beam, the characteristic steep temperature curves do not allow any prediction. Zones of enlarged hardness were reduced by different heat-treatments.     

Schweißen im Behälter- und Apparatebau

Welding in Tank and Plant Construction Since a relatively limited number of plastics types are used, the welding tasks involved in chemical apparatus engineering can be accomplished with just a few welding techniques are well-known and welding can be carried out efficiently, reliably and economically by this means. In this respect, however, a number of important principles have to be observed. The DVS guidelines provide vital information on questions relating to calculation, design, personnel, equipment, material and testing. Extrusion welding enjoys prime importance as regards quality and economic aspects.     

Modellierung des Ermüdungsrisswachstums in Schweißverbindungen unter Berücksichtigung von Schweißeigenspannungen

The present paper contains research results determined within the framework of a project called IBESS (?Integrale Bruchmechanische Ermittlung der Schwingfestigkeit von Schweißverbindungen“) by the Materials Mechanics Group of the Technische Universität Darmstadt [1]. Aim is to calculate the fatigue life of welded joints by taking into account the effect of residual stresses and the influence of the weld toe geometry. Here, the fatigue life is regarded as period of short fatigue crack growth. Two and three dimensional finite element models, with cracks as initial defects, are constructed for this purpose. Fatigue crack growth analyses are performed by using the node release technique together with the finite element program ABAQUS. The welding residual stresses as well as the plasticity induced crack closure effects are considered. Structural calculations are performed in order to introduce residual stress fields in finite element models. The calculated compressive residual stress field matches the measured one especially in the weld notch area. The effective cyclic J‐integral (ΔJ_eff) is used as crack tip parameter in a relation similar to the Paris equation for the calculation of the fatigue life. For this purpose, a Python code was written for the determination of ΔJ_eff at every crack length phase. The calculated fatigue lives were compared with experimental data and a good accordance between both results was achieved. The impact of welding residual stresses on ΔJ_eff as well as on the fatigue life during short crack growth was investigated. As expected, results revealed that at lower stress amplitude, a compressive residual stress field is favorable to the fatigue life, whilst a tensile residual stress field is unfavorable. The influence of residual stresses can be neglected only for large load amplitudes.     

Schweißverhalten von Aluminium und seinen Legierungen

Weldability of Aluminium and Al Alloys Aluminium is behind steel the most used metal. Advantages of Al-materials are the low weight, the favourable strength and deformation properties, the good corrosion resistance and the good electrical and thermal conductivity. Welding of aluminium encounters difficulties due to high melting oxide layer at the surface, the strong heat dissipation from the welding region, the tendency to form craters or pores and the high heat extension. This contribution describes the basic facts and special features of arc welding and weld properties of aluminium and Al-alloys during welding.     

Untersuchung von Schmelz‐ und Rekristallisationsprozessen sowie Riss‐, Oxid‐ und Porenbildung beim Schweißen

Investigation of melting and re‐crystallization processes and of crack, oxide and pore formation during welding In the paper some known investigation methods of the primary re‐crystallization and of the structure of the crystallization front are discussed. All these methods have disadvantages, in some cases they cause a limited validity or are expensive. The presented new method is based on the micrographic recording of the melting and solidifying metal surfaces. The method overcomes the above‐mentioned disadvantages. It is able to investigate in the welding seams not only the primary re‐crystallization processes but also the secondary ones including the formation of cracks and pores and the appearance of non‐metallic inclusions like oxides. The facilities of the method are demonstrated on some examples of use.     

Gezielte Entwicklung von Magnesiumlegierungen mittels angewandter Thermochemie

No abstract The principle and the methodology of focused alloy development by means of applied thermochemistry are described. The Calphad method is described briefly. As an example, calculations used for applications are shown in the system Mg‐Al‐Sc. In more detail the development of creep resistant alloys in the system Mg‐Mn‐(Sc, Gd, Y, Zr) is discussed. One aim is to produce a sufficiently large quantity of efficient precipitations in the structure in order to improve the mechanical properties with a minimum of expensive alloying addition. The large number of possible combinations of the alloying elements in the system Mg‐Mn‐(Sc, for Gd, Y, Zr) on the one hand and the time and cost of technological experiments on creep stability on the other hand require a preselection of the systems and the alloy compositions. Thermodynamic phase equilibrium and phase amount diagrams were calculated, which give indications on the selection of the promising alloying elements. A priority list of three quaternary systems is produced: Mg‐Mn‐Gd‐Sc, Mg‐Mn‐Sc‐Y and Mg‐Mn‐Y‐Zr. For technical investigations the alloy MgMn1Gd5Sc0.8 (wt.%) is most promising, furthermore the alloys MgMn 1Gd5Sc0.3 and MgMn1Y5Sc0.8 seem promising. Very many other alloys could be eliminated as doubtfull or useless with this method. The entire quaternary Mg‐Mn‐Y‐Zr system was disqualified because of characteristics of the phase diagram, which are harmful for the desired microstructure. This focused alloy development saves time and cost‐intensive technical investigations.     

Schneiden und Schweißen mit dem CO2-Laser

Cutting and Welding Using a CO₂-Laser. Among the various types of lasers the CO₂ laser is particulary suitable for materials working. It has a very high efficienty (15–20 %) and a high output power (up to several kW). When the laser light is focused by means of a lens or a mirror, a power density of more than 10⁹ W/cm² is attained in continuous operation. The laser need not be applied in vacuum. The CO₂ laser is a suitable cutting tool for numerous materials, e. g. for metals such as titanium or steel, for combustible materials such as paper, textiles, wood, and plastics, and also for hard and brittle materials such as aluminium oxide and silicon carbide. If the metals are cut in an oxidizing atmosphere, the cutting speed may be increased. The cutting width however is determined by the size of the laser spot. In addition, experiments are reported in which the CO₂ laser was used for welding steel, titanium, plastics, quartz, and glass. The advantages of the laser for this application are discussed. Another important field of application is the growth of single crystals. In several fields the laser is in competition with the electron gun. Therefore, the laser technique is compared with the electron beam technique.     

Einfluss der Oberflächenbehandlung auf das Korrosionsverhalten von Magnesiumlegierungen

For the coating of magnesium alloys, especially those that should be used in medical technologies, fluoride coatings are tested and used. These coatings should enable a predictable degradation behavior for future implant materials. Presently the pre‐ and post‐treatment processes vary, so an optimum to achieve defined thicknesses of the fluoride layers is still not established. The present investigations are based on the process parameters known from the literature, they are discussed and compared. Different pre‐treating times of the base material (pure magnesium and a magnesium‐aluminum‐calcium alloy as a comparison) in 200 g/l sodium hydroxide combined with 96 hours of immersion in 40 % fluoride acid are examined.     

Langzeitfestigkeit von Schweißverbindungen an Deponiedichtungsbahnen aus Polyethylen

Long-term Strength of Welded Joints in Polyethylene Water Proofing Sheets for Refuse Dumps Waterproofing of refuse dumps is nowadays mainly carried out with sheets made from HDPE. Welding is the technique used for joining the sheets. These welded joints are the weakest point in the system. Whereas non-destructive tests give relatively little information on the quality of the weld, peel and tenside creep tests provide a good degree of differentation. The test results available show that it is not so much the welding technique that influences long-term strength, but that it is the creep strength of the base material and the excessive rise in stress due to notch effects at the changes in cross-section which determine the long-term quality of the welded joint.     

Elektrodenauswahl für das WIG‐Unterwasser‐Schweißen

Selection of electrode for GTA‐Underwater Welding Reproducible good weld quality and economical benefit of underwater‐welding require a complete automation. For this purpose Gas Tungsten Arc‐welding (GTA or TIG) offers numerous advantages, especially for the root and the following hot pass run. Disadvantages of GTA‐welding are the low weld deposit rate and the limited lifetime of the tungsten electrode. Already small wear damages cause wide alterations of the arc under high surrounding pressure, so that a suited choice of the electrode enables to increase the productivity considerably. Therefore the influence of the electrode features on welding process, arc stability, arc ignition, weld geometry and electrode wear has been investigated. For quick and elementary selection of electrode an assessment catalogue was elaborated.     

Zeitstandverhalten von Schweißverbindungen aus HDPE bei der Einwirkung von Chemikalien

Long Term Creep Tests with HDPE Pipes under the Influence of Several Fluids The results of long term creep tests with unwelded and welded HDPE pipes under the influence of several fluids are presented. It is shown that the resistance factors determined by the investigations of tubes are also valid for welded constructions, when the long term weld factors in DVS 2205, Blatt 1 are taken in consideration.     

Die Anwendung von Schallemission für die Schweißnahtkontrolle

The use of Acoustic Emission as a weld Quality Monitor . The results obtained from our experiments with spot welding, gas tungsten-arc welding, and submerged-arc welding demonstrate the potential of acoustic emission monitoring as a nondestructive test for welds. The real-time nature of the acoustic emission data sets this technique apart from other nondestructive methods used for weld inspection. Since the acoustic emission from weld defects is transient, empirical calibration is required for each application. Defects can be located approximately but the type of defekt cannot be determined.