Abstract: This paper presents a sequential research works on visual information acquirement and intelligent control of arc weld pool dynamics and seam formation during pulsed GTAW (Gas Tungsten Arc Welding) in robotic welding process. The visual information acquirement methods are focused in computer vision sensing, image processing and characteristic extraction of the weld pool surface from the single-item pool images by particular algorithms for robotic welding process. Based on acquired visual characteristics of weld pool and established neural network and knowledge models for predicting dynamical characteristics of weld pool during robotic welding, corresponding control methods, such adaptive control, self-learning and other composite intelligent control strategies are developed to control welding pool dynamics during pulsed GTAW by welding robot. Some experiments and applications of intelligent control methods in welding robot systems are shown in the paper.
Abstract: Welding of dissimilar materials is frequently accompanied by structural and technological difficulties and not always successful. The knowledge resulting from dissimilar welding experiments can be used to further identify directions and suitable technological parameters for optimal results.
This paper report on the difficulties encountered on friction welding of nodular cast irons with low alloyed steels, due in principle to the low deformation capacity and the microstructural differences.
It was shown through experiments that low friction times and high axial pressure leads to a significant plastifiation of the cast iron, while the low alloyed steel remains practically undeformed. The early (premature) plastifiation of the cast iron leads to a radial expulsion of the base structure associated with a continuous transport of the graphite nodules in the joint plan. As a result, a new graphite film forms which hinders a metallic contact between the parts and a welded joint.
Qualitative and quantitative electron microscopy observations reveal carbon and alloying elements diffusion phenomena on the interface of the dissimilar materials to be joined.
Abstract: Incoloy 903 overlays have been used to provide hydrogen environment embrittlement (HEE) resistance to welds in nickel alloy 718 structures. This is problematic because application of the required overlays has a history of high rejection and rework due to interpass microfissuring. Kovar has been identified as a potential hydrogen resistant replacement for Incoloy 903. A weldability study was initiated to compare the hot crack (microfissure) resistance of the two alloys to determine if substitution of Kovar for Incoloy 903 has the potential to improve the fabricability of HEE overlays. Varestraint testing indicates that Kovar has much higher crack initiation strains for both HAZ and weld metal cracking. Crack initiation strains were approximately 2% for Kovar while Incoloy 903 crack initiation strains were only 0.25% . Maximum crack lengths (MCL) observed on Kovar Varestraint tests were 0.12mm and 0.58mm for base and weld metal respectively, while 903 MCLs were 0.56mm and 2.3mm. Gleeble hot ductility testing indicates that Kovar has a nil ductility range of 7 degrees C while Incoloy 903 has a range of approximately 45 degrees C. The larger range observed for 903 is an indication of its greater crack susceptibility. Fabricability was correlated to material microstructure using optical microscopy, scanning electron microscopy and microprobe analysis.
Abstract: Tensile residual stresses introduced by conventional welding processes diminish the crack resistance and the fatigue lifetime of welded components. In order to generate beneficial compressive residual stresses at the surface of a welded component, various post-weld treatment procedures are available, like shot peening, hammering, etc. These post-weld treatments are, however time and cost extensive. An attractive alternative is to generate compressive stresses over the complete weld joint in the course of the welding procedure by means of so-called Low Transformation Temperature (LTT) filler materials. The volume change induced by the transformation affects the residual stresses in the weld and its vicinity. LTT fillers exhibit a relatively low transformation temperature and a positive volume change, resulting in compressive residual stresses in the weld area.
In-situ measurements of diffraction profiles during real welding experiments using Gas Tungsten Arc (GTA)-welding process were realized successfully for the first time. Transformation temperatures during heating and subsequent cooling of LTT welding material could be assessed by means of energy dispersive diffraction using high energy synchrotron radiation. The results show that the temperature of martensite start (Ms) is strongly dependent on the content of alloying elements. In addition the results indicate that different phase transformation temperatures are present depending on the welding depth. Additional determination of residual stresses allowed it to pull together time and temperature resolved phase transformations and the resulting phase specific residual stresses. It was shown, that for the evaluation of the residual stress state of LTT welds the coexisting martensitic and austenitic phases have to be taken into account when describing the global stress condition of the respective material in detail.
Abstract: Bonding interface in aluminum (Al) and silicon nitride (Si3N4) clad fabricated by explosive welding has been investigated by transmission electron microscopy (TEM). The nanocrystalline region was clearly observed at the interface between Al and Si3N4. Electron diffraction pattern and energy dispersive X-ray spectroscopy (EDS) measurements across the interface revealed that this nanocrystalline region consist of the only aluminum.
Abstract: Friction welding is one of the most reliable joining techniques for various materials due to a high reproducibility, lack of ecological treats and a relatively narrow thermomecanical affected zone. It offers multiple applicative opportunities in various fields – from automotive and manufacturing industries as well as for special destinations – leading to incontestable technical and economical advantages. However, the friction welding of materials with compositional and/or structural gradients is a critical problem due to the high probability of cracks occurrence, the formation of intermetallic phases that increase the brittleness and a high gradient for residual stresses.
The paper refers to the particularities of graded welds through which the rotational energy will be transformed by friction into heat in the surface layer with a certain structural morphology, as well as in the core with different characteristics. Experiments have been performed in order to assess the process parameters which provide the optimal friction energy for an adequate plastifiation of the materials. Macro and optical micrographic investigations corroborated with electron microscopy and hardness measurements provide information concerning the hardening of the thermomecanically affected zone and the formation of seam as result of expulsion of the carburized layer.
Abstract: Ti-Sn binary alloys (Ti-5 to 20 mol% Sn) were diffusion-bonded to high carbon steel between 1073 and 1273 K for 3.6 ks in a vacuum to investigate the influence of the alloy composition on the interfacial microstructures. Ti-5 and 10 mol% Sn alloys were attached firmly to the steel at a bonding temperature of 1273 K. A continuous TiC layer was formed along the interface, while voids were observed between the TiC layer and the steel. Although the joints with Ti-15 and 20 mol% Sn alloys were also prepared at 1273 K, these joints separated near the interface after the bonding treatment. The TiC layer was formed in the separated surface of Ti-Sn alloy, and Fe in the steel diffused into the Ti-Sn alloy. This indicates that the Ti-15 and 20 mol% Sn alloys established contact with the steel at elevated temperatures until just before the separation. The specimens bonded at 1173 K also denoted the same tendency. However, the Ti-15 mol% Sn/steel joint bonded at 1073 K showed a shear strength of more than 50 MPa. The mechanism and the application of the interface separation are discussed on the basis of the microstructural observations.
Abstract: The fabrication of microelectronic and micromechanical devices leads to the use of only very small amounts of matter, which can behave quite differently than the corresponding bulk. Clearly, the materials will age and it is important to gather information on the (changing) material characteristics. In particular, Young’s modulus, yield stress, and hardness are of great interest. Moreover, a complete stress-strain curve is desirable for a detailed material characterization and simulation of a component, e.g., by Finite Elements (FE). However, since the amount of matter is so small and it is the intention to describe its behavior as realistic as possible, miniature tests are used for measuring the mechanical properties. In this paper two miniature tests are presented for this purpose, a mini-uniaxial-tension-test and a nanoindenter experiment. In the tensile test the axial load is prescribed and the corresponding extension of the specimen length is recorded, both of which determines the stress-strain- curve directly. The stress-strain curves are analyzed by assuming a non-linear relationship between stress and strain of the Ramberg-Osgood type and by fitting the corresponding parameters to the experimental data (obtained for various microelectronic solders) by means of a non-linear optimization routine. For a detailed analysis of very local mechanical properties nanoindentation is used, resulting primarily in load vs. indentation-depth data. According to the procedure of Oliver and Pharr this data can be used to obtain hardness and Young’s modulus but not a complete stress-strain curve, at least not directly. In order to obtain such a stress-strain-curve, the nanoindentation experiment is combined with FE and the coefficients involved in the corresponding constitutive equations for stress and strain are obtained by means of the inverse method. The stress-strain curves from nanoindentation and tensile tests are compared for two mate-rials (aluminum and steel). Differences are explained in terms of the locality of the measurement. Finally, material properties at elevated temperature are of particular interest in order to characterize the materials even more completely. We describe the setup for hot stage nanoindentation tests in context with first results for selected materials.
Abstract: Aluminum alloys containing lithium are attractive to the aerospace industry. The high specific strength and stiffness of these alloys improves lift efficiency, fuel economy and performance and increases payload capability. The objective of this study was to compare the fabricability of six different aluminum base alloys. Three were Li containing alloys, two variants of AL 2195 (Al-4Cu-1Li) and a lithium enhanced analog of AL 5083 (Al-4Mg-2Li). Three were materials in common usage, Al 2219, Al 2014 and Al 5083. Fabricability was assessed using Gleeble thermomechanical testing, Varestraint testing and differential scanning calorimetry (DSC). Results indicate that Alloy 2195 is more susceptible to hot cracking than both Al 2219 and Al 2014. Cracking sensitivity is a strong function of chemical composition within specification ranges for Al 2195. Results also indicate that the lithium containing analog of Al 5083 is more hot crack susceptible than its parent material. Fabricability was correlated to material microstructure using optical microscopy, scanning electron microscopy and microprobe analysis. Hot cracking in all materials was associated with persistent, continuous liquid films produced by weld thermal cycling, aggravated by base material structure. Measures of several characteristic temperatures using the Gleeble simulator were fully consistent with Varestraint results. The maximum crack length in the Varestraint test correlates well to the liquidus temperature for the alloy less the nil ductility temperature. The temperature difference is equivalent to the thermal gradient associated with welding times the maximum crack length.
Abstract: The purpose of the paper is to present the mechanism of deformation and failure of welded joints during the process of retrofit. The considered joints are made of conventional steels used in the power industry, as well as new-generation steels. The problems of failure and the design of such joints are considered in the paper. The model of welded joint was built with properties of low alloyed chrome-molybdenum steel and a new-generation steel of P/T91 type. All metallurgical zones, i.e. the weld and the heat-affected zone were considered, modelled and discussed. A simulation of thermomechanical loading was conducted under various loading conditions to explain the causes of the failure of energy structures, which appears more frequently during their loadings or steam dropping (i.e. in conditions of changes of thermal loadings) than during their stable work. The results of calculations were confirmed by experimental metallurgical studies. It is observed that the brittle and soft zones of coarse-grained ferrite with minimal impact strength, resulting from internal and external operation parameters, appear in regions where oscillation of the highest von Mises and normal stresses takes place. The relevant literature does not describe the analysis of the phenomenon of acceleration of the failure of the welded structure leading to its failure, while taking into account the complex thermomechanical conditions of their operation. This mechanism is based on the accumulation of structural weakening of the weld as the result of the diffusion process within the axially symmetrical joint, the growth of the brittle zone and their additional degradation, and the influence of the oscillation of maximal normal and von Mises stresses.
It was noted that in the process of thermomechanical loading, the state of failure initiation in circumferential and longitudinal directions appeared in the weld. A few illustrations presenting the mechanism of the deformation and failure of the welded joint are presented. The effects of the distortions of the weld are also discussed in the paper.