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Carbon arc-air gouging is a common technology when repairing defects in welded structures. Often this technique is applied in repeated cycles even on the same location of the joint. Due to the multiple heat input by gouging and subsequent re-welding, the residual stresses are strongly influenced. This can become crucial when microstructure and mechanical properties are adversely affected by multiple weld reparations. Knowledge about the relation of gouging and residual stresses is scarce but important when high strength steels, which are sensitive to residual stresses, are processed. The present study shows the effect of repair welding on a high strength steel structural element. The weld and the heat affected zone were subjected to multiple thermal cycles by gouging and subsequent repair welding. The residual stresses were determined by X-ray diffraction at different positions along the joint. The results showed that the residual stress level has increased by the repair cycles. This is most pronounced for the heat affected zone. Adapted welding procedures may prevent detrimental residual stress distributions.
In the paper by using molecular dynamics method we investigate behavior of nickel-phosphorus compound in amorphous-like state under conditions of shear loading at the constant velocity. Samples with an amorphous layer of pure nickel and nickel-phosphorus compound were considered. The analysis showed that forces of shear resistance in the sample with an amorphous layer containing phosphorus in about 3 times less than the sample with a layer of pure nickel. Thus, it was shown that nickel-phosphorous coating in amorphous-like state may exhibit low friction properties, and, therefore, serve as the solid lubricant material.
The nonlinear analysis became a common tool to precisely assess the load-bearing behavior of steel beam and column members. The failure level is significantly influenced by different types of imperfections, among geometric also structural imperfections (residual stresses). Here are still gaps in the knowledge. Nowadays, 3-D welding simulation developed to a level where it could provide reliable estimation of weld-induced distortion and residual stresses. Nevertheless, modelling and computational effort are still in a less practicable range. In this study a simplified procedure to implement residual welding stresses in continuous large scale members is proposed and the influence on the ultimate limit state of slender members in compression is evaluated for two common structural steel grades. The results showed significant improvements in the utilization of load bearing capacity compared with simplified design methods. The comparatively general approach in this study offers
potential for future optimization.
Joining of dissimilar material has become highly popular research subject in the automobile industry due to the reduced weight and thereby increasing the fuel efficiency. Infrared thermography can be used as a natural tool to measure the temperature near the welding region and correlate the distribution of temperature to the weld quality. In the present work the quality of the dissimilar welded sample is identified using the temperature distribution in the vicinity of the weld pool region. A numerical model for CMT continues welding process has been modeled and simulated for the first time and compared with the experimental measurement.
In-line laser thermography for crack detection at elevated temperature: A Numerical modeling study
(2016)
The detection and characterization of cracks prior to damage is a technologically and economically highly significant task and is of very importance when it comes to safety-relevant structures. The evaluation of a components life is closely related to the presence of cracks in it. Laser thermography has already high capability for the detection of surface cracks and for the characterization of the geometry of artificial surface flaws in metallic samples. Crack detection in metallic samples at high temperature is highly significant in present manufacturing scenario. During the casting process of billets, surface cracks form, due to the suboptimal cooling rates. These cracks reduce value of the billet and must be removed using machining process after cooling. This secondary process increases cost of manufacturing. In this work we developed a heat transfer model for laser thermography to study the thermal contrast variation with increase in surface temperature using finite element method (FEM). Here we are mainly concentrating the capability of the scanning laser thermography in crack detection which are in elevated temperature and numerical modeling study of thermal contrast variation of crack with respect increase in metal surface temperature. This study is important to prove the capability of laser thermography for crack detection in elevated temperature. Since we are using High power CW Laser to local heating of the metal surface which can give relatively high thermal contrast even at elevated temperature compare to other heating source. Here we are modeled and simulated 2D laser scanning across a surface breaking crack and developed an algorithm to produce the vicinity of crack. The algorithm we developed applied for various surface temperature data. And validated the credibility of the algorithm with experimental data.
Selecting a welding process for a given application is crucial with respect to the sustainability of part manufacturing. Unfortunately, since welding processes are evaluated by a number of criteria, preferences for one or the other process can be contradictory. However, the prevalent procedure of weight assignment for each criterion is subjective and does not provide information about the entire solution space. From the perspective of a decision maker it is important to be able to assess the entire set of possible weightings and answer the question which welding process is optimal for which set of weights. This issue is investigated by means of a weight space partitioning approach. Two welding processes are considered with respect to three criteria that reflect their economic and environmental performance. In order to find the most sustainable welding process the underlying weight space partition is evaluated.
Increasing performance and energy efficiency of Gas Metal Arc Welding by a high power tandem process
(2016)
Standard Gas Metal Arc Welding (Standard GMAW) and a high power Tandem GMAW (TGMAW) process are evaluated with respect to energy efficiency. Current, voltage and overall equipment power are measured and energy consumption is determined. The new key performance indicator Electrical Deposition Efficiency is introduced to reflect the energy efficiency of GMAW processes. Additionally, wallplug efficiency of the equipment is determined in order to identify the overall energy consumption. Results show that energy efficiency as well as economic process performance can be significantly increased by application of the TGMAW process. Furthermore findings indicate that wall-plug efficiency of the equipment is independent of power level and material transfer mode. A metal plate of 30 mm thick structural steel is joined by Standard GMAW and TGMAW to demonstrate the total energy savings for a real weld. Electricity consumption is reduced by more than 20 % using the high power TGMAW process.
Spot welding is one of the most important joining technologies, especially in the automotive industry. Hitherto,the quality of spot welded joints is tested mainly by random destructive tests. A nondestructive testing technique offers the benefit of cost reduction of the testing procedure and optimization of the fabrication process, because every joint could be examined. This would lead to a reduced number of spot welded joints, as redundancies could be avoided. In the procedure described here, the spot welded joint between two zinc-coated steel sheets (HX340LAD+Z100MB or HC340LA+ZE 50/50) is heated optically on one side. Laser radiation and flash light are used as heat sources. The melted zone, the so called “weld nugget” provides the mechanical stability of the connection, but also constitutes a thermal bridge between the sheets. Due to the better thermal contact, the spot welded joint reveals a thermal behavior different from the surrounding material, where the heat transfer between the two sheets is much lower. The difference in the transient thermal behavior is measured with time resolved thermography. Hence, the size of the thermal contact between the two sheets is determined, which is directly correlated to the size of the weld nugget, indicating the quality of the spot weld. The method performs well in transmission with laser radiation and flash light. With laser radiation, it works even in reflection geometry, thus offering the possibility of testing with just one-sided accessibility. By using heating with collimated laser radiation, not only contactfree, but also remote testing is feasible. A further convenience compared to similar thermographic approaches is the applicability on bare steel sheets without any optical coating for emissivity correction. For this purpose, a proper way of emissivity correction was established.
We describe the fabrication of a polycarbonate (PC) micro-structured polymer optical fiber (mPOF) and the writing of fiber Bragg gratings (FBGs) in it to enable strain and temperature measurements. We demonstrate the photosensitivity of a dopant-free PC fiber by grating inscription using a UV laser. We further show that PC Bragg gratings can be extended up to at least 3% without affecting the initial functionality of the micro-structured fiber. The response of PC FBGs to temperature up to 125°C is also investigated. Polycarbonate has good mechanical properties and its high temperatureresistance might extend the range of application of polymeric FBGs.
CONRAD-2 is an imaging instrument using low-energy (cold) neutrons. The instrument is installed at the end of a curved neutron guide which avoids the direct line of sight towards the reactor core. This ensures a very low background of high-energy neutrons and. photons at the sample position. The cold neutron beam provides a wavelength range which is suitable for phase-and diffraction-contrast imaging such as grating interferometry and Bragg edge mapping. The instrument is well suited for high resolution imaging due to the high efficiency of the very thin scintillators that can be used for the detection of cold neutrons. An instrument upgrade was performed recently as a part of an upgrade program for the cold neutron instrumentation at HZB. The parameters of the instrument as well as some research highlights will be presented.