In laser Transmission Welding (LTW), the laser beam passes through the transparent part and is dissipated as heat in the absorbent material through the use of laser-absorbing pigments such as carbon black (CB). This energy is then conducted further into both parts. Melting and subsequent solidification occur at the interface causing a weld to form between the two parts. Gluing or welding structures to the back of automotive Class-A panels often results in the appearance of undesirable surface deformations on the Class-A side. Through control of the laser welding and material parameters, it may be possible to use contour LTW as a means of joining structures to the back of absorbent Class-A panels without creating these unwanted surface defects. A series of lap welds was made using a range of CB levels, laser powers and polypropylene part thicknesses. A profilometer was used to measure the size and shape of the defects generated on the surface of the black part. Two types of defects were observed: ribs and sink marks. It was observed that lower powers combined with higher carbon black levels generally resulted in smaller defects. The type of defect depended on the boundary conditions between the two parts and the flow of polymer that had thermally expanded during welding (flash). If weld flash flowed into gaps between the two plates, rib defects were always observed. If flash flowed elsewhere and no gaps existed between the plates, sink marks occurred. Finite element modeling was used to qualitatively validate these observations. / Thesis (Master, Chemical Engineering) -- Queen's University, 2010-07-02 14:34:41.201

The welding of austenite stainless steel often results in large amount of welding distortion due to its high thermal expansion coefficient and low thermal conductivity. This has created great difficulty in the dimensional control of the welded stainless steel structure, ending up with high manufacturing cost. Researches on the welding distortion of stainless steels were very limited, especially for large weld structures with complex component shapes. The studies of this thesis were initiated with focus on the stainless steel nozzle-to-shell-can weld structures, a very typical structural configuration for pressure vessels used in petrochemical and nuclear power generation industries. Both the experimental and the FEA (finite element analysis), i.e. computational simulation, approaches were taken in the studies which addressed the influences of the welding fixture, the welding sequence, and the welding process on the distortion caused by stainless steel nozzle-to-shell welding. The investigations employed single and multi-nozzle weld test models (called mockups in the thesis) or FEA models. Manual GTAW (gas tungsten arc welding) and SMAW (shielded metal arc welding) processes were selected to represent the most common practice for stainless steel nozzle welding. The FEA simulations were conducted with ABAQUS program using sequentially coupled transient analysis method with lumped weld passes to achieve high computing efficiency. The investigations on the effect of the welding fixture concluded that the contour fixtures introduced in the thesis be effective for reducing the welding distortion for both the single and the multi-nozzle welding. The contour fixtures tend to localize the welding distortion, hence yield less impact on the global distortion of the whole weld structure. The rib-bar fixture, a more common fixture type for multi-nozzle welding, was found resulting in a big jump in the shell plate distortion when the fixture was removed. The studies on the influence of the welding sequence revealed that a progressive approach was more favorable for distortion control under the given nozzle-to-shell weld structure configurations. The best sequence suggested is to start welding at one nozzle, firstly on shell OD (outside diameter) side then on ID (inside diameter) side, then proceed to next neighboring nozzle. The effect of the welding direction of each weld pass was found affecting only the nozzle angular distortion. The experimental data showed that the manual GTAW process developed much higher shell plate distortion than the SMAW process. The reason would be that a higher percentage of the welding heat had been consumed on the base metal. The influence of the weld bead size didn???t appear to be significant. In the FEA study on the effect of the size of the lumped weld pass, the increase in weld bead size even resulted in a decrease in weld distortion. From the FEA simulation point of view, using large lumped pass would be a highly efficient choice without compromising too much in the precision of the distortion prediction. The FEA study confirmed that a decrease in cooling time after welding would result in more welding distortion. The large scale multi-nozzle mockup with rib-bar fixture demonstrated a maximum out-of-plane shell distortion of 16.4mm after the welding of 10 nozzles with GTAW+SMAW process, which suggests that additional measures should be developed to further control the welding distortion.

A scanning reference electrode technique (SRET) apparatus has been designed and commissioned to investigate the corrosion of friction stir spot welds (FSSW) made in AZ31 magnesium alloys. The operational parameters of the apparatus have been calibrated to give good spatial resolution. By combining the SRET data with material flow data and immersion test data it was found that the FSSW process caused the formation of distinct noble and active regimes within the weld area. The noble region was aligned with the stir zone (SZ) and was caused by a dynamically recrystallized grain structure which is void of dislocations / twins, and β Mg17Al12. Localized corrosion attack was observed in both SRET and immersion testing along the thermo-mechanically affected zone (TMAZ). The same effect was consistently observed with a flat versus concave shoulder tool, and dwell times of 1s and 4s.

A scanning reference electrode technique (SRET) apparatus has been designed and commissioned to investigate the corrosion of friction stir spot welds (FSSW) made in AZ31 magnesium alloys. The operational parameters of the apparatus have been calibrated to give good spatial resolution. By combining the SRET data with material flow data and immersion test data it was found that the FSSW process caused the formation of distinct noble and active regimes within the weld area. The noble region was aligned with the stir zone (SZ) and was caused by a dynamically recrystallized grain structure which is void of dislocations / twins, and β Mg17Al12. Localized corrosion attack was observed in both SRET and immersion testing along the thermo-mechanically affected zone (TMAZ). The same effect was consistently observed with a flat versus concave shoulder tool, and dwell times of 1s and 4s.

The need for rehabilitation of bridges has become a critical challenge due to aging and an increase in traffic loads. Many of these bridges are exceeding their design fatigue life. Since many of these bridges are structurally deficient, they need to be rehabilitated or replaced by a new bridge. The most susceptible and weak parts of steel bridges to cracks and fatigue are the welds, due to the presence of high stress concentrations, tensile residual stresses, and imperfections as a result of the welding process. Inspection and repair of welds are difficult and elimination of welded details is not possible in steel bridge construction. Ultrasonic impact treatment (UIT) is a promising and innovative post-weld treatment (PWT) method for improving the fatigue performance of existing welded steel and steel-concrete composite structures such as highway bridges. The fatigue resistance of treated joints is enhanced by improving the geometry of the weld toe, and introducing compressive residual stresses. However, a lack of tools for quality assurance has slowed UIT’s adoption by bridge authorities. The current study was undertaken to examine the fatigue performance of structural steel welds subjected to UIT at various levels, including intentional under-treatment and over-treatment, and to relate the fatigue performance of the treated welds to geometric and metallurgical properties measured to control the treatment quality. The last objective was to use the laboratory results to develop acceptance criteria for the quality control of UIT in bridge applications. Fatigue tests of non-load carrying fillet welded attachments were conducted on properly treated, under-treated, and over-treated weld toes. Statistical analyses of the fatigue life data were performed and crack growth was monitored using the alternating current potential drop (ACPD) method. Measurement of local properties (such as weld toe geometry, local hardness, and residual stresses) and examination of the weld toe microstructure were also performed on the untreated and treated welds. The effects of weld toe geometry on the local stresses in the untreated and treated welds were also investigated using elastic finite element analysis (FEA) to obtain the stress concentration factor (SCF) for the different treatment cases and to examine the changes in the SCF for the different weld toe geometries. Based on the statistical analysis performed in this research, the results illustrated that UIT significantly improved the fatigue lives of weld details regardless of the investigated level of treatment quality. The fatigue lives of welded details under constant amplitude (CA) loading and constant amplitude loading with under-loads (CA-UL) were increased up to 30 and 27 times respectively. On average, the fatigue life of the treated weld details was slightly lower under CA-UL than under CA loading. Treatment quality had little impact on the mean of the S-N curves. However, it did impact the design (95% survival probability) S-N curves, with the curve associated with a proper treatment slightly higher than the curves for poor or unknown treatment quality. Local near-surface microhardness and compressive residual stresses were greatest for the over-treated welded details, followed by the properly treated and then the under-treated welded details. Increasing the treatment speed resulted in a greater reduction in the surface microhardness and compressive residual stresses than decreasing the treatment intensity. Finite element analyses showed that changes in weld toe geometry due to UIT can cause a decrease in the SCF near the surface of the treated weld toe. The SCF was the lowest for the properly treated steel specimens and slightly higher for the under-treated specimens. For the over-treated specimens, the SCFs were nearly as high as for the untreated weld. The SCF increases as the thickness of the flange increased up to 19 mm. With further flange thickness increase to 38 mm, the SCF did not change substantially. The work presented herein demonstrated that indent depth measurements from the base metal side, commonly used for quality control, may not identify over-treatment on their own. Indent depth measurements from both the weld and the base metal sides, obtained by measurement of weld toe impressions, offer a good alternative means for identifying over-treatment. However, for identifying under-treatment, indent depth measurements should be used in conjunction with visual inspection for traces of the original weld toe.

Thesis (doctoral)--Universität Osnabrück, 1992. / Includes bibliographical references.

Thesis (doctoral)--Universität Osnabrück, 1992. / Includes bibliographical references.

A computer controlled weld quality assurance system has been developed to detect weld defects ultrasonically whilst welding is in progress. This system, including a flash analogue to digital converter and built-in memories to store sampled data, a peak characters extractor and a welding process controller, enabled welding processes to be controlled automatically and welding defects to be detected concurrently with welding. In this way, the weld quality could be satisfactorily assured if no defect was detected and the welding cost was minimised either through avoiding similar defects to occur or by stopping the welding process if repair was necessary. This work demonstrated that the high temperature field around the weld pool was the major source of difficulties and unreliabilities in defect detection during welding and, had to be taken into account in welding control by ultrasonic means. The high temperatures not only influence ultrasonic characteristic parameters which are the defect judgement and assessment criterion, but also introduce noise into signals. The signal averaging technique and statistical analysis based on B-scan data have proved their feasibility to increase 'signal to noise ratio' effectively and to judge or assess weld defects. The hardware and the software for the system is explained in this work. By using this system, real-time 'A-scan' signals on screen display, and, A-scan, B-scan or three dimensional results can be printed on paper, or stored on disks, and, as a result, weld quality could be fully computerized.

Thesis deals with individual technology of new welding processes "specifically FRONIUS TPSi". The greatest emphasis is on welding technology, especially on the differences between the individual methods. The experimental part of the thesis is focused on finding suitable parameters for welding filled welds.Everythingwith respect to required welds quality welded by robotic welding. The samples were evaluated in terms of penetration depth, amount of heat introduced and the number of defects.

No description available.