1,721,005 research outputs found
Advanced hybrid laser welding of pipeline steels using novel technologies
No full textLaser beam welding using a high energy density source is a promising method for joining thick plates due to its deep penetration and high productivity, although there are drawbacks such as poor gap-bridging ability, and fast cooling which can lead to weld metal brittleness in more hardenable carbon steels. These limitations can be overcome by adding a filler wire into the weld pool. Meanwhile, novel technologies including beam wobbling and hot-wire feeding have shown the potential to improve the mechanical properties of laser welds. The aim of the present work is to pursue the development of a state-of-art hybrid laser welding process as a root pass for the welding of high strength low-alloy pipeline steels with improved weld quality, mechanical properties, and increased productivity.
The first part of the current research focused on the effect of hot-wire feeding ER70S-6 wire on the microstructure and mechanical properties of API X80 steel laser welds. A high wire feed rate along with preheating of the wire limited the formation of brittle microconstituents such as bainite and martensite in the weld metal, leading to an acicular-ferrite-dominated microstructure. The weld metal strength and toughness met the industrial requirement, although the hardness exceeded the maximum value allowed.
Further study was conducted utilizing a wobble laser during wire-fed laser welding process. Beam wobbling technique provided the potential to increase the gap-bridging ability. However, increased amount of bainite was found due to the dilution effect with more melted base material in the fusion zone, and this led to an increase in the hardness and strength of the weld metal.
Utilizing a hot-wire feed together with a wobble laser with a small spot size improved the fusion zone microstructure homogeneity and reduced the weld metal hardness. The preheating of the substrates reduced the cooling rate and resulted in the formation of more ferrite. Heat-affected zone hardness was also reduced with the preheating. A larger root gap was found to limit the dilution of alloying elements from the filler material, providing sufficient nucleation sites for the formation of acicular ferrite.
The influence of wire chemistry was then investigated with the optimized welding parameters. Introducing an ER70S-2 wire with a lower carbon equivalent into the weld pool led to a coarser weld metal microstructure, and resulted in a lower hardness and a sufficient toughness which met the API standard. Considering the high productivity and satisfactory mechanical properties of the laser welds, the welding procedure proposed by this work indicates strong potential for utilizing laser welding for more applications in the pipeline industry
Prediction of Bead geometry in Gas Metal Arc Welding by Statistical Regression Analysis
Full text linkManufactures in fabrication industry have long depended on Gas Metal Arc welding as one of the most reliable and economical techniques for joining parts. The need for reducing costs and addressing a skills shortage on the shop floor, often limits productivity in the manufacturing industry involving welding. Developing a welding procedure for instance, involves a lot of time and cost, and currently there is a gap in welding industry where few available methodologies can predict even the basic features of weld geometry based on input parameters. To provide a methodology for prediction, the aim of this thesis focusses on developing a statistical model for the weld inputs and outputs in the Gas Metal Arc Welding process to predict the various geometries of the weld bead. To study the effect of welding process parameters—such as wire feed speed, voltage, travel speed and gas type—on the resultant bead geometry such as bead width, penetration, reinforcement height, reinforcement area and penetration area a factorial design of experiment was used. Low carbon electrode (ER 70S-6) of two different diameters was used, and a total of 242 welds were made with 121 for each wire diameter. Two cross sections were cut from each weld bead and the geometries were measured, and a linear regression analysis was performed to develop a statistical model for each of the bead geometry based on experimental data. Analysis of variance (ANOVA) indicated the significant squared and interaction variables for each of the bead geometry with 95% confidence interval. The trends of geometry for each diameter varied with gas type. Residual analysis revealed that all assumptions inherent in the regression analysis were satisfied. Finally the statistical models were validated in bead on plate, fillet and V-groove joint positions. A total of 8 fillet tests and 5 groove tests were performed for both the wire types and it was found that predicted values were in good agreement with the measured values for bead on plate and fillet conditions, whereas welds with a V-groove joint geometry had a significantly under-predicted penetration area due to increased heat transfer and faster cooling down of weld with a higher equivalent sample thickness
Effects of Processing Parameters on Friction Stir Welded Lap Joints of AA7075-T6 and AA6022-T4
No full textFriction stir welding (FSW) is a solid-state welding process that has a number of advantages over traditional fusion welding techniques when attempting to join aluminum or dissimilar material workpieces. It is expected to play a large role in the automotive industry, where aluminum alloys are becoming more prevalent in mass-production vehicles.
The research in this thesis evaluates overlap FSW joints between thin sheets of AA7075-T6 and AA6022-T4 when the welding parameters of tool geometry and welding speed are varied. The resulting joints are characterized by optical microscopy, overlap shear tests, microhardness tests, and temperature measurements. The effect of a post-weld heat treatment is also examined. The main objective of the research are to determine a tool geometry that can produce good quality welds over a wide range of operating conditions, for use in an industrial setting.
Friction stir welds of good quality are made successfully at speeds of up to 500mm/min, and it is found that weld microhardness and joint strength are greater at faster welding speeds; whereas temperatures in the weld area are lower at faster welding speeds. Five different tool geometries are tested, and the tool design that delivers the best performance is a one that uses a concave shoulder shape, and a pin with a tapered profile, threads, and 3 flats. A post-weld heat treatment at 180°C for 30 minutes is found to increase joint strength by approximately 10%.
Future studies involving transmission electron microscopy, corrosion testing, and fatigue testing are recommended in order to supplement the results presented in this thesis
Characterization of Micro-Plasma Wire Arc Additive Manufacturing: Anisotropy and Layer Height Investigation
No full textDirected energy deposition describes the process of deposition of molten metal in wire or powder form with a focused energy beam source in a layer-by-layer fashion to create a final part. The use of an arc as heat source and wire as feedstock material for directed energy deposition, also known as wire arc additive manufacturing, has become increasingly popular in recent years due to its high productivity, high versatility, availability, cost, and its ability to produce large and complex parts.
However, due to the additive nature of the process and the high heat input involved, anisotropy is a recurring problem arising in printed parts, which leads to different tensile properties in the travel and build directions. Hence, the first section of this work looks into the mechanical properties and microstructures of a thin-walled AISI316LSi austenitic stainless-steel component fabricated by wire arc additive manufacturing using the micro-plasma arc welding process, which is a low heat input process. While properties were mainly uniform, the effect of anisotropy was found to have a significant influence on the modulus of elasticity, with values ranging from 79.5±6.8 GPa along the build direction to 105.2±20.7 GPa in the travel direction. This difference was found to be due to the strong preferential orientation of grains during solidification along the direction corresponding to the build direction, which was also confirmed by electron back scatter diffraction. This was also confirmed by theoretical calculations.
The second portion of the work deals with the investigation of the effect of vibratory weld conditioning on the grain size for titanium and stainless-steel layers using the current process. This was motivated by the need to break down the orientation of columnar grains witnessed and transform them into random equiaxed grains. Tests were conducted through the deposition of five layers for each material and the use of a shaker device and a signal generator, which was used to conduct tests based on v-square waves with different amplitudes and frequency ranges. Results revealed that fine grains were achieved when close to the substate, while only frequency was found to have a significant effect on secondary dendrite arm spacing and grain size for stainless-steel and titanium, respectively.
The final section of this work deals with correcting layer height deviations, which arise as a result of the heat accumulation of the wire arc additive manufacturing process. The performance of the automatic voltage control, which automatically adjusts the Z-position of the torch during deposition based on arc voltage measured, was initially investigated based on gain and correction speed. Results revealed very high correlations between Z-position and bead height, particularly for a gain of 1.0 (R=0.96) and a max speed of 65 mm/min (0.995). This proved the high reliability of the automatic voltage control when maintaining the voltage measured with the desired voltage but still does not account for surface inconsistencies. Hence, layer height deviations were measured and corrected with an accuracy of 0.03 mm through the modification of the wire feed speed, obtained by determining the exact volume of material added during deposition for different wire feed speeds. Also, in this section, optimal bead overlay parameters were determined based on best fusion and flat surface, revealing to be 15 % for substate welding and 25 % for subsequent layer deposition
Sulphide Stress Cracking in X80 Pipeline Steel Welds
Full text linkThe utilization of high strength linepipes provides various benefits, including reducing construction cost, and operational cost. On the other hand, the girth welds for such pipelines are welded onsite, and simultaneously require strength overmatching of the weld joint, high fracture toughness, and low hardness, regardless of whether these mechanical properties are potentially at odds with each other. Moreover, hardness in the welds must be maintained below HV250 for high strength pipe grades used in natural gas pipelines if they are used for severe sour service applications, where severe sour is defined as SSC region 3 in the NACE standard MR0175/ISO 15156, as the range where H2S partial pressure exceeds 1 kPa, and below a pH of 3.5, and above 100 kPa and below a pH of 5.5. Thus, it is very difficult to employ high strength pipeline materials such as those exceeding API X70 grade for such applications.
This particular hardness criterion has been standardized by NACE standard MR0175/ISO 15156 and European Federation of Corrosion publication number 16 (EFC No.16) to avoid sulphide stress cracking (SSC) initiation, and was determined by experience and testing; however, the tests had been performed several decades ago on rather different steel chemistries than those used today. Since recent steels and weld metals have significantly different chemical composition and grain size, thus this hardness criterion has become controversial. Meanwhile, some recent research papers mention that acicular ferrite is one of the fine microstructures and argued that intragranular can improve resistance to hydrogen embrittlement. Thus, the present work evaluates these issues on API X80 grade weld metals. This involves establishing a control method of acicular ferrite volume fraction in the weld metals. The relationship between kinds of microstructures and SSC susceptibility for GMA weld metals is then compared to validate a critical hardness value. The actual microstructural features which provide a hydrogen trap mechanism in particular are demonstrated for the acicular ferrite microstructure in the GMA weld metal, by using a combination of electron microscopy and hydrogen micro-printing. The role of microstructure on SSC susceptibility and the influence of hydrogen charging on strength and hardness properties are then compared.
The results show that it was possible to produce X80 grade weld metals with differing intragranular ferrite (acicular ferrite) volume fractions, by controlling the relationship between the titanium and oxygen ratio. The most effective balance between titanium and oxygen contents for maximizing intragranular ferrite corresponds to an ideal stoichiometry of Ti2O3. Next, SSC tests are performed in accordance with NACE MR0175/ISO 15156 (using a test solution) based on the (3 and 4 points bending method) on this material with varying intragranular ferrite volume fraction. These results confirm that weld metals exceeding 98% intragranular ferrite volume fraction and low grain boundary ferrite are able to pass the SSC test even if their hardness value exceeded 250HV. Furthermore, in order to reveal the roles of intragranular ferrite microstructure for SSC susceptibility, the hydrogen microprint technique combined with Scanning Electron Microscopy observation was used to compare the microstructures of specimens which passed and failed the SSC test. It was confirmed that grain boundaries within the intragranular ferrite are effective hydrogen trapping sites, and it appeared that nano-carbides prevent motion of dislocations which carrying diffusible hydrogen. Moreover, intragranular ferrite exhibits high toughness due to fine grains which promote crack deflection more effectively than grain boundary ferrite. Thus, it is suggested that intragranular ferrite can increase SSC resistance based on these mechanisms resulting from the desirable microstructural features. Finally, mechanical properties, such as tensile behaviour was investigated following hydrogen charging for two weld metals with differing intragranular ferrite volume fractions. The stress-strain curves for weld metals involving a higher intragranular ferrite volume fraction were less affected by hydrogen compared to the case of low intragranular volume fraction, and this is attributed to the difference of hydrogen trapping as revealed by thermal desorption analysis. In conclusion, this research proposes that intragranular ferrite dominant structures in high strength steels can decrease SSC susceptibility and stabilize mechanical properties in sour environments by providing strong trapping sites
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Low Cost Thermal Imaging System for Welding Applications
No full textA significant component of welding research is devoted to studying the relationship between welding parameters and material properties. The fundamental link between process parameters and material properties is established through the thermo-physical history of the material. Currently, there are very few measurement techniques that are suitable for quantifying the local thermal history induced by welding, and as a result, the application of fundamental materials science principles is severely limited in the welding industry. The goal of the following report is to develop a low cost CCD based thermal imaging camera which can be used to provide thermal feedback information for welding applications. The details regarding the design and operation of the thermal imaging system and calibration equipment are discussed. In addition, the performance characteristics of the thermal camera system is investigated under a variety of operation conditions. It was found that the newly developed system is capable of measuring the temperature of steel alloys across the 800°C to 500°C range required for direct T8/5 time measurement and microstructure prediction applications
Evaluation of Mechanical Properties in Pipeline Girth Welds Using Instrumented Indentation
Full text linkThis study aims to develop a new method to characterize mechanical properties using instrumented indentation. The indentation results were correlated with observed microstructure in weld metal of gas metal arc welding (GMAW) pipeline joints with their
mechanical properties such as: hardness and yield strength. The focus of this work was on a representative API-X80 grade weldment using different commercial welding consumables. Microstructure analysis was conducted on the as-deposited and reheated regions of the weld in each pass. The microhardness distribution was mapped for the whole weld cross section and correlated with the observed microstructure. A nearly at tip indenter was used to measure the load versus displacement response during micro-scale instrumented indentation test, in order to determine the yield strength for different areas in the weld
region. This technique had previously been used as provided by the inventor to determine the apparent yield strength for a wide range of engineering materials. However, the detailed physics of the method were not described. In the second phase of this work, a finite element model was built to understand the mechanics of contact using this indenter geometry and to derive the full stress-strain relationship from the indentation test data (based on load-displacement response) using an inverse approach, and to correlate these to the experimentally measured yield strength for pipeline steels. To establish the inverse approach, the model was tested in the first stage for common materials such as steel and aluminium alloys. This inverse approach technique aims to determine the tensile properties by analyzing the full load-displacement curves. In addition, by further analysis of the FE modelling results, a new technique to determine yield strength based on the load-displacement curve and indenter geometry was developed and validated with the experimental results. This technique, utilizing the nearly-flat indenter geometry along with cavity expansion theory and/or slipline theory, allows one to determine yield strength directly from an experimental load-displacement curve. The advantage of using this technique is that there is no need to use regression fitting or other assumptions of the material properties to estimate yield strength, or require FE modelling. Different steel and aluminium alloys were tested and the results correlated well with the yield strength measured by conventional tensile testing. Validating this indentation method as a localized strength measurement technique is essential, so that it can be used to conveniently test different regions of a weld joint. Once the method was validated, the yield strength was measured for different API-X80 pipeline steel welds using the nearly at tip indenter. The indentation based measurements of the apparent yield strength were correlated with the microstructure and hardness values in each zone. Moreover, the indentation technique was able to characterize narrow zones in
the weld and heat effected zone (HAZ) such as reheated weld metal, deposited weld metal, coarse grain heat effected zone (CGHAZ), and fine grain heat effected zone (FGHAZ). Digital Image Correlation (DIC) was also used to map the strain distribution during transverse tensile testing for the welds. DIC data provided further evidence of the strain distribution post-yielding, along with more information regarding the effect of strength mismatch level on strain distribution through the welding joint. The result of the strain map also was correlated with the formed microstructure in the WM and HAZ. The DIC data were used to construct local stress-strain curves based on the iso-stress condition assumption. The tensile yield strength results were compared with the indentation yield strength in different zones through and across the weld zone and the results showed good
agreement between them and suggest that the nearly at tip indentation technique could be used as a tool to determine the level of strength mismatch between a weld metal and base metal. In addition, the convenience of the indentation technique allows one to determine strength mismatch in both the longitudinal and transverse directions revealing that the estimation of the strength mismatch using the all-weld metal could overestimate the strength mismatch level up to 10%, which may lead to non-conservative design. Finally, further study of the mechanical properties of the weld metal in the longitudinal and transverse directions indicated that the weld metal has anisotropic properties. The indentation results were compared with conventional tensile test results for all-weld metal and cross-weld specimens; results from the latter specimens also supports anisotropy of
strength in the weld. These results were also supported by hardness mapping and detailed microstructral cheracterztion of the weld metal. This understanding is essential to improve the integrity and reliability of welds and testing procedures for pipeline applications
Influence of current pulse profile on metal transfer in pulsed gas metal arc welding
No full textThe increase in the requirements of safety and reliability demanded from the normalization institutes by the issue of normalizing standards, led to the need of the development of materials of superior properties. Owing to the development of metallurgy, the development of materials with extremely high values of strength and toughness was made possible. However, employing these materials in industrial applications is limited by the deterioration of their properties after the material is processed using techniques such as welding.
To cope with this challenge, several welding process have been developed. One of these processes is the pulse gas metal arc welding (GMAW-P), in which the arc current is periodically pulsed in order to achieve metal transfer and effectively join the material being dealt with, and this decrease the energy input to the base metal. Due to the advance of electronics and transistor technology there are a large number of current pulse profiles commercially available, with different degrees of complexity, designed for specific applications. Determining the balance between the complexity and benefits for the various pulse profiles and process modifications available is the main motivation for the research presented in this thesis. Specifically, the metal transfer of two commercially available current profiles was studied using high-speed imaging and high speed data acquisition of the electrical signal during welding, for different welding conditions. The results showed that the mode of metal transfer differs for the investigated profiles and that for the same pulse profile, as the pulse parameters are modified the metal and heat transfer changes, altering weld bead features such cross-sectional area and penetration
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