Systematic study of effect of cross-drafts and nozzle diameter on shield gas coverage

A shield gas flow rate of 15–20 L min21 is typically specified in metal inert gas welding, but is often adjusted to as high as 36 L min21 by welders in practice. Not only is this overuse of shield gas wasteful, but uncontrolled high gas flows can lead to significant turbulence induced porosity in the final weld. There is therefore a need to understand and control the minimum shield gas flow rate used in practical welding where cross-drafts may affect the coverage. Very low gas coverage or no shielding leads to porosity and spatter development in the weld region. A systematic study is reported of the weld quality achieved for a range of shield gas flow rates, cross-draft speeds and nozzle diameters using optical visualisation and numerical modelling to determine the shield gas coverage. As a consequence of the study, the shield gas flow has been reduced to 12 L min21 in production welding, representing a significant process cost saving and reduced environmental impact with no compromise to the final weld quality

Assessing the Importance of Sequencing Laboratory Welding Practicums

The effects of mastering 1F (flat position-fillet) welds on carbon steel using a sequenced pattern of welding was examined. Participants (N = 71) were randomly assigned a specific sequence of welding between Gas Metal Arc Welding (GMAW) and Shielded Metal Arc Welding (SMAW). The hypothesis of teaching beginning welders the GMAW process before the SMAW process to improve the ability to master American Welding Society (AWS) welds was tested. A welding pretest was administered to determine any prior welding knowledge. Sixty minutes of welding instruction taught proper welding safety, machine set-up, arc length, travel speed, work angle, and correct bead formation. Two practice sessions of 60 minutes were given for participants to become familiar with each welding process, followed by two testing sessions of ten minutes. During the testing period, individuals were assessed on their ability to produce an AWS 1F (flat position- fillet) lap weld in a given sequence. Welds were graded according to the AWS rubric for fillets with a total of four criteria categories based on the following criteria: a) presence of cracks or porosity, b) complete fusion, c) fillet leg size is specified minimum, and d) undercut - not to exceed 1/32 inches. The study indicate that students perform welds that meet AWS quality standards when using the GMAW process, however, the results were not statistically significant

Role of the micro/macro structure of welds in crack nucleation and propagation in aerospace aluminum-lithium alloy

Al-Li alloys offer the benefits of increased strength, elastic modulus and lower densities as compared to conventional aluminum alloys. Martin Marietta Laboratories has developed an Al-Li alloy designated 2195 which is designated for use in the cryogenic tanks of the space shuttle. The Variable Polarity Plasma Arc (VPPA) welding process is currently being used to produce these welds [1]. VPPA welding utilizes high temperature ionized gas (plasma) to transfer heat to the workpiece. An inert gas, such as Helium, is used to shield the active welding zone to prevent contamination of the molten base metal with surrounding reactive atmospheric gases. [1] In the Space Shuttle application, two passes of the arc are used to complete a butt-type weld. The pressure of the plasma stream is increased during the first pass to force the arc entirely through the material, a practice commonly referred to as keyholing. Molten metal forms on either side of the arc and surface tension draws this liquid together as the arc passes. 2319 Al alloy filler material may also be fed into the weld zone during this pass. During the second pass, the plasma stream pressure is reduced such that only partial penetration of the base material is obtained. Al 2319 filler material is added during this pass to yield a uniform, fully filled welded joint. This additional pass also acts to alter the grain structure of the weld zone to yield a higher strength joint

Mechanical Properties of Welded Deformed Reinforcing Steel Bars

Reinforcement strength, ductility and bendability properties are important components in design of reinforced concrete members, as the strength of any member comes mainly from reinforcement. Strain compatibility and plastic behaviors are mainly depending on reinforcement ductility. In construction practice, often welding of the bars is required. Welding of reinforcement is an instant solution in many cases, whereas welding is not a routine connection process. Welding will cause deficiencies in reinforcement bars, metallurgical changes and re-crystallization of microstructure of particles. Weld metal toughness is extremely sensitive to the welding heat input that decreases both of its strength and ductility. For determining the effects of welding in reinforcement properties, 48 specimens were tested with 5 different bar diameters, divided into six groups. Investigated parameters were: properties of un-welded bars; strength, ductility and density of weld metal; strength and ductility reduction due to heat input for bundled bars and transverse bars; welding effect on bars’ bending properties; behavior of different joint types; properties of three weld groove shapes also the locations and types of failures sections. Results show that, strength and elongation of the welded bars decreased by (10-40%) and (30-60%) respectively. Cold bending of welded bars and groove welds shall be prevented

High frequency Alternating Current (AC) tangent delta

Power cables are constantly subjected to thermal, electrical and mechanical stresses during their service life which leads to ageing of the insulation material. Ageing of the insulation means that the insulator degrades or gets older. When the insulation is degraded it does not have the same physical properties as compared to the new cables, which basically means that the risk of failure has been increased. So, it is necessary to carry out a diagnostic test to resolve the possible problem that may occur in cable insulation so that precautionary action can be taken to avoid unnecessary in-service failure of the underground cable [1]. Dissipation factor or tangent delta measurement in either time or frequency domain are used to identify water tree degraded XLPE cables with high moisture content or moisture ingression through poor jointing, diagnostic tests based on [2], [3]. In practice, standard laboratory electrical test equipment utilizing power frequency (PF) i.e. 50Hz to 60Hz testing is used. Hence, large static transformer was installed in the laboratory to cater any large capacitance test sample for high voltage testing. These existing tangent delta diagnostic test methods are bulk assessment technique that cannot sectionalize or pin point the water tree or any other conductive defect. In addition, both this tangent delta measurement technique which utilizes standard 0.1Hz and 50Hz equipment is not capable to extract the tangent delta values of accessories from the bulk assessment. It can only diagnose the overall condition of the complete cable system that includes cable, joints and terminations. Since medium voltage underground (MVUG) cables in utilities are in service for many years, it is crucial to develop a method to sectionalize or identify the defective components

Single sided single pass submerged arc welding of austenitic stainless steel

The weld metal produced from a series of high productivity welds of 316LN austenitic stainless steel plate was examined to evaluate the effects of the use of a higher heat input process (> 2.5kJ/mm).This high heat input process was aimed at maximising single sided weld metal penetration in a single pass using simple square edge preparations and minimising time consuming handling operations. The evaluation was undertaken by correlating the local microstructure with the local toughness and microhardness of the cap, middle and root of the weld. It was established that the intermetallic phases / carbides present did not appear to have a significantly adverse effect on either corrosion or toughness. The phases observed and confirmed by the use of SAED were predominantly chi (χ) with some sigma( σ). No identifications were made of M23C6 which was observed in other studies of 316LN welds. A series of impact tests with variations in the notch positions showed that the thickness of the delta ferrite had an effect on the weld metal toughness. As a result of this work it was established that similar volume fractions of delta ferrite did not necessarily produce similar levels of weld metal toughness, but ferrite thickness did appear to have a contributory effect. Welding of 316LN stainless steel with a single sided single pass submerged arc welding process was satisfactorily undertaken up to 20mm plate thickness without preheat or post weld heat treatment. The ability to achieve this resulted in significant economic savings within the process for ship panel production combined with satisfactory weld metal properties

The philosophy and principles of welding design

The welding designer requires a philosophy for his design responsibilities and a thorough understanding of the basic principles in the subjects covering his responsibilities. The education of the welding designer must provide a spectrum of studies which will define these responsibilities and which will help him to understand and apply the science and technology relevant to his work

Multi-Objective Optimization in Friction Welding Process Parameters on EN353 Alloy Steel using Taguchi based GRA

Joining of similar materials by varying the input factors on a continuous drive friction welding process is done in this study. The intention is to find the optimal solution in friction welding input process parameters. Among several types of welding processes, friction welding forms good metal joints. The process factors considered for this material joining practice are Upset Time (UT), Heating Time (HT), Heating Pressure (HP), Upset Pressure (UP), chemical composition and measurements of the materials. Frictional joints provide better mechanical properties, hence it is attracted by researchers. Here, EN353 is used as the specimen, for its extensive usage in the automobile and manufacturing sectors. Axial shortening, hardness testing and the temperature during welding are evaluated, compared and optimized using Taguchi Design of Experiments (DoE) scheme using L27 orthogonal array and Grey Relational Analysis (GRA)

A review of agricultural waste activated carbon and effect on adsorption parameters

Activated carbon is widely used in industrial wastewater treatment operation because of the effectiveness in removing the dye. As the conventional activated carbon on market nowadays is expensive, lots of research was done on agricultural materials to replace the activated carbon. Sugarcane bagasse is listed as one of the agricultural material alternatives. This review compiles the characterisation of sugarcane bagasse and several agricultural wastes-based activated carbon including proximate analysis, ultimate analysis, and activated carbon surface characteristic used to remove dye in textile wastewater. Nevertheless, the adsorption parameters are the main focus as it affecting the removal of dye. The expansion review regarding activated carbon performance implies that dye removal efficiency depends on the amount of adsorbent dosage, contact time, pH solution, dye concentration applied during the adsorption process. This review shows the need of thoughtful information on adsorption parameters with an expanded list of various types of agricultural-based activated carbon and various dye removal