Hydrogen assisted cold cracking (HACC) represents a significant threat to the integrity of welded steel constructions. HACC occurs at near ambient or lower temperatures and may be delayed for several hours, or even days, after welding is completed. Significant advances in developing thermomechanically controlled processed steels, with reduced carbon and alloy content, resulted in steels that are more resistant to HACC in the heat affected zone (HAZ). This has enabled the use of lower preheat temperatures during fabrication of welded structures. However, the improvement in the resistance of the HAZ to HACC has shifted the problem of cold cracking to the weld metal. The objective of this current work was to establish the effect of flux cored arc welding (FCAW) welding parameters, such as welding current, contact-tip to workpiece distance (CTWD) and shielding gas type, on diffusible hydrogen (HD) content for single run, bead-onplate welds using low strength seamed and seamless gas shielded rutile wires of E71 T-1 classification. The work has shown that under most conditions investigated, the weld metal HD levels for the seamed rutile wire were above the 10 ml/100g specified by the consumable’s classification (H10). The measured range of diffusible hydrogen for the H10 wire was 8.3 to 17.0 ml/100g, with the highest hydrogen content being obtained at the lowest welding current of 280 A, shortest CTWD of 15 mm and deposited using 75Ar-25CO2 shielding gas. In contrast, the seamless wire met requirements of the H5 classification (HD ≤5 ml/100g) for all welding conditions investigated, with a range of HD levels of 0.9 to 3.5 ml/100g. v In general, lower HD levels were achieved when using CO2 shielding gas, although the effect is less significant with the H5 seamless rutile wire. The work included an investigation of arc characteristics under typical welding conditions, using high speed digital imaging and laser backlighting, in order to provide information on metal transfer and arc length. Several tests were carried out using the H10 consumable. It was established that the amount of heat generated by resistive heating of the wire prior to melting can exert a strong influence on the weld metal HD content and is more pronounced in welds deposited using 75Ar-25CO2. The measured arc length was reduced significantly when welding under CO2 shielding gas. Despite suggestions in the literature there was no evidence of a change in metal transfer mode to spray transfer on increasing the welding current from 280 to 320 A, transfer mode was globular for all conditions used. Following the weld metal diffusible hydrogen testing and welding arc imaging work, the weld metal susceptibility to cold cracking was assessed using the gapped bead-on-plate (G-BOP) test at different preheat temperatures. For this part of the work, identical welding parameters those used for the diffusible hydrogen testing were selected. It was found that the H5 wire weld deposits did not reveal any cracking at ambient temperature, whereas all the welds deposited using the H10 wire exhibited cold cracking with no preheat. Weld metal deposited using 75Ar-25CO2 shielding gas resulted in higher susceptibility to cold cracking than with CO2, which correlated to lower HD levels the CO2 tests. Besides the higher hydrogen content, it was also found that higher weld metal hardness corresponded to the greater crack susceptibility in the welds deposited using 75Ar-25CO2 shielding gas. These factors are considered to contribute to higher susceptibility to transverse cold cracking compared with CO2 shielding gas. vi Although the overall results indicate that the weld metal susceptibility to cold cracking corresponds to the relevant levels of HD, this relationship was found to be ambiguous in welds deposited at the shortest CTWD of 15 mm, using CO2 shielding gas at all welding currents investigated. While the amount of diffusible hydrogen was marginally increased from 11.7 to 12.8 ml/100g, resulting from the welding current increase from 280 to 320 A, the amount of cold cracking at room temperature was significantly decreased from 89 to 25 %RTC. This is explained by a significant difference in the cross section of the weld beads, suggesting a need to more closely evaluate the G-BOP testing, particularly examining the effects of weld bead profiles on the weld susceptibility to HACC. Preheat was found to decrease the amount of cold cracking in the H10 welds and it was concluded that preheat significantly reduces the main contributor to decrease the HD in the weld metal. Although the cracking susceptibility of welds using 75Ar-25CO2 shielding gas decreased more slowly with an increasing preheat temperature, compare to those deposited using CO2, no cracking was observed at 120 °C in welds under both shielding gases. This indicates that the same welding consumable (H10) deposited using different shielding gases can result in a different response to preheat temperature. Based on the results of this work, a number of changes are proposed to hydrogen testing standards AS 3752-1996 and ISO 3690-2000, particularly with respect to the effects of CTWD and shielding gases on levels of diffusible hydrogen in weld metal deposited using gas shielded rutile flux-cored wire.

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The effect of welding parameters on levels of diffusible hydrogen in weld metal deposited using gas shielded rutile flux cored wires

Precipitation hardening aluminum alloys are used in many industries due to their excellent mechanical properties, including good weldability. During a welding process, the tensile strength of the joint is critical to appropriately exploit the original properties of the material. The welding processes are still under study, and gas metal arc welding (GMAW) in pulsed metal-transfer configuration is one of the best choices to join these alloys. In this study, the welding of 6061 aluminum alloy by pulsed GMAW was performed under two heat treatment conditions and by using two filler metals, namely: ER 4043 (AlSi5) and ER 4553 (AlMg5Cr). A solubilization heat treatment T4 was used to dissolve the precipitates of β”- phase into the aluminum matrix from the original T6 heat treatment, leading in the formation of β-phase precipitates instead, which contributes to higher mechanical resistance. As a result, the T4 heat treatment improves the quality of the weld joint and increases the tensile strength in comparison to the T6 condition. The filler metal also plays an important role, and our results indicate that the use of ER 4043 produces stronger joints than ER 4553, but only under specific processing conditions, which include a moderate heat net flux. The latter is explained because Mg, Si and Cu are reported as precursors of the production of β”- phase due to heat input from the welding process and the redistribution of both: β” and β precipitates, causes a ductile intergranular fracture near the heat affected zone of the weld joint.

Comparative in Mechanical Behavior of 6061 Aluminum Alloy Welded by Pulsed GMAW with Different Filler Metals and Heat Treatments.

In this work we have shown that how combining the multi-criteria decision making (MCDM) process with fuzzy logic can be very efficient to solve decision problems in which there are criteria of different natures.
 This combination is applied here to solve a real decision problem of great interest in structural steel welding which is used widely in industry. The problem consists of selecting the best electric arc welding processes for a plate thickness greater than 12 mm. Because facing a decision problem of this type involves, on one hand, quantitative criteria (thermal efficiency, productivity, minimum power, etc.) and on the other hand, qualitative criteria (fatigue of welder, process learning, reliability, etc.), not only will it be necessary to apply a MCDM process, but it should also be combined with fuzzy logic with the aim of being able to perform the extraction of knowledge through a survey focused on experts. The Analytic Hierarchy Process has been used in order to obtain the weight of the criteria and, through the Technique for Order Preference by Similarity to Ideal Solution. The alternatives are also evaluated. There are no previous studies related to welding processes using the MCDM process with fuzzy logic.

Assessment of Arc Welding Process Through the Combination of TOPSIS–AHP Methods with Fuzzy Logic

The effect of torch weaving on the microstructure, tensile strength, impact resistance and fracturing in robotic welded joints using varing welding speeds, voltages and currents was evaluated through fractography using scanning electron microscopy, tension, impact and micro-hardness tests and optic microscopy. Results indicated that linear, sinusoidal and circular weavings favored an increase of the width of the HAZ as well as a slight increase in yield resistance accompanied by hardening in comparison with a triangular weave. The latter favored larger impact energy in the HAZ with less width, containing coarse-grained ferrite due to lower tensile strength and Vickers hardness. A circular weave generated the highest level of hardening and the lowest energy absorbed in the HAZ as a consequence of an increase in yield strength related to the fine needles of acicular ferrite. A linear weave favored the greatest width of the HAZ compared with other weld weavings due to heat accumulation along the fusion line of the welded joint. Hardening and loss of toughness were evaluated through fractographic analysis showing mixed fractures mainly composed of brittle fracturing made by transgranular cleavages with facets containing well defined river patterns.

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Effect of Torch Weaving on the Microstructure, Tensile and Impact Resistances, and Fracture of the HAZ and Weld Bead by Robotic GMAW Process on ASTM A36 Steel

In this paper, a three-dimensional finite element model has been developed to simulate dynamically the gas metal arc welding (GMAW) process of aluminium alloy sheets. The numerical simulation was conducted using a non-linear transient thermal analysis by changing the welding parameters: namely arc power and welding speed. A moving Gaussian distributed heat source is implemented. All major physical phenomena associated with the GMAW process, such as thermal conduction and convection heat losses are taken into account in the model development. The developed model can calculate the temperature field and predict the weld geometry profile during the welding process. The measurement of weld bead profile from the GMAW experiments was used to validate the developed finite element model. The numerical study reveals that the arc voltage and welding speed have a significant influence on the temperature distribution, weld pool size and shape, and weld bead geometry. The results show that there are good agreements with the weld bead profile between the experimental observation and finite element simulation.

Thermal analysis on a weld joint of aluminium alloy in gas metal arc welding

This work was part of a nationwide program for the development of new high-performance steels with 70 ksi (485 MPa) minimum yield strength, improved toughness, and lower manufacturing costs through the elimination of preheat for welding. The purpose of the present work was to evaluate the fusion zone hydrogen-induced cracking susceptibility of single-pass weld deposits made using four different welding processes at equivalent diffusible hydrogen levels. The gapped bead-on-plate test was used to compare shielded metal arc (SMAW), submerged arc (SAW), gas metal arc (GMAW), and flux cored arc (FCAW) welding processes. Equivalent net heat inputs were produced and the weld cross-sectional areas were normalized at different arc energies, including the heat transfer efficiency for each process. The minimum predicted preheats were different, lower for SAW than for GMAW, FCAW, and SMAW at similar diffusible hydrogen levels and heat inputs. This difference was attributed to the different solidification microstructures and weld bead geometries. Preheating guidelines based on the SMAW process remained the most conservative, confirming the validity of the past practice of using SMAW to find minimum preheats. It was concluded that preheat recommendations should not be extrapolated from one welding process to another. The information generated was used for manufacturing recommendations for welding high-performance steels.

Y. Adonyi

Papers

Welding process effects in weldability testing of steels : Welding aluminium