Welding Current Values Research Articles

Enhancing the Microstructure and Mechanical Properties of Cast Aluminum Matrix Composites Through Nano-Al2O3 Reinforcement via CMT and PMC Welding

In this investigation, two distinct composites were produced using the liquid metallurgy vortex-route method with an aluminum AA6013 matrix and 0.5% and 1% nano-Al2O3 reinforcement by weight. The effects of nano-Al2O3 ceramic particles added into the AA6013 alloy to increase the hardness and strength of the composite were investigated. Samples for welding were prepared from cast composite materials with dimensions of 250 × 110 × 60 mm. The obtained specimens were joined via butt welding at 110-A and 120-A current intensities, and at a constant speed of 400 mm/min by cold metal transfer (CMT) and pulse multi-control (PMC) welding methods. Using the use of micro- and macrostructural evaluations, hardness measurements, and tensile testing, the impacts of each welding parameter on the mechanical characteristics and weld structure of the cast composites were examined. The composite material with 0.5% nano-Al2O3 reinforcement that was joined at 120 A utilizing the CMT process achieved the highest tensile strength value at 96.6 MPa. At 110-A and 120-A welding current values, 0.5% reinforced composite demonstrated the maximum strength in both welding techniques.

Tension and Impact Analysis of Tungsten Inert Gas Welded Al6061-SiC Composite

An aluminum 6061 (Al6061) metal matrix composite (MMC) reinforced with silicon carbide was prepared by stir casting. Specimens of the required dimensions were welded using the tungsten inert gas (TIG) method. ER5356 (Al-5%Mg) was chosen as the appropriate filler material for TIG welding. The input current parameter was varied (150, 170 and 200nA) while maintaining the other welding parameters at constant values. An assessment of the mechanical (tensile and impact strength) and microstructure properties of the TIG-welded Al6061 MMC with 6 wt. % silicon carbide particles was accomplished. An 8.27% improvement was observed in ultimate tensile strength (UTS) for the 150 A TIG-welded sample. UTS and elasticity decreased linearly with an increase in welding current but exhibited higher values than in non-welded specimens. The microstructural analysis of the welded MMCs showed a mixed mode of failure, with equiaxial dimples being dominant in lower-weld-current specimens. Compared to non-welded specimens, a 40% increase in impact strength was observed for the 150 A TIG-welded specimens, which decreased with an increase in the welding current value. SEM analysis revealed ductile striations and continuous river patterns, resulting in mixed failure.

Estimation of the Welding Current in Gas Tungsten Arc Welded (GTAW) Process with a Specified Width Penetration for AISI 304 Stainless Steel

Gas tungsten arc welding (GTAW), also known as tungsten inert gas welding, is now widely used in the manufacturing industry. In GTAW, the weld size, especially the weld depth, is one of the important parameters of the weld. Many studies demonstrated that the weld depth significantly affects the quality of the weld joint and is greatly influenced by the welding current. Thus, estimating the welding current value for the desired weld depth is necessary. This paper presents a method to estimate the welding current in the GTAW process with a specified width penetration for AISI 304 stainless steel with 0.8[Formula: see text]mm thickness. The welding current determination of the GTAW process is performed by using the inverse method. In this work, the inverse method utilized is the Levenberg–Marquardt (LM) method. The advantage of the method is that the algorithm is simple, easy to apply, and has a high convergence rate. Two examples are considered to demonstrate the proposed method. The relative error between the desired width penetration of the weld value and the estimated width penetration of the weld by the LM method is very small (0.2% for case 1 and 0.08% for case 2). The results show that the proposed method efficiently estimates the welding current for a specified width penetration in the GTAW process.

Investigation of processes in spark gap during contact butt welding of railway rails

Modern equipment for contact butt welding of railway rails has a huge number of technological parameters (input factors) that affect the quality of the resulting welded joint. These parameters do not allow the optimal welding modes to be fully selected. The study of the processes occurring in the spark gap (formation of microcontacts, increase or decrease in their cross-sectional area, explosion of the resulting microcontacts) will make it possible to determine the amount and uniformity of the invested heat during rail welding. The acquired knowledge will reduce the number of influencing factors on quality of the welded joint. Using a large amount of statistical data, the authors have investigated the processes occurring in the spark gap during contact butt welding of railway rails of R65 type of DT350 category on K1000 rail welding machine. When studying the influence of movable plate movement of the welding machine during reflow on the strength of welding current, it was determined that there is no dependence between these values. The random nature of distribution of the welding current values is assumed, changing as a result of the processes of formation, existence and explosion of a large number of tiny electrical contacts. Based on the analysis of frequency distribution, it is certain that distribution of the spark gap resistance does not correspond to the normal distribution law. As the sample increases, the deviation from it increases; visually the histogram describes the gamma distribution. With the help of the gamma distribution law, the welding current forces during contact butt welding were modeled by reflow. The research results are planned to be used to simulate thermal processes and formation of the structure of weld metal and the zones of thermal influence of railway rails welded joint.

Electron beam welding of 7075 aluminum alloy: Microstructure and fracture properties

Aluminum alloys have widespread use due to their superior properties. However, it exhibits complex effects with its various intermetallic compounds formed by clustering of other elements in its welded joints. In this study, electron beam welding (EBW) of the 7075 aluminum alloys (Al-alloys) was examined. Two different welding current values ​​are used in the welding process (20 and 25 mA). The welded specimens were subjected to tensile testing to determine their mechanical properties. In addition, the fractured surfaces were examined. The microhardness, scanning electron microscopy (SEM) and energy-dispersive spectroscopy analysis (EDS), EDS line analysis and EDS mapping were conducted. The microstructural evolution of EB welded plates investigated in the base metal (BM), heat affected zone (HAZ), and fusion zone (FZ), using SEM. Results indicated that after EBW, microhardness increment was observed in the FZ as compared to the BM and HAZ. The lowest hardness was observed in HAZ. The intermetallic formations in the FZ, and the effects of the elements on fracture were tried to be identified. At parameters 20 mA and 25 mA, 7075 alloys weld joints were brittlely broken. It was observed that the amount of Cu element increased at a high rate in the fracture zones of 7075 alloys (25 mA), it was clumped and active in fracture. Furthermore, it is estimated that a new formation is observed in the FZ of the welded joint made at 20 mA current in this study. As a result of the literature review, it is predicted that a natural structure similar to the one found in our study was not encountered for 7075 alloys or other welded Al-alloys. This formation was named, and the theory was put forward about the formation. The size, shape, chemistry and how this new formation came about were analysed.

Analysis of mechanical and microstructural properties of gas metal arc welded dissimilar aluminum alloys (AA5754/AA6013)

The use of aluminum alloys to reduce weight in air, sea, and land vehicles is increasing rapidly, especially due to their low density, excellent corrosion resistance, good forming properties, and increasable mechanical properties. A common aspect of all the relevant industrial sectors is the need to join the many parts of manufactured technological products using welding methods. In these industries, using welding to join different aluminum alloys having high mechanical properties is especially important. However, despite the widespread usage areas, it is very difficult to join different aluminums and their alloys via welding. In this study, AA5754/AA6013 aluminum alloys were joined using automatic gas metal arc welding (GMAW). Welded joints were created at two different welding speeds (150 and 180 mm/min) and three different welding current values (130, 140, and 150 A). This study investigated the microstructural and mechanical properties of AA5754/AA6013 aluminum alloy sheets joined at different welding speeds and current values. The macro and microstructures of samples taken from the welded joints were examined. In addition, hardness, tensile, bending, and fatigue tests were performed on the welded samples. The findings obtained as a result of the tests were evaluated and interpreted in light of the literature.

Shielded metal arc welding of AISI 409M ferritic stainless steel: study on mechanical, intergranular corrosion properties and microstructure analysis

PurposeThe purpose of this study is on AISI 409 M ferritic stainless steel (FSS) which is developing a preferred choice for railway carriages, storage tanks and reactors in chemical plants. The intergranular corrosion behavior of welded SS 409 M has been studied in H2SO4 solution (0.5 M) with the addition of NH4SCN (0.01 M) with different heat input. As this study is very important in context of various chemical and petrochemical industries.Design/methodology/approachThe microstructure, mechanical properties and intergranular corrosion properties of AISI 409 M FSS using shielded metal arc welding were investigated. Shielded metal arc welding with different welding current values are used to change the heat input in the joints resulted in the microstructural variations. The microstructure of the welded steel was carefully inspected along the width of the heat-affected zone (HAZ) and the transverse-section of the thin plate.FindingsThe width of heat affected zone (3.1,4.2 and 5.8 mm) increases on increasing the welding heat input. Due to change in grain size (grain coarsening) as HAZ increased. From the microstructure, it was observed that the large grain growth which is dendritic and the structure become finer to increase in welding heat input. For lower heat input, the maximum microhardness value (388HV) was observed compared with medium (351 HV) and higher heat input (344 HV), which is caused by a rapid cooling rate and the depleted area of chromium (Cr) and nickel (Ni). The increase in weld heat input decreases tensile strength, i.e. 465 MPa, 440 MPa and 418 MPa for low, medium and high heat input, respectively. This is because of grain coarsening and chromium carbide precipitation in sensitized zone and wider HAZ. The degree of sensitization increases (27.04%, 31.86% and 36.08%) to increase welding heat input because of chromium carbide deposition at the grain boundaries. The results revealed that the higher degree of sensitization and the difference in intergranular corrosion behavior under high heat input are related to the grain growth in the HAZ and the weld zone.Originality/valueThe study is based on intergranular corrosion behavior of welded SS 409 M in H2SO4 solution (0.5 M) with the addition of NH4SCN (0.01 M) with different heat input which is rarely found in literature.

Establishing empirical relationship between welding current and weld metal deposition rate for submerged arc welding process

Any increase in the productivity of the SAW process will immensely benefit the welding industry, as this process is widely used. Since the traditional single wire DCEP SAW system is extensively used even today, any enhancement using this system will benefit the industry in a big way, if this traditional SAW system is part of the improvisation process. This paper establishes a relationship between welding current and productivity (deposition rate), for the three common sizes of both solid wires and metal cored tubular wires at different current values over its full range, through bead-on-plate experimentation. At each preset current value, the weld bead was optimized for acceptable visual quality, by adjusting arc travel speed and voltage, then the wire feed rate-making optimized bead was noted. The weld metal deposition rate was calculated for establishing A vs WMDR empirical relationships. From the results, ten progressive strategies have been established which can improve welding productivity from 21 to 211% from baseline. The established relationships, strategies can be effectively used, to estimate/optimize the productivity based on the welding current values, for each wire type and diameter.

The effect of welding current on nugget geometry, microstructure and mechanical properties of TWIP steels in resistance spot welding

In this study, two twinning induced plasticity (TWIP) steels with different chemical compositions were produced by casting-hot rolling-cold rolling processes. A total of four groups of samples were formed by obtaining cold-rolled and heat-treated samples giving optimum mechanical properties. Welding parameters of resistance spot welding (RSW) of all samples were determined, and weldability at different welding current values was examined. Button shapes, microstructures, microhardness, and tensile shearing tests of welded joints were investigated. In addition, scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analyses of broken samples were examined as a result of tensile testing. It has been concluded that non-heat-treated samples have higher tensile strength than heat-treated samples. In heat-treated steels; a marked increase in ductility was observed. It has been observed that RSWs of non-heat-treated steels have higher tensile load bearing capacity (TLBC) value.

Оценка стойкости кремнийорганического герметика к действию сверхнизких температур

Properties of the silicone sealant VGO-1 have been examines, which is used for production of glue-welded joints made by contact welding. Conditions to produce glue-welded joints are described. It was found out that when welding steel specimens of the 08kp brand on the VGO-1 sealant there was a decrease in a welding current value and an increase in welding time and the electrode compression force. The study of sealant structures after storage at different temperatures demonstrated that the used sealant had good resistance at –30 °С in the air environment and satisfactory resistance at –50 °С in a liquid environment. The swelling index of the sealant after 20 day storage in medicinal ethyl alcohol was no more than 4.5%.

INFLUENCE OF TACK WELDS DISTRIBUTION AND WELDING SEQUENCE ON THE ANGULAR DISTORTION OF TIG WELDED JOINT

In this paper the influence of tack welds distribution and welding sequence on angular distortion of the Tungsten Inert Gas (TIG) welded joint was tested. Additionally, the effect of welding current on angular distortion was assessed. For research X2CrTiNb18 (AISI 441) stainless steel (2.5 mm thick) was chosen. During research specimens were prepared with different distributions of tack welds. Then they were welded by different welding sequences with the use of different welding current values. After welding the angular distortion of each specimen was measured by using the coordinate measuring machine. In the next step specimens were cut. Cross-sections were polished and the metallographic macroscopic testing was conducted to check the geometry of performed welds. Performed experiments allowed determining the optimal tack weld sequence and welding parameters for welding thin stainless steel sheets.

X-Ray Radiography of Micro-alloyed Steel Joined by Submerged Arc Welding

In this study, micro-alloyed steels were welded by using submerged arc welding method. Different welding current values ​​in experimental studies were carried out according to Table 3. After welding process, the radiographic tests were examined by using X-Ray. The experimental results indicated that microalloyed steel could be joined by using the submerged arc welding technique with sufficient strength. The results of the radiographic tests indicated that with increasing welding current, the amount of deep penetration increased in all samples.

Effect of Welding Current on Weld Formation, Microstructure, and Mechanical Properties in Resistance Spot Welding of CR590T/340Y Galvanized Dual Phase Steel

During resistance spot welding, the welding current is the most important process parameter, which determines the welding heat input and then has a great influence on the welding quality. In present study, the CR590T/340YDP galvanized dual phase steel widely used as automobile material was carried out using resistance spot welding. The effect of welding current on the weld formation, microstructure, and mechanical properties was studied in detail. It was found that the quality of weld appearance decreased with the increase of welding current, and there was a Zn island on the weld surface. The microstructure of the whole resistance spot welded joint was inhomogeneity. The nugget zone consisted of coarse lath martensite and a little of ferrite with the columnar crystal morphology, and the microstructure of weld nugget became coarser when the welding current was higher. There was an optimum welding current value and the tensile strength reached the maximum. This investigation will provide the process guidance for automobile body production.

Resistance Spot Welding of Steel Sheets of the Same and Different Thickness

Resistance welding ranks among progressive and in practice often used manufacturing techniques of rigid joints. It is applied in single‑part production, short‑run production as well as in mass production. The basis of this method is in the utilization of the Joulean heat, which arises at the passage of current through connected sheets at collective influence of compressive force. The aim of the carried out tests was the determination of the dependence between the rupture force of spot welds made using steel sheets of the same and different thickness for different welding conditions. For carrying out of this aim 360 assemblies were prepared. The sheets (a total of 720 pieces) of dimensions 100 × 25 mm and thickness of 0.8 mm, 1.5 mm and 3.0 mm were made from low carbon steel. In the place determined for welding the test specimens were garnet blasted and then degreased with acetone. The welding of two specimens always of the same (0.8+0.8 mm, 1.5+1.5 mm a 3.0+3.0 mm) and different (0.8 + 1.5 mm, 0.8+3.0 mm a 1.5+3.0 mm) thickness was carried out using the welding machine type BV 2,5.21. At this type the welding current value is constant (Imax= 6.4 kA). The welding time (the time of the passage of the current) was changed in the whole entirety, namely 0.10 s, 0.15 s, 0.20 s, 0.25 s, 0.3 s, 0.4 s, 0.6 s, 0.8 s, 1.0 s, 1.3 s, 1.6 s and 2.0 s. The compressive force was chosen according to the thickness of the connected sheets in the range from 0.8 to 2.4 kN. From the results of carried out tests it follows that using the working variables recommended by the producer we obtain the quality welds. But it we use the longer welding times, we can obtain stronger welds, namely up to 21 % compared to welds made using working variables recommended by the producer.

Proposta e Análise de Dispositivo para Otimização do Destacamento da Gota na Soldagem MIG/MAG

Resumo Diversas forças atuam na soldagem MIG/MAG durante a transferência da gota fundida presente na extremidade do arame-eletrodo à poça de fusão, as quais variam de acordo com o modo de transferência adotado. No modo globular, o destacamento da gota ocorre quando a sua força peso supera a tensão superficial produzindo gotas maiores que provocam maior instabilidade durante sua transferência. Já no modo goticular, os altos valores de corrente intensificam a atuação da força de Lorentz. Esta força magnética proporciona o desprendimento da gota ainda em pequenas dimensões, resultando numa maior estabilidade. Entretanto, este modo de transferência caracteriza-se por um relativo grande aporte de energia, o que inviabiliza uniões de chapas finas ou fora de posição. Busca-se então neste trabalho, analisar a viabilidade da utilização de uma força magnética externa que auxilie o processo de destacamento da gota e propor um dispositivo para esta finalidade. O intuito é obter um modo de transferência semelhante ao goticular, porém, com baixos valores de corrente de soldagem sem, entretanto, a necessidade de emprego de fontes eletrônicas. Deste modo, espera-se obter uma maior estabilidade no processo aliada a uma menor quantidade de calor entregue a peça. Contudo, os resultados obtidos mostram a inviabilidade tecnológica quanto à utilização do dispositivo proposto, dada à necessidade de emprego de níveis de correntes extremamente elevados (da ordem de 100.000A) para sensibilizar a gota na ponta do eletrodo.

Prediction of magnetic interference from resistance spot welding processes on implantable cardioverter-defibrillators

The purpose of this study is to create a tool capable of reliably predicting the magnetic fields (MFs) generated in resistance spot welding (RSW) processes. If the MF is greater than 10 G, the MF activates a reed switch in the implantable cardioverter-defibrillator (ICD) and temporarily inhibits detection of ventricular tachycardia and ventricular fibrillation. The MF is predicted from two variables: (1) distance between ICD and electrodes; (2) welding current. Firstly, a linear regression model is attempted. The MF is the dependent variable, 1/distance and welding current are the independent variables. The ANOVA table shows that the regression is significant at the 0.01 level but the residuals analysis reveals nonlinear behaviour. An artificial neural network (ANN) is proposed because the ANNs are capable of mapping nonlinear systems. The inputs are two-component vectors, the first component is the distance value and the second component is the welding current value. The training of the ANN uses supervised learning mechanism. Therefore, each input must come with its respective desired output (target) that is the recorded MF. The available data set is randomly split into a training subset (to update weight values) and a validation subset (to guard against overfitting by means of cross validation). The number of neurons in the hidden layers is selected considering the overfitting phenomenon: the number of neurons in the hidden layers that minimizes the validation mean square error (MSE) is 4. With the selected ANN (2-4-4-1) the aim of the present study (to reliably predict the MF) is achieved because this ANN produces good results in prediction from input vectors non-used in the training.

Effect of auxiliary preheating of filler wire on dilution in gas metal arc stainless steel surfacing using RSM

Weld surfacing is a process for producing surfaces with good corrosion resistant properties by means of depositing/laying of stainless steels on low carbon steel components with an objective of achieving maximum economy and enhanced life. The aim of the work presented here was to investigate the effect of auxiliary preheating of the solid filler wire in mechanised gas metal arc welding (GMAW) process (using a specially designed torch to preheat the filler wire independently, before its emergence from the torch) on dilution. Four factor five level central composite rotatable design was used to develop relationship for predicting dilution, which enables to quantify the direct and interactive effects of four numeric factors, namely, wire feed rate, arc voltage, welding speed and electrode extension, and one categorical factor, preheat current. Mathematical models developed show that dilution increases with increasing wire feed rate, arc voltage and welding speed, whereas it decreases with increasing electrode extension and preheat current. External preheating of the filler wire resulted in greater contribution of arc energy by resistive heating due to which significant drop in the main welding current values and hence low dilution levels were observed. Numerical optimisation was carried out and the optimal solutions generated indicate that for same levels of dilution, higher deposition rates are achievable in the auxiliary preheating arrangement. Conformity experimental runs show a good agreement with the numerical results.

Effect of Auxiliary Preheating of the Filler Wire on Quality of Gas Metal Arc Stainless Steel Claddings

Weld cladding is a process for producing surfaces with good corrosion resistant properties by means of depositing/laying of stainless steels on low-carbon steel components with an objective of achieving maximum economy and enhanced life. The aim of the work presented here was to investigate the effect of auxiliary preheating of the solid filler wire in mechanized gas metal arc welding (GMAW) process (by using a specially designed torch to preheat the filler wire independently, before its emergence from the torch) on the quality of the as-welded single layer stainless steel overlays. External preheating of the filler wire resulted in greater contribution of arc energy by resistive heating due to which significant drop in the main welding current values and hence low dilution levels were observed. Metallurgical aspects of the as welded overlays such as chemistry, ferrite content, and modes of solidification were studied to evaluate their suitability for service and it was found that claddings obtained through the preheating arrangement, besides higher ferrite content, possessed higher content of chromium, nickel, and molybdenum and lower content of carbon as compared to conventional GMAW claddings, thereby giving overlays with superior mechanical and corrosion resistance properties. The findings of this study not only establish the technical superiority of the new process, but also, owing to its productivity-enhanced features, justify its use for low-cost surfacing applications.

Modelling of the effects of welding conditions on dilution of stainless steel claddings produced by gas metal arc welding procedures

Abstract This paper highlights an experimental study carried out to analyse the effects of various gas metal arc welding (GMAW) and universal gas metal arc welding (UGMAW) process parameters on dilution in single layer stainless steel cladding of low carbon structural steel plates. Four factors five levels central composite rotatable design was used to develop relationship for predicting dilution, which enables to quantify the direct and interactive effects of four numeric factors i.e. wire feed rate, open circuit voltage, welding speed and nozzle-to-plate distance and one categorical factor preheat current. This comparative study reveals that dilution obtained in UGMAW process is significantly lower than that with GMAW process due to external preheating of the filler wire in UGMAW process which results in greater contribution of arc energy by resistive heating and as a consequence, significant drop in the main welding current values in case of UGMAW process is observed. Mathematical models developed show that dilution increases with increase in wire feed rate, open circuit voltage and welding speed, whereas it decreases with increase in nozzle-to-plate distance and preheat current. Furthermore the optimization capabilities in design-expert software were used to numerically optimize the input parameters and the optimal solutions generated indicate that for same levels of dilution, higher deposition rates were achieved in UGMAW process thus establishing that UGMAW process can prove to be a good choice for low cost surfacing applications.

18-8不銹鋼熔接棒によるα系鋼材熔接の基礎研究(第1報)

There are many practical applications in which austenitic stainless steel electrodes are utilized in welding of various plain carbon and alloy steels (low and high). Since the above joining is also considered as a so-called disimilar-metal welding from the standpoint of chemical composition, it is naturally expected that penetration ratio of base metal to the austenitic stainless steel weld metal, in other words, the degree of dilution of austenitic stainless steel weld metal by base metal may depend upon several welding conditions, and the mechanical and chemical properties may accordingly he different, too. The present researches were conducted in order to disclose, systematically and fundamentally, the relation between the above properties of austenitic stainless steel weld metal and several welding conditions.In this report, relations between polarity and welding current, amount of penetration and penetration ratio of base metal to the austenitic stainless steel weld metal, hardness of austenitic weld metal and microstructure, etc., in each single bead which was deposited on four kinds of carbon steel plates with plain 18-8 austenitic stainless steel electrode consisting of commercial 18-8 austenitic stainless steel core wire of 4mm. in diameter and suitable coating, were studied. D. C. (straight and reverse polarity) and a. c. were used. Welding speed and thickness of base metal were constant, 203mm/min and 10 mm respectively. The results obtained are as follows :(1) In either d. c. (straight and reverse polarity) or a. c., the amount of penetration and penetration ratio, . etc., have a tendency to increase linearly as welding current increases. And also in this case, the above penetration ratio increases from about 20 to 60 percent as welding current increases from 95 to 175 amperes.(2) Penetration ratio has a little, tendency to increase, as carbon content of base metal increases.(3) Penetration ratio with a. c. is smaller than with d. c., in the same welding current value. (4) There is only a little difference between penetration ratio in d. c. straight polarity and that' in d. c. reverse polarity. In this experiment, however, penetration ratio in d. c. straight polarity is larger than that in d. c. reverse polarity for the range of lower welding current value. On the contrary, penetration ratio in d. c. reverse polarity is larger than in d. c. straight polarity for the range ot higher welding current value.(5) Vickers hardness of austenitic stainless steel weld metal is comparatively high. And Vickers hardness of austenitic weld metal containing high carbon base metal is lower for the range of smaller penetration ratio, but higher for the iange of larger penetration ratio, than that of the metal containing low carbon base metal.(6) Micro-hardness at the side of austenitic stainless steel weld metal of the vicinity of weld fusion line increases linearly as welding current increases, and is higher in the weld metal containing high carbon base metal than in the one containing, low carbon base metal. Consequently, the tendency of the above micro-hardness change is clearly different from that of Vickers hardness change.(7) Both maximum Vickers hardness of heat-affected zone and micro-harness of the vicinity of weld fusion line of that same zone in base metal, tend to decrease as welding current increases.