Authors

Carpenedo, Marcelo^a. mcarpenedo@gmail.com

Gonzalez, Arnaldo R^b. ruben@mecanica.ufrgs.br

Reguly, Afonso^a. reguly@ufrgs.br

^a Department of Materials Engineering, Federal University of Rio Grande do Sul. Av. Bento Gonçalves, 9500 – Sector 4 – Building 74 – Room 211, 91501-970, RS, Brazil.

^b Department of Mechanical Engineering, Federal University of Rio Grande do Sul. Rua Sarmento Leite, 425 – Room 202 – 2º floor, 90046-902, RS, Brazil.

Corresponding author: Tel.: +55 51-982178461; E-mail address: mcarpenedo@gmail.com

Abstract

Semi-automatic welding process is widely used in industry, but has limitations for the correct positioning of weld beads regarding length and spacing when applied intermittently. This paper provides a statistical research that reports the existence of important dimensional variations in the final product of this welding process, analyzing the main factors that have influence over these variances and the lack of resources to mitigate their undesirable effects on the welded joint. To mitigate this gap on manufacturing process is proposed the application of the welding markings, which are physical reliefs applied on the welding edge of the joint aimed to orient the welder to apply the weld beads in the correct position, precisely. The experimental results showed a solution that provides significant gains to the welding process, presenting a welding technique that mitigates most of the process variables with influence over the dimensional precision of the weld beads.

Keywords

Semi-automatic weld, Weld length, Dimensional precision, Process improvement

Abbreviations

L_p – Specified weld bead length

L_pm – Verified weld bead length applied with joint markings usage

L_psm – Verified weld bead length applied without joint markings usage

L_e – specified weld bead spacing

MAG – Metal Active Gas

1-Introduction

The semi-automatic welding process, despite widely used in the industry, has an intrinsic characteristic which is the movement control of the welding gun strictly dependent by the operator’s skills. This condition generates restrictions regarding the achievement of desired dimensions of the weld beads, which has relation to their length and spacing.

Currently there is a lack of continued studies about the resulting structural effects of the dimensional precision of the weld beads for the semi-automatic welding process applied intermittently, having no references about the state of art and solutions to improve this manufacturing condition. Welding standards provide some degrees of variation about weld beads length and spacing, but for some applications these range of tolerance can bring some structural negative effects for the joint.

This scenario shows the need for improvements in this welding process, conducing to the development and evaluation of new techniques to apply intermittent weld beads under semi-automatic handling. The weld markings, which are physical reliefs applied on the weld edge of the joints, are the solution proposed to orientate the welder about the correct weld bead positioning, eliminating variances from the process that causes imprecise weld start/end placing.

This study has been based in a statistical investigation about the improvements obtained by using the weld markings compared to the state of art through ANOVA assessment, applying experiment project factorial 3² with three replications. The study was carried out evaluating the welding markings effects on the weld bead length and positioning, comparing the same analysis applied to the state of art of semi-automatic welding technique, both using conventional MAG – Metal Active Gas – and intermittent weld beads.

ANOVA methodology was carried out to statistically determine the significance of each factor evaluated, weld bead length – L_p and weld bead spacing – L_e, by p value analysis. Once welds applied with markings and by state of art have no relation between their results, inference statistic method was carried out to compare the dimensional accuracy of the welds applied by both process regarding its dimensional variations.

2-Revision

Along welding process evolution regarding consumables and equipment, especially considering the guidance to the process automation and robotization [3-4], the semi-automatic MAG welding process continues to be widely used in the industry [5], mainly when applied for intermittent weld beads. However, one characteristic remained unchanged since the beginning of the application of this welding process in the industry, which is the weld edge shape.

Weld quality regarding dimensional precision, when applied by semi-automatic process, is a direct result of the welder’s skills [6], once the weld bead positioning is defined by superficial references, tooling or just by the welder ability and experience to handling the weld gun. These characteristics have direct relation to the joint quality, reported by [7] and [8] as an aspect with increasing industrial concerning in the current economic scenario.

Complementing the welding quality concerning, [9] and [10] carried out studies to verify the influence of the weld bead positioning on the structural rigidity and mechanical resistance of the component, which demonstrated direct influence on these characteristics on the mechanical resistance of the welded joint.

Following the recommendations of welding standards [11] and [12] the weld sizes and lengths should not be lower than that specified in project, reinforcing the need to achieve the correct weld length and, consequently, the mechanical requirements designed for the joint. These criteria conduces designers and engineers to search for solutions, on shop floor or design departments, to achieve the standardized recommendations for a safe, but more related to a standardized, product delivering.

The weld marking is a solution developed to fix the issues about weld length and positioning accuracy, designed to be applied to intermittent welds in sheet metal parts, thin or thick, cut by laser process. It is a relief added on the welding edge of the parts during the component cutting process, which serves as a guidance for the welder about the correct places to start and end the weld bead.

This paper provides data about a welding solution to prepare and use sheet metal parts to be welded in order to improve the dimensional accuracy of the weld bead length and positioning, applying physical markings directly on the design phase, enabling the welder to follow visible features to ensure the weld bead positioning during welding process.

In this work the three-level factorial experimental project was carried out to investigate the influence of the factors L_p and L_e on the weld bead body produced by GMAW, followed by RSM analysis, applied to predict the weld bead length with respect to the tested factors (L_p and L_e), followed by a statistical inference analysis, aiming to verify the dimensional variation among both welding techniques.

3-Experimental Procedure

The current research considers the analysis of two factors regarding the weld beads, L_p (weld bead length) and L_e (weld bead spacing) and three dimensional levels for each factor, evaluating the interaction between them and the effects of each combination on the responses for the weld bead positioning accuracy based on its technical specifications. Both factor’s evaluations were performed using two welding techniques for semi-automatic MAG process:

1 – The proposed welding technique, where the positioning of the intermittent welds are defined by the presence of markings, orienting the welder about the exact places to start and end the weld beads, as show in the figure 1.a;

2 – The current welding technique used on the industry, applying intermittent welds direct on the welding edge of the sample, as shown in the figure 1.b.

The analysis has the aim of compares the results to quantify statistically how the joint markings can improve the current welding process regarding its dimensional precision.

Figure 1: Weld edges characteristics for both welding techniques: a) proposed, using the joint markings and b) current, with flat weld edge.

Figure 1.a shows the welding edge with the presence of the markings, conferring references for the welding gun positioning about weld start and end positions. Figure 1.b shows the weld edge in the current shape, with a flat and simple edge.

For the statistical analysis was performed a factorial design at three-levels with three replications 3², submitting the results to analysis of variance – ANOVA to analyse the relevance of each factor assessed and concluding by statistical inference method, observing how the factors are influenced by itself, without influence of neighbouring. The factor’s levels selected for the experimental procedure were defined based on common values for intermittent weld bead dimensions regarding length and spacing applied in the industry, applied in plates with thickness of 3 mm, defining the weld bead lengths of 30, 40 and 50 mm and spacing of 30, 50 and 70 mm, as show in Table 1.

Table 1. Coded and natural levels of the design factors and levels.

Table 2 shows the factors and levels combination and their respective responses, where L_pm is the response for weld bead length with pre-markings and L_psm is the response for weld bead length for the current technique. Both L_pm and L_psm responses are results from L_p and L_e combination, according each run step.

Table 2: Summary of the 3^k factorial design combinations and the experimental results of L_pm and L_psm.

            The samples were welded using cold rolled sheet metal plates based on standard ASTM A1011 Grade 50 (American Society for Testing and Materials, 2008), obtained as strips with width, length and thickness by 25,0 mm, 400,0 mm and 3,17 mm, respectively. Material chemical composition and mechanical properties are shown in tables 3 and 4, respectively, as specified in the standard.

Table 3: Base metal chemical composition for the steel Grade 50. Values in weight %. Source: ASTM A1011 (2008).

Table 4: Base metal mechanical properties the steel Grade 50. Source: ASTM A1011 (2008).

            The samples were cut by a laser cutting machine brand Cincinnati, model n° CL-7A and welded by semi-automatic method using an ESAB source of power, model LAI 400 with nominal power by 14,6 KVA.

Figure 2 shows the geometry of the samples prior welding, where Figure 2.a shows the basic strip intended for the current welding technique, with flat welding edge. The strip 2.a double combined results in the final sample for the current technique evaluation, without markings. The strip 2.b in combination with 2.a result in the final sample for the proposed technique evaluation, with the joint markings. The strip 2.c shows a strip with markings resulted from the cutting process.

Figure 2: Basic samples resulted by the laser cutting process.

Welding parameters applied on the samples welding are shown in Table 5.

Table 5: Welding parameters used for the samples welding.

            The final welded samples resulted with width and length by 50,0 mm and 400,0 mm, respectively, as shown in the figure 3, where can be seen a sample for the weld done using the markings.

Figure 3: a) Example of sample specification for welding process; b) Example of final welded sample.

In figure 3.a is shown the design specification and dimensions for one of the samples assessed, while in figure 3.b is shown a sample welded, both for the proposed welding technique, with joint markings.

The weld beads were measured using callipers with resolution of 0,05 mm and the results are shown in table 2. The measurements considered the length between the two visible extremities of the weld beads, as shows the figure 4.

Figure 4: Weld bead length measured.

4-Results and Discussions

            From results shown in Table 2, applying ANOVA, were obtained the following data about the current and proposal welding techniques concerning weld bead length and spacing accuracy.

4.1-New Welding Technique Proposed.

An ANOVA was carried out to quantify the relative importance of each factor (L_p and L_e) and also its interaction on L_pm results. The contribution percentage was calculated by the ratio of the sum of squares (SS) of the selected factors and total sum of squares. Table 6 shows the results for ANOVA performed with the acquired data considering an interval of confidence by 95%. The factors are physically significant when its percentage of contribution is greater than the error associated, which is by 5% I this assessment.

The results showed in this table are in agreement with the results obtained in figure 7 in which L_p (mm) is the most important factor affecting L_pm (84,1%). L_e (2,5%) itself has no significant influence on the L_pm response for the selected range of factors, followed by L_p x L_e interaction (1,2%). The total contribution rate of L_p and L_e factors over L_pm was 87,8%.

Table 6. ANOVA of L_pm values.

The significance of L_p indicates the effectiveness of welding markings about weld beads dimensional control, where both length and spacing are not affected one by other. The only visible influence on weld beads length variation occurs regarding its own length L_p, which is defined by design.

            Figure 5 shows L_pm results according to L_p specification, showing the accuracy level of the proposed welding technique regarding L_p.

Figure 5. Main factor L_p effects over L_pm.

On Figure 5 is observed that in all three levels (L_p) evaluated L_pm remained slightly above L_p specifications, with the responses keeping the variances increasing inversely proportional to the levels increasing. This means that as greater is L_p greater is the weld bead dimensional accuracy.

            Figure 6 shows the average variation of L_pm as function of L_e values.

Figure 6. Main effects plot for L_pm x L_e.

            On Figure 6 can be observed that L_pm tends to keep its dimensionally constancy as L_e increases, remaining without influence from external factors. A subtle characteristic can be observed due a smooth L_pm decreasing with L_e increasing, following the trend in figure 5 and reinforcing that no external factors affects the weld bead variation in length when applied using markings.

            Figure 7 shows Lpm variations according its specification L_p in combination to L_e.

Figure 7. Interaction factors (L_p x L_e) plot for L_pm.

Figure 7 shows that L_pm follow a dimensional variation proportionally constant to L_p increasing, independently of L_e specification, with L_pm remaining lightly above L_p except for a specific point where L_p = 50 mm combined with L_e = 70 mm resulted in L_pm beneath L_p, representing 33,3% of sampling for L_p = 50 mm (runs 9, 16 and 17 from table 2).

Figures 5, 6 and 7 showed that the responses for each L_pm level evaluated follow a constant dimensional variation according to L_p specification, not affected by L_e and remaining above L_p specified values predominantly in most of cases.

4.2-Current Welding Technique.

An ANOVA was also carried out to quantify the relative importance of each factor (L_p and L_e) and also its interaction on L_psm results for the current wending process, without markings.

The results showed in this table are in agreement with the results obtained in figure 10 in which L_p (mm) is the most important factor affecting L_pm (86,3%). L_p x L_e interaction has an relevant significance (4,4%) and L_e (1,3%) itself has no significant influence on the response L_pm for the selected range of factors.

Table 7 shows the analysis of variance (ANOVA) for L_p data by the current welding techqnique, without markings usage.

Table 7. Analysis of variance for L_psm data.

Can be observed that, for the welds done without markings (current welding process), L_p factor is also significant for L_psm control. However L_p x L_e, despite has no statistical significance for the responses evaluated, is considerably near to the significance border control. This condition takes to the consideration that its statistical no significance can be revised, considering it for this situation as significant due the results obtained for the comparisons L_psm x L_e and L_psm x L_p x L_e shown on Figures 8 and 9, respectively, where L_p presents a tendency to influences L_psm dimensions as function of L_e.

The significance of L_p x L_e, besides L_p, indicates the influence of external factors related to the dimensional variations not only from L_p, but also L_e. In this case, as there is no physical limitations to define the weld bead start and end, its positioning is defined regarding nearby geometries as weld beads or other joint components, besides the welder skills.

            Figure 8 represents L_psm positioning according to L_p specified, showing the lack of dimensional precision of current welding technique regarding L_p values specified. These results reflect the current scenario of the semi-automatic welding process applied on intermittent welds.

Figure 8. Main factor L_p effects plot for L_psm.

On Figure 8 can be observed that all weld beads – L_psm – resulted in lengths below that specified for L_p, showing that the current welding technique has a consistent tendency to not achieve project specifications for the weld bead length. Only in one case (run 22 from table 2) L_psm remained above Lp specification, representing 11,1% of cases for L_p = 30 mm. Is observed also the responses keeping the variance increasing directly proportional to the levels increasing (inverse of observed in figure 5), what means that as greater is L_p greater is the weld bead dimensional variation.

Figure 9 shows the average L_psm values as function of L_e values.

Figure 9. Main effects plot for L_psm x L_e.

            Can be observed on Figure 9 that L_psm average values increase as L_e increases, showing a direct relation between the weld bead length and its neighbor elements. Compared to the figure 6 L_psm averages are consistently lower than L_pm, showing an evidence of the dimensional accuracy discrepancy between the two welding techniques.

Figure 10 shows L_psm variation according its reference L_p in combination to L_e.

Figure 10. Interaction plot for L_psm x L_p and L_e.

Figure 10 shows that, for L_e values by 50 mm and 70 mm, L_psm follows a proportional variation regarding L_p, however for L_e value of 30 mm is observed a dispersion on L_psm values, what indicates the influence of external factors on L_psm length.

The most important observation in Figure 10 is that 100% of L_psm cases resulted below L_p specifications, meaning 100% of welds out of specification.

The influences of external factors are connected to L_psm regarding welder’s perception during weld application, once the presence of geometrical characteristics near the next weld bead to be applied has an important influence on the welder decisions regarding start/end weld bead points.

4.3-Statistical Inference

Despite the promissory results with the joint markings usage is important take in count that the significance of the results are dependent of the welding methods evaluated, having no relation between them. To analyse this relation and compare its influence above the efficiency of the proposed welding technique the data results was assessed under inference statistic approach, looking for the effective difference between the two methods evaluated. The analysed numbers are shown in Table 8, derived from table 2.

Table 8: Data for statistical inference analysis.

            Is observed that the averages µ are significantly greater for L_psm regarding L_pm, what confirms the superior accuracy of the welding technique using the pre-marking.

Figure 11 shows the graphic analysis for both factor levels evaluated, comparing L_pm x L_psm for L_p = 30 mm, 40 mm and 50 mm on figures 11.a, 11.b and 11.c, respectively.

Figure 11. L_pm and L_psm comparison for the three levels analysed.

            Can be seen a predominant data pattern for both weld techniques assessed and no dependent of the level evaluated, where the weld beads applied with the pre-markings remained above the specified line in most cases, meaning the predominant achievement of the project specifications. For the three levels evaluated was observed that in a confidence level of 95% the averages from L_pm exceed L_psm.

            This analysis corroborates the superiority of the weld beads applied with the joint markings usage, granting a higher dimensional precision to the component, which invariably means a quality increasing in process costs and mechanical properties of the component.

5-Conclusion

The welds applied without the joint markings resulted in 3,7% of the samples with L_psm values above L_p, which means that 96,3% of the samples not reached the design specifications. The opposite situation was observed for the welds applied with the markings, where 92,6% of the samples reached the design specifications.

            The dimensional variation of the weld length, when evaluated without the markings usage, resulted in numbers significantly influenced by weld bead extension and its distance to the next joint elements. L_p and L_e specification affects directly L_psm values, which are dependent of the length, position and weld edge extension. When the dimensional variation is evaluated with the joint marking usage L_e don’t affect L_pm values, what makes L_pm variation independent of the weld bead distance with the next element.

            The usage of the joint markings allows the welder to have references about physical limitations dedicated exclusively to orient him to the correct weld gun start and end positioning, ensuring that the weld start point be positioned lightly before the weld marking and the end point be lightly after the marking. These characteristics offer to the semiautomatic welding process a higher dimensional precision to the dimensional accuracy of the weld bead length and spacing when compared to the current process, resulted from the controlling methods currently available.

Authors’ contributions

MC: formal analysis, writing – original draft, methodology. ARG and AR: data curation, supervision, writing – review & editing.

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

6-References

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