The influence of double pulse on porosity formation in aluminum GMAW

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Abstract

Pulsed GMAW (P-GMAW) has been recognized as an efficient alternative for minimizing porosity in aluminum welding. The double-pulsed GMAW (DP-GMAW) technique is a variation of the pulsed GMAW technique, in which the pulsing current aimed to metal transfer control is overlapped by a thermal pulsation, which in turn means pool control (similar to pulsed GTAW). Despite some advantages claimed by this reasonably new technique, one could expect more porosity when compared with P-GMAW. Thus, in this work a comparative study on the performance of these techniques at different parameter combinations and conditions favorable to generate porosity is presented. Porosity quantification was assessed by gravimetric and radiographic procedures, while the pores were characterized by macrographic analysis. The results indicate that the DP-GMAW technique maintains the capability of porosity minimization credited to the conventional P-GMAW technique and its advantages can be better exploited in industrial applications.

Introduction

The choice of a specific welding process must take into account some requirements, such as joint quality, levels of productivity and the costs involved in equipment operation and acquisition. Unfortunately, the search for improvements on one of these necessities may lead to the detriment of others. For instance, trials on increasing productivity can lead to drop in quality, and vice-versa. Thus, to better qualify the term, quality would be the result of the capability of the process in adequating to operational barriers, which are inherent to each situation. In the case of aluminum welding, one remarkable barrier would be the tendency for porosity generation.

It is widely recognized that hydrogen is the dominant cause of porosity in aluminum alloy welds. Gases such as oxygen and nitrogen present lower solubility in molten aluminum in comparison to hydrogen [1] and, as a consequence, they usually play no important role in porosity generation. However, in a micro gravity environment with vacuum, these gases can assume importance. In a study on aluminum welding in this environment, Fujii et al. [2] carried out experiments with electron beam and GTAW processes and some unexpected results emerged. These workers report that in all runs, including the ones carried out in bead-on-plate condition, several pores were observed. This porosity was linked to the formation of Al2O gas bubbles at high vacuum (the oxygen would come from the superficial oxide layer).

One of the most comprehensive reviews on porosity in aluminum alloy welding was published two decades ago, by Devletian and Wood [3], and it can still be considered up-to-date. These reviewers cite that the ratio between the maximum solubility of hydrogen in superheated molten and in the metal at the solidification temperature is far higher for aluminum than for any other structural materials, despite the disagreement among researchers on solubility data. These authors believe that the varying solubility data is the influence of alloy additions. They also show several examples of the alloying elements influences on the porosity concentration after welding. That means that porosity in aluminum welds is very much related to the bead chemical composition.

The most important source of hydrogen entering the arc column of welds includes: hydrogen contained with the filler metal and parent metal; hydrogen bearing contamination or hydrated oxide films on the surface of the filler and base metals; hydrogen or water vapor in the shielding gas [3]. Thus, cleanliness of wire and workpiece surfaces is extremely important for reaching aluminum sound welds.

However, the mechanisms of pores formation in aluminum welding is based not only on entrapped hydrogen. Hooijmans and Den Ouden [4] remark that the hydrogen concentration in the weld pool at any moment is the result of two mutually independent processes: inflow and outflow of hydrogen. That is, porosity is a consequence of a dynamic equilibrium between the rate of hydrogen absorption and rejection. And the control of this equilibrium is not an easy or predictable task. That is the reason why Devletian and Wood [3] state that size, shape, distribution and amount of hydrogen pores generated in the weld are generically dependent upon the solidification mode, cooling rate, welding parameters, bead shape, shielding gas and external pressure. And no conclusive results have been presented upon the effect of, for instance, the welding parameters.

Porosity contents in welds can be measured by different approaches, such as radiographic, gravimetric and microscopic procedures. Morais [5] used the gravimetric method to evaluate the influence of different GMAW metal transfer modes on porosity generation in an AA5052 alloy. He concluded that controlled metal transfer (pulsed GMAW) leads to lesser porosity than welds carried out with the standard transfer modes. The worst results were found with short-circuiting transfer.

Recently, a new welding technique was developed as a derivation of the pulsed GMAW (P-GMAW), in which a low frequency current pulsation, typical of pulsed GTAW, is superimposed a pulsed current for metal transfer control, characteristic of the P-GMAW. This new technique has been named double pulsed GMAW (DP-GMAW) or P-GMAW with thermal pulsation. Commercially it has been referred to by different names, such as Inpulse, Alu-Plus, etc. The DP-GMAW can be considered a means of increasing productivity without quality drops, a fact not always conceived, as shown in the first paragraph.

The principle of the most popular version of the DP-GMAW is explained through the schematic oscillogram presented in Fig. 1. Double pulsation is represented by the alternation of low frequency periods of thermal base (τb) and thermal pulse (τp). During each of the thermal periods, there is a coexistence of a current pulsation at higher frequency, in which the pulse current and time (duration) (Ip and tp) are kept the same and the base current and time (Ib and tb) are adjusted as a function of the desired average currents at each thermal period. The variation of the mean current between τb and τp (Imτb and Imτp, respectively) is synchronized with a wire feed speed variation (WFSτb and WFSτp, respectively), so that the arc length is maintained constant. There is a resultant weighted mean current (Imt) and a weighted mean wire feed speed (WFSt).

The role of a low frequency current pulsation (thermal pulsation) is to control the weld pool. While the heat input is higher (thermal pulse), there is an effective fusion of the parent metal. On the other hand, when the heat input is lowered (thermal base), surface tension and viscosity of the molten metal get higher, making it more difficult for the pool to melt through. There must be equilibrium between the τb and τp duration so that the volume and shape of the weld bead do not get patchy [6].

However, the consequent cyclic variation of the arc dimensions at low frequency can, in theory, mimic arc instabilities, breaking the harmony of the original technique (P-GMAW) and making DP-GMAW more prone to porosity. Thus, it is worthwhile to exploit better the DP-GMAW technique, especially concerning its ability for generating pores, before claiming the advantages of this new technique as a productivity and quality booster. This work aimed to carry out a comparative study between the techniques P-GMAW (considered appropriate concerning porosity generation) and DP-GMAW (claimed for higher productivity), in relation to porosity on AA5052 welding.

Section snippets

Materials and methods

As the objective of this work is to study comparatively the ability of the two techniques for generating pores, conditions more prone to this defect were looked for methodologically, yet not in excess; either too little or too much porosity could disguise the effect of the techniques. However, at the same time, the welds should be close to real conditions, in order to transport the results of this study to workshops. According to Devletian and Wood [3], welding in overhead position is more

Step 1

Table 3 presents the monitored mean values of current and voltage and wire feed speed. It is also presented the values of the parameter Imt/TS calculated from the monitored current and the adjusted travel speed (considering the precision of the gun manipulator, the TS as set can be considered correct). The resulting values of Imt/TS suggest that the heat input was practically the same for all welds. The arc voltage around 24 V for the first run and around 25 V for the capping run depict constancy

Conclusions

In the conditions established in this work, it can be deduced that:

  • The proposed methodology for comparing two welding techniques in relation to porosity generation in aluminum, based on the correspondence of the thermal conditions and on the promotion of pores at a regular amount, is adequate;

  • The double pulsed GMAW (DP-GMAW) technique, in spite of having theoretically higher potential for porosity generation, does not increase the porosity susceptibility in aluminum welding, when compared with

Acknowledgements

The authors would like to acknowledge the importance of the PADCTIII research program, though the CEMAT 620094/97-4 project, due to its financial support. They are grateful to ALCAN and White Martins Gases Industriais, for providing the parent metal and shielding gas, respectively.

Dr. Celina Leal Mendes da Silva is a lecturer at Federal Center for Technological Education of Pará, Brazil. She is graduated as mechanical engineer and received her Dr. Eng. degree in 2003 at Federal University of Uberlandia, Brazil.

References (13)

  • R.J. Shore et al.

    Effects of porosity on high strength aluminum 7039

    Welding J.

    (1970)
  • H. Fujii et al.

    Gas tungsten arc welding under microgravity

    Trans. JWRI

    (2001)
  • J.H. Devletian, W.E. Wood, Factors affecting porosity in aluminum welds—a review, Welding Research Council Bulletin...
  • J.W. Hooijmans et al.

    A model of hydrogen absorption during GTA welding

    Welding J.

    (1997)
  • F.C. Morais, Influence of operational factors on porosity formation in aluminum MIG welding, MSc. Thesis, Federal...
  • C.L. Mendes da Silva et al.

    Assesment of the thermal pulsation periods on the weld bead formation in aluminum welding by the DP-GMAW technique

There are more references available in the full text version of this article.

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    In contrast, a lower thermal base produces higher surface tension and viscosity of molten metal delaying the melting of the weld pool [21]. An equilibrium between duration of τb and τp should be maintained to obtain a regular shape and volume of the weld bead [13]. Firstly, the bead-on-plate weld study using DP-GMAW process was performed on low carbon steel IS 2062 plates of 20 mm thickness.

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Dr. Celina Leal Mendes da Silva is a lecturer at Federal Center for Technological Education of Pará, Brazil. She is graduated as mechanical engineer and received her Dr. Eng. degree in 2003 at Federal University of Uberlandia, Brazil.

Dr. Americo Scotti is a professor in the Mechanical Engineering Faculty at Federal University of Uberlandia, Brazil. He is Mech. Eng., M.Sc. and received his P.hD. degree at Cranfield University (UK) on welding technology. He has been lecturing, researching and consulting on welding for 23 years. He also has 2 years of industrial experience with WMGI, a Brazilian branch of Praxair Inc. His current research interests are related to the application of novel arc welding technologies into industrial fields.

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