Soldadura gmaw

Materials & Design
Materials and Design 28 (2007) 1623–1631 www.elsevier.com/locate/matdes

Analysis of the influence of shielding gas mixtures on the gas metal arc welding metal transfer modes and fume formation rate
I. Pires
a

a,*

, L. Quintino a, R.M. Miranda

b

IST, Seccao de Tecnologia Mecanica, Av. Rovisco Pais, 1049-001 Lisboa, Portugal b ´ Universidade Aberta, R. EscolaPolitecnica 147, 1269-001 Lisboa, Portugal Received 12 July 2005; accepted 8 February 2006 Available online 30 March 2006

Abstract The development of shielding gases for welding applications has been of increasing interest for two main reasons: to improve the productivity and the operating characteristics of the process and to reduce the healthy and safety problems due to fume and particleemissions. The present paper outlines some of the most important features of seven shielding gas mixtures used and gives information about the influence of these mixtures on the process characteristics, namely on the metal transfer modes and fume emissions. The focus of this work is an experimental study of shielding gases aimed at analysing arc stability and transfer modes, as well as, fume formation,having in view the achievement of a better working environment for welders. Ó 2006 Elsevier Ltd. All rights reserved.
Keywords: D. Shielding gas mixtures; Metal transfer modes; Fume formation rate

1. Introduction The gas metal arc welding (GMAW) process has been dominating the welding construction world for several years now. This fact is related to its high flexibility, which allows the weldingof a great variety of materials and thickness, and to its considerable potential for automation and robotization [1]. Nevertheless, applications of the process lack scientific background and are often based on empirical approaches, which can solve some specific problems, but do not result in more general solutions. The need to improve the process performance requires a clear and detailed study ofthe basic phenomenon and of the mechanisms involved in this process. The physical phenomena involved in this welding process are of complex nature, requiring knowledge of electricity, magnetism, hydrostatics and fluid and gas dynamics [2].
Corresponding author. Tel.: +351218419001; fax: +351218419058. E-mail addresses: inespires@ist.utl.pt (I. Pires), rmiranda@univ-ab.pt (R.M. Miranda). 0261-3069/$- see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2006.02.012
*

The development of welding shielding gas mixtures in recent years has been based on the need to establish a stable arc, to obtain a smooth molten metal transfer and to reduced fume emissions, which will improve process performance, productivity and control, and will reduce the risk of fusiondefects. Process productivity and quality, essential factors when considering automation and robotization, are strongly dependent of the shielding gases and mixtures used. It is well known that the chemical composition and the amount of the fumes produced depend on the welding process, filler and shielding gas composition, and of the welding parameters. 2. Background 2.1. Metal transfer modes The metaltransfer mode is influenced by the type of the filler wire, voltages and current intensities range, electrode polarity and shielding gas. For a better understanding of

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the effects of the several factors it is necessary to consider the physics of the transfer process [3,4]. There are, basically, two theories to describe the metaltransfer: static force balance theory and pinch instability theory [5–8]. Pinch instability theory only explains the general tendency for the decrease of the droplet size with the increase of the current intensity and the conditions for the instability of the molten metal column. The static force balance theory allows a better explanation of arc phenomena, and is the one that will be considered...