Journal of Materials Processing Technology 179 (2006) 219–224 Emission spectrometry evaluation in arc welding monitoring system Sadek C.A. Alfaro ∗ , Diogo de S. Mendonc ¸a, Marcelo S. Matos The University of Brasilia, ENM-FT, UnB, Campus Universit´ ario 70.910-900, Brasilia, DF, Brazil Abstract This work describes exploratory experimental procedures implemented for the development of a non-intrusive and real-time sensor for weld defect tracking which uses emission spectrometry for measuring the electromagnetic content of the plasma-weld pool interface in the GMA welding arc. The welding process monitoring is carried out by calculating the iron (Fe) and the manganese (Mn) electronic temperatures within the welding arc column, admitting that the observed region is at local thermodynamic equilibrium. The temperature was calculated by utilising the relative intensity method, which is based on the Boltzmann and the Saha Laws and on the definition of the emission line intensity. The calculated electronic temperatures of the two elements were correlated with the position of welding defects, which have been introduced for simulation purposes. These simulated defects resulted in abrupt changes in the average and standard deviation temperature values, thus providing an indication of the presence of a defect. © 2006 Elsevier B.V. All rights reserved. Keywords: Spectrometry; Welding; Plasma; Welding monitoring 1. Introduction For many years, several different monitoring techniques have been studied, attempting to develop a general technique capa- ble of dealing with the inherent complexity of the arc welding processes. Such studies aimed mainly at developing methods of quality of the welds on-line controlling, in order to prevent the needs for the costly and time consuming post weld inspec- tion processes. The innovations generated by these studies are based on the physical phenomena involved in the arc welding processes, mainly those related to the plasma arc and its influ- ence on the weld pool [1]. The applied techniques range from numerical simulation of the arc [2], image analysis [3], sound spectrum analysis and electromagnetic emission analysis [4–6] to the use of intelligent systems, based on neural networks and fuzzy logic [7]. This work describes a study on the possibility of utilising the electromagnetic emission of the arc welding plasma column for monitoring the presence of weld defects. The proposed method uses the electronic temperature, calculated from the intensities of the emission lines in the electromagnetic spectrum within the visible region, as an indicator of change in the expected quality. Some specially chosen emission lines can also give indication ∗ Corresponding author. E-mail addresses: sadek@unb.br (S.C.A. Alfaro), diogo@ps5.com.br (D. de S. Mendonc ¸a), msmatos@unb.br (M.S. Matos). of contamination of the weld bead with hydrogen. This makes it possible for a control system to act before a deleterious effect can affect permanently the quality of a weld. 1.1. Emission spectrometry and plasma characterisation The term “Spectrometry” stands for a set of experimental techniques used for measuring the electromagnetic spectrum that results from phenomena such as absorption, emission or diffrac- tion of electromagnetic radiation by atoms or molecules. These techniques are generally analytical and, as such, may produce relevant data for the analysis of welding processes [8]. Accord- ing to the Quantum Theory, atoms and molecules can only exist in a steady energy states, which are characterised by discrete amounts of energy that are specific to each atom or molecule. When a change of energy state occurs in an atom or a molecule, their electrons absorbs or emits the specific amount of energy, which is strictly necessary for taking it from one energy state to another. Such change of energy state is generally accom- panied by emission or absorption of light, which wave length, λ, is related to the energy of both states [9], according to the Eq. (1) E i - E n = hc λ (1) in which E i is the energy in the lower state, E n , the energy in the higher state, h, the Planck constant and c is the light speed. 0924-0136/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2006.03.088