Ivan Polajnar

Expulsion detection system for resistance spot welding based on a neural network

Resistance spot welding is one of the most important welding procedures. Therefore, a strong emphasis is placed on the quality of the welds. One of the phenomena that causes the deterioration in quality is the eruption of molten material, the so-called expulsion. Expulsion can be avoided with appropriate parameter selection. The problem, however, lies in the fact that the best quality welds are made with parameters just below the expulsion area. Therefore, for any successful control scheme an efficient and dependable expulsion detection is needed. A linear vector quantization (LVQ) neural network system is proposed to achieve this goal. The network is analysed with different sensor combinations and different materials. The results show that the LVQ neural network is able to detect the expulsion in different materials. The experiment also points to the welding force signal as the most important indicator of the expulsion occurrence.

Overview of resistance spot welding control

Abstract This paper presents an overview of resistance spot welding control. The presentation of the physical background of the resistance welding process is followed by the description of the main problems concerning the appurtenant control theory. Solutions to these problems are presented primarily according to the measured signals used and to the type of control strategies.

Estimating the strength of resistance spot welds based on sonic emission

Abstract Audible sound signals detected during the resistance spot welding (RSW) of zinc coated steels were investigated in order to assess their suitability for estimating the strength of the weld. A new sonic emission indicator was introduced and compared to a commonly used emission count indicator. A new method of spot weld strength estimation based on the two indicators is presented. The advantage of the method is that it makes it possible to establish the stage when the electrode is worn out. The method enables the development of improved RSW process control algorithms.

Influence of welding current shape on expulsion and weld strength of resistance spot welds

Abstract The present paper presents the influence of welding current shape on weld strength of resistance spot welds of zinc coated mild steel sheets. The influence is analysed at different levels of the electrode wear. Welding currents with different peak values and different RMS (root mean square) values were used in the experiment. The results show that welding current with high peak values implies higher weld strength.

A method for measuring displacement and deformation of electrodes during resistance spot welding

A new method for measuring the time-dependent displacement and deformation of electrodes during resistance spot welding (RSW) is described. The method allows assessment of the non-stationary thermal expansion of the electrodes. By measuring the electrode indentation close to the weld-piece surface, the method enables a better estimation of indentation than the present methods that rely on measurement of the displacement of the outside part of the electrodes or their holders. The method is based on using a digital video camera to acquire image sequences of electrode caps in the RSW process. A regular pattern of shallow bores is drilled into the caps to enhance the contrast of the acquired images and facilitate image processing. The distances between the bores are analyzed from the image sequences to determine the temporally and spatially resolved displacement and deformation of the caps. The analysis revealed that the cap deformations in some welding regimes can reach up to 20% of the maximum cap displacements, which are approximately 200 µm. The employed image processing algorithms are presented as well as examples of results that demonstrate the applicability of the method.

Indentation shape parameters as indicators of spot weld quality

This contribution presents a laser-based method for three-dimensional (3D) measurements of the shape of an electrode indentation. The method is based on the illumination of the indentation with structured light and the detection of the image of the illuminated indentation by means of a digital camera. Image processing algorithms are employed to determine the 3D shape of the indentation. A batch of welds obtained from an automotive production process has been measured and analysed for characteristic geometrical indentation parameters such as diameter, depth, presence of cracks, splash, etc. The potential of indentation shape parameters to serve as indicators of spot quality is discussed.

3D Measurement of Electrode Contact Area in Resistance Spot Welding of Coated Steel

The document discusses the possibility of contact area estimation during resistance spot welding employing post-weld 3D measurement of electrode indentation. Experimental welding of a common type of mild zinc-coated steel was conducted using two different electrode tip types varying welding time, RMS current and number of successive welds. The other parameters (clamping force, voltage, pre- and post-weld time, weld-piece thickness) were constant. The results show that this approach gives valuable information on the conditions under which the weld-spot was formed.

Feasibility study of in-process weld quality control by means of scanning laser profilometry

We have developed a scanning laser system based on the optical triangulation principle for the shape measurement of fusion weld surfaces. The system integrates a triangulation module consisting of a laser line projector and a digital video camera with a mechanical scanning stage and an industrial computer. The system is small and rugged, suitable for application in industrial environment. The system can sample a weld surface at a rate of up to 30 profiles per second achieving 0.1mm accuracy. Software was developed which analyses the captured weld surface shape in real time determining the characteristic shape parameters (length, width, height, cross section, volume, starting position,...) which are then used for automated classification of the welds into acceptable and unacceptable. The software also detects surface defects such as undercutting, holes or melt splash, etc. The system has been tested in a robotized welding cell for automotive parts in an industrial production facility. Weld classification obtained by the system was compared to an independent classification determined by a trained weld inspector on the basis of visual inspection and to another one determined on the basis of metallographic analysis of the weld. Using the metallographic based classification as the reference we find that the developed weld inspection system can achieve better classification reliability than a trained visual weld inspector.

Influence of the type of workpiece adjustment and energy input on the quality of a resistance projection weld

The paper describes how the technological issue of assuring stability of circumferential welds in mass production was solved and shows how resistance projection welding substituted for fusion arc welding. In the introduction, key technological reasons for replacing the welding process used are given. The study is supported by a detailed description of physical characteristics of resistance projection welding that assures the desired quality of a particular product. The initial experiments and analyses carried out, allowed limiting the range of the investigation conducted to a comparison of efficiency of welding with two different types of workpiece adjustment. The experimental section further includes a description of welding with front and edge adjustments and the choosing of suitable welding parameters. This is supported by the results of a quality evaluation of welds obtained in a visual examination, tensile testing, and a metallographic analysis.

Mathematical modelling of GMAW process using sound pressure measurements

A burning of the electric arc produces strong side effects such as UV light, heat, vibrations of the burning electrode, acoustic emission, and audible sound. In addition to variable welding voltage and current, a distinctive characteristic of arc welding processes is the pressures of audible sound phenomena which reflect the variable conditions in the arc to the highest extent. Measurements and analysis of audible sound generated by the GMAW are presented in this paper. Results indicate that more information about the welding process is embedded in the sound signal as in the welding current signal or/and in the welding voltage signal. An additional advantage of sound implementation into the control algorithms is simplicity of its detection. The mathematical model for the calculation of sound from the welding current is presented. The model can be implemented for the calculation of welding process parameters in real–time applications.