Correlation between Peel Test and Real Time Ultrasonic Test for Quality Diagnosis in Resistance Spot Welding

Soldagem & Inspeção. 2016;21(3):282-289 http://dx.doi.org/10.1590/0104-9224/SI2103.04 Technical Papers Correlation between Peel Test and Real Time Ultrasonic Test for Quality Diagnosis in Resistance Spot Welding Angelo F. Andreoli1, Andriy M. Chertov2, Roman Gr. Maev2 1 2 Faculdade de Tecnologia de Sertãozinho – FATEC, Sertãozinho, SP, Brazil. Institute for Diagnostic Imaging Research, Windsor, ON, Canada. Received: Sept. 14, 2015 Accepted: June 07, 2016 E-mails: . br (AFA), (AMC), (RGM) Abstract: This paper aimed to study the correlation between Real Time Integrated Weld Analyzer measurements and peel test results. The experiment involved study of weld nuggets for different combinations of sheet thicknesses, welding time and current of mild steel samples. The intent was to assess the potential use of a real time integrated ultrasonic system in the industrial environment allowing nondestructive evaluation of 100% of the spot welds produced. The results showed high correlation between nugget penetration into the sheets and nugget size measured ultrasonically and destructively, for all evaluated parameters. Key-words: Resistance Spot Welding (RSW); Real-Time Integrated Weld Analyzer; Peel test; Weld quality. Correlação entre Teste de Destacamento e Teste Ultrassônico em Tempo Real para Diagnóstico da Qualidade em Soldagem a Ponto Resumo: Este trabalho teve como objetivo estudar a correlação entre medidas obtidas por Real Time Integrated Weld Analyzer e resultados de teste de destacamento. O experimento envolveu o estudo das lentilhas de solda para diferentes combinações de espessura de chapas, tempo de soldagem e corrente de soldagem, para amostras de aço de médio teor de carbono. A intenção foi avaliar o potencial uso do sistema ultrassônico integrado em tempo real no ambiente industrial permitindo a avaliação não destrutiva de 100% das juntas soldadas. Os resultados mostraram alta correlação entre penetração das lentilhas e diâmetro medidos ultrassônicamente e destrutivamente, para todos os parâmetros avaliados. Palavras-chave: Soldagem a ponto por resistência (RSW); Real-Time Integrated Weld Analyzer; Teste de destacamento; Qualidade de solda. 1. Introduction In resistance spot weld quality assessment, the main criteria evaluated are: diameter of the weld nugget and shear strength. The nugget weld size formed during the welding process can be evaluated by peel test, and there is a correlation between the diameter of the nugget and joint strength [1]. Due to its relative ease, peel test is used as a key criterion of quality in production especially in the automotive sector. One of the disadvantages of using the peel test method is the need for the destruction of the component to be inspected [2], so the evaluation is done by selective sampling. Several non-destructive quality assessment methods for resistance spot welding have recently been proposed, including the Real Time Integrated Weld Analyzer (RIWA), by Tessonics. Such ultrasonic system can evaluate liquid metal penetration and weld nugget size automatically and in real time. This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License which permits unrestricted non-commercial use, distribution, and reproduction in any medium provided the original work is properly cited. The RIWA system is a device capable of performing quality inspection of spot welds during the welding process. A high-frequency transducer is integrated into the welding gun electrode; this transducer generates ultrasonic waves that pass through the cooper electrode cap into the welded plates. These waves are reflected and received by the same transducer. The range of interest is the area between the two plates where the weld will be produced represented as wave reflection 2 and 4 in Figure 1. Several pulses are emitted during the welding process. The time interval between reflections arriving from front and back wall of the sample are called time of flight (TOF) through the sample [3]. The TOF is inversely proportional to Correlation between Peel Test and Real Time Ultrasonic Test for Quality Diagnosis in Resistance Spot Welding temperature. The more material is heated the bigger delay the back wall reflection will experience, a characteristic curve forming the melting process of the plates is shown in Figure 2. Here the top line is the dynamics of the front wall and the bottom line in the middle of the image is representing dynamics of the back wall. During welding, the heat generated in the plates reduces the speed of sound in metal and causes back wall reflections to arrive later. The process reverses when current is shut off. Figure 1. Model of ultrasound setup for RIWA. Figure 2. Ultrasonic B-scan. Soldagem & Inspeção. 2016;21(3):282-289 283 Andreoli et al. The union of all A-scans obtained during the process is treated by software and generates a B-scan that can be seen as a signature of the welding process. Each column in the B-scan represents an A-scan obtained at a certain time during welding. Figure 2 shows an example of a B-scan where the A-scans were taken every 2 milliseconds, in that the image is clear to see that the melting and solidification of the material between the plates. Thus, weld quality can be interpreted through its ultrasonic signature [4]. There was discovered strong correlation between some ultrasonic parameters and the nugget diameter. One of those parameters is time of flight of the ultrasound through the total stack. The more it is heated the higher TOF value is. 2. Experimental Procedures Several specimens were welded using mild steel plates (carbon 0.15 to 0.30 wt. %) with thicknesses of 0.7, 1.35, 1.80 to 1.85 millimeters (mm), apart from using samples with two (2T) or three sheets (3T) joined, also with different thicknesses combination. In order to achieve different weld nugget sizes varying from stick welds to expulsion, representing the extremes in spot welding, the current flow (kilo-amperes) during the welding process varied as well the welding time, but the compression force (pounds) was kept the same, as shown in Table 1. For each test, approximately 20 specimens were welded. The purpose of intentionally produce welds with nugget sizes varying from stick weld to expulsion was to analyze if the ultrasonic system would be able to build confidence data even in situations where welds are not produced according to established standards. Test Plate thickness 1 (mm) Plate thickness 2 (mm) Plate thickness 3 (mm) Time (60 Hz cycles) Lowest current (kA) Highest current (kA) Force (lb)* Table 1. Experimental model. 1 2 0.7 0.85 1.8 1.85 0 1.35 18 20 5.0 6.0 8.6 9.0 605 603 *Force in pounds’ unit. The samples were welded using a resistance spot welder, Medweld 3000s model, Centerline Co., at the Dissimilar Materials Laboratory of the University of Windsor. The equipment is connected to a RIWA system, which uses a 10 (...truncated)