A Study of Nondestructive Weld Quality Inspection and Estimation during Resistance Spot Welding

The resistance spot welding process has been widely used for sheet metal fabrication in various fields including automotive industry due to its convenience and low cost. A minimal number of weld spots is required to maintain the strength and, in turn, increase the productivity. The destructive tests, in general, are used to examine the qualities of welded zone, but these cannot directly be performed during welding. In order to overcome this problem, a new approach, the real time non-destructive resistance spot welding inspection method, is proposed. This approach monitors the dynamic resistance, and estimates the nugget formation and weld quality during welding. Introduction Resistance spot welding has occupied an important position in the sheet metal fabrication process. The reliability of each spot has been becoming increasingly critical to produce structures of reliable strength with a minimal amount of parts and spots. Weld inspection, in general, adopts both destructive and non-destructive methods using randomly extracted samples. In current production systems, however, the quality of only some of the extracted sample spots can be estimated through such traditional methods, thus making it difficult to directly examine and fix problems occurred in all of the weld parts. Real-time weld quality inspection is thus necessary in order to solve these problems, and development of an estimation algorithm and formation of a system have become very important. Recent researches have attempted to develop an artificial intelligence(AI) system, which uses a fuzzy or neural network and complements the defects of extant estimation systems. The lack of perceptibility due to the process automation of such systems is predicted to be adequately supplemented. After a few introductory researches[1], the resistance spot weld quality has been estimated based upon several studies using electrical or mechanical measurement such as welding voltage and current, dynamic resistance, and the rate of electrode separation as monitoring variables[2,3]. Various examinations of nugget formation have also been performed[4,5] through analytical and experimental methods. Dickinson et al.[6], in specific, observe the relationship between the dynamic electrical factors and the formation of the weld nugget based upon the pattern changes of the dynamic resistance. In addition, researchers continue to examine the weld quality using the above dynamic factors[7,8]. A research has recently been conducted using the neural network on resistance spot welding, and Brown et al.[9] use this method to estimate the nugget diameter, which is closely related to weld shear strength, while Dilthey et al.[10] employ a similar method to estimate shear strength. Relative dynamic resistance is used in this study to estimate the weld quality and classify the various patterns of the dynamic resistance, and the relationship of the classified patterns and weld quality are Corresponding Author: Sehun Rhee Key Engineering Materials Online: 2004-08-15 ISSN: 1662-9795, Vols. 270-273, pp 2338-2344 doi:10.4028/www.scientific. net/KEM.270-273.2338 © 2004 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific. net. (Semanticscholar.org-13/03/20,17:38:23) Title of Publication (to be inserted by the publisher) consequently analyzed. As a result, an algorithm of weld quality estimation, one of the artificial neural networks called the ‘ART1’ network, is developed. Resistance Spot Welding Process Resistance spot welding is based upon Joule heating by passing a short-time large current across the electrode. Welding process is proceeded through four steps as follows. First, a welding part takes squeeze cycle, which is pressurized and stabilized by pressure of electrode prior to the welding current flow. If this period passes, welding current is applied through the workpiece. The contact resistance between the metal sheets produces heating, and the temperature of contact area between the metal sheets rapidly increases once heating begins. Nugget in the shape of stone checkers, which is molten pool, is thereby formed on the joining surface. This process facilitates the melting nugget to be stably solidified after welding current is intercepted and pressurization continues for a certain period of time. Electrode is detached upon the completion of this process, and it finishes the welding process on the welding part and moves to the next welding point to repeat the process. Dynamic Resistance The resistance spot welding process is controlled by welding time, welding force, and welding current in specific, and this resistance change is known as the dynamic resistance in resistance spot welding. Dynamic resistance is a variable, which can be measured while welding is in process with important welding process parameters containing welding process information. In addition, the dynamic resistance can be measured through electrical signal monitoring, and has the advantage of comparatively easier way of measuring than other mechanical signals. Fig. 1 shows nugget formation mechanism in accordance with general dynamic resistance form. Relation with dynamic resistance and nugget formation retains a significant implication: the biggest advantage of this value is neither the post-welding result nor the establishment of welding condition prior to welding, but the result ‘during’ the progress of welding. Step I shows a sudden resistance decrease by surface breakdown at an early current. It is difficult to see the decrease since it disappears very quickly under the general welding condition where preheating is not available. The resistivity increases in the work piece as the temperature rises on the faying surface in step II. An extreme point called α peak, which forms homeostasis, thus appears. In step III, resistance rises continuously with a resistivity increase, and melting begins in contact area between two sheets in step IV. The extreme β peak appears as the decreasing resistance value by shortening current path distance is higher than the increasing resistance value by rising resistiviy. After this point, the dynamic resistance is decreased by faster shortening current path distance by plastic deformation, and an expulsion is occurred when the splash force of molten metal is bigger than the supporting force of surrounded metal of the nugget in step V. Therefore, the dynamic resistance is a process variable containing weld quality information, and a weld quality estimating algorithm using dynamic resistance was developed in this study. Key Engineering Materials Vols. 270-273 2339