Resistance Welding
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RESISTANCE WELDING Fundamentals and Applications RESISTANCE WELDING Fundamentals and Applications Hongyan Zhang Jacek Senkara Boca Raton London New York A CRC title, part of the Taylor & Francis imprint, a member of the Taylor & Francis Group, the academic division of T&F Informa plc. Published in 2006 by CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487–2742 © 2006 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group This edition published in the Taylor & Francis e-Library, 2006. “To purchase your own copy of this or any of Taylor & Francis or Routledge’s collection of thousands of eBooks please go to www.eBookstore.tandf.co.uk.” No claim to original U.S. Government works ISBN 0-203-49752-X Master e-book ISBN ISBN 0-203-61639-1 (OEB Format) International Standard Book Number-10:0-8493-2346-0 (Print Edition) (Hardcover) International Standard Book Number-13:978-0-8493-2346-1 (Print Edition) (Hardcover) Library of Congress Card Number 2005049419 This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. A wide variety of references are listed. Reasonable efforts have been made to publish reliable data and information, but the author and the publisher cannot assume responsibility for the validity of all materials or for the consequences of their use. No part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www.copyright.com (http://www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC) 222 Rosewood Drive, Danvers, MA 01923, 978–750–8400. CCC is a not-for-profit organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe. Library of Congress Cataloging-in-Publication Data Zhang, Hongyan, 1962– Resistance welding: fundamentals and applications/Hongyan Zhang and Jacek Senkara. p. cm. Includes bibliographical references and index. ISBN 0-8493-2346-0 1. Electric welding. I. Senkara, Jacek. II. Title. TK4660.Z43 2005 671.52213–dc22 2005049419 Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com Contents Chapter 1 Welding Metallurgy 1 Chapter 2 Electrothermal Processes of Welding 19 Chapter 3 Weld Discontinuities 58 Chapter 4 Mechanical Testing 107 Chapter 5 Resistance Welding Process Monitoring and Control 146 Chapter 6 Weld Quality and Inspection 196 Chapter 7 Expulsion in Resistance Spot Welding 229 Chapter 8 Influence of Mechanical Characteristics of Welding Machines 287 Chapter 9 Numerical Simulation in Resistance Spot Welding 339 Chapter Statistical Design, Analysis, and Inference in Resistance 373 10 Welding Research Index 427 1 Welding Metallurgy Welding is a metallurgical process—all aspects of a welding process can be, more or less, related to metallurgy of the materials involved in welding, either the base metal or the electrodes. There are a number of books dedicated specifically to welding metallurgy.1–7 Although most of them are on fusion welding, the general metallurgical principles are applicable to resistance welding. In this chapter, the metallurgical principles governing the various aspects of resistance spot welding are discussed. They are critical in understanding the formation of the structures of a resistance spot welding (RSW) welded joint, the mechanisms of defect formation, and their impact on a weld’s strength. For additional information regarding the metallurgy in resistance spot welding steels and aluminum, the reader is referred to the recommended readings listed at the end of this chapter. 1.1 Solidification in Resistance Spot Welding The cast structure of ingots in the sheet materials used in RSW, such as steels or aluminum alloys, is deliberately modified by hot or cold working, such as rolling and heat treatment operations. In the process, grains are refined through cold working and recovery/recrystallization, and structures are homogenized through solution annealing or quenching and tempering. However, such operations are difficult to perform in welding, especially in RSW, as melting and solidification occur between two sheets. Welding parameters, such as hold time and post heating, may be altered to a certain extent, in order to treat the welded structures. However, due to the steep temperature gradient in a weldment, the high cooling rate, and the short time span, such a treatment is not comparable to the controlled heat treatment processes of the parent sheets. Therefore, the microstructures and properties of a weldment are generally not as optimized as in the base metal. During welding, solidification of a liquid nugget is similar to that in a metal casting. It consists of two steps: nucleation of a solid phase and subsequent crystal growth, same as solidification in an ingot mold. The crystallization process is controlled by the heat dissipation into the base metal and the electrodes. The direction and rate of cooling, in addition to the alloy’s composition, decisively affect the type, size, and orientation of the crystals formed. During solidification of a liquid nugget a change of alloy composition takes place in the crystals being precipitated, compared to the original composition of the Resistance Welding 2 alloy In the case of a very rapid cooling of a spot weldment, the diffusion rate in the precipitated crystals is not sufficient to achieve an equilibrium composition distribution, and crystals consisting of layers of different chemical compositions are formed. A crystal nucleus that solidifies first is rich in some elements and lacks others, while the outermost layer of the crystal is just the opposite. This process is called microsegregation. The difference in composition between the core and outer layer of a crystal increases with increasing distance between the liquidus and solidus lines in the phase diagrams, and decreases with increasing diffusion rate and the time span for solidification. In addition to microsegregation, which occurs in the scale of crystals, segregation also takes place as the solid-liquid interface advances into the liquid, as solidification proceeds, and results in enrichment in concentration in the remaining melt of alloying elements. Some of the elements form eutectics of lower melting temperature that exist in the liquid state, mainly around the central portion of a nugget after it is cooled to a temperature below the solidus but above the eutectic temperature. Examples of such eutectics are Al-Cu, Al-Mg, and Al-Mg-Si in aluminum alloys, and certain compounds such as sulfur and phosphorous eutectics in steels. Because of their lower melting temperatures, they are the last bits of liquid to solidify, mainly existing at grain boundaries, as they are rejected from solid solution due to reduced solubility during cooling. Grains surrounded by such liquid at the boundaries can be torn apart as the liquid has no strength when they are stretched, either by external loading or thermal stresses as in the case of fusion welding. However, such cracking rarely occurs in RSW, due to the pressure from the electrodes if proper electrodes and welding schedule are used. The formation of different crystals, including dendrites and globular and cellular crystals, is determined by the composition and heat transfer through the liquid-solid interface. Solidification occurs when the liquid nugget reaches the liquidus temperature of the alloy and there is a net heat loss in the liquid; i.e., the heat dissipated from the liquid is greater than that into the liquid. Under proper welding conditions, the water- cooled electrodes act as a large heat sink during welding. The parent sheet metal also transfers heat from the sides. Partially melted grains in the mushy zone at the nugget- HAZ (heat-affected zone) boarders may serve as nuclei for solid grains to grow. Further cooling results in columnar grains in directions approximately normal to the fusion line. The remaining molten metal in the central portion of the nugget solidifies last and forms equiaxed grains due to cooling when the liquid volume is considerably small after its surroundings are solidified. Depending on welding schedules and other conditions, various structures can be formed in a weld nugget. The ideal scenario is when solidification occurs in a uniform manner Welding Metallurgy 3 FIGURE 1.1 Schematic structure of a solidified nugget created through idealized uniform cooling. from all the sides of the surrounding solid, in both the electrode and sheet directions. A schematic of such a solidified nugget is shown in Figure 1.1. Shrinkage cracks or voids, if created, tend to be close to the last liquid to solidify. Section 6.1 shows that such an imperfection in the central portion of a nugget has no apparent adverse effects on the performance of a weldment. When cooling is not even, such as when improper welding schedules are used or undesirable thermal barriers exist, cracks or voids may result, especially when there is a large volume of shrinkage. Figure 1.2 shows a nugget with long columnar grains grown in the electrode direction, i.e., from the top and bottom, and relatively small and short grains from the sides of the nugget. This could happen when the electrodes are overcooled. As the solidification rate is faster in the vertical direction, the last bit of liquid is squeezed to the location close to the original faying interface of the sheets.