ULTRASONIC NON-DESTRUCTIVE EVALUATION FOR SPOT WELDING IN THE AUTOMOTIVE INDUSTRY

Transcription

1 ULTRASONIC NON-DESTRUCTIVE EVALUATION FOR SPOT WELDING IN THE AUTOMOTIVE INDUSTRY N. Wylie 1, S.R. Wylie. J.D. Cullen and A.I. Al-Shamma a Liverpool John Moores University, RF and Microwave Group, Byrom Street, Liverpool, L3 3AF ABSTRACT The ultrasonic pulse-echo method has been used within the automotive industry for many years. This approach to non-destructive evaluation (NDE) is however an offline method as testing is carried out after welding, during which production must be halted. Research is continually being conducted to determine spot weld quality during production in order to lower material waste and energy costs. This research proposes an online, portable, throughtransmission, continuous wave ultrasonic non-destructive evaluation system, to monitor the quality of spot welds in real time. Ultrasonic transducers are mounted on the upper and lower electrode arms of the spot welding machine. Results are shown that indicate an increase in received signal amplitude upon formation of the spot weld, with a reduction on the occurrence of splash, indicating a poor weld. Results are compared to those taken from destructive tests. Keywords: non-destructive evaluation, spot welding, ultrasonic 1. INTRODUCTION Spot welds are used to join sheets of metal which typically have thicknesses between 0.5mm and 3mm. One sheet of metal is placed on top of another and a current is passed through them using electrodes which are located above and below the two metal sheets [1]. The heat for resistance spot welding is generated by the resistance to the flow of current between the steel sheets, as shown in Equation (1). This contact resistance is dependent upon the surface condition of both the electrode caps and the steel [2]. where H is the heat generated in Joules I is the current in Amperes R is the resistance in Ohms and T is the time the current flows in seconds. (1) The force that the electrodes provide improves the electrical contact and helps contain the molten metal within the joint. The two metal sheets are then fused together at the point and this area is known as a weld nugget, as shown in Figure 1. 1 N.Wylie@ljmu.ac.uk 252

2 Fig. 2: Resistance Spot Welding (Inset: Spot Welding Machine) Weld quality is an important aspect in the automotive industry where the durability of a car is very much dependent upon the weld quality within the body-in-white structure. It is possible that the parts being welded may not fuse completely, or that the area of the nugget may be smaller than required for a strong joint. A weld s quality can be determined by its geometric or physical features, its strength or performance or by inspection of the process during welding. Surfaces with impurities such as grit and grease can lead to unevenly distributed contact between the electrode caps and the sheet steel. This in turn, results in an uneven contact resistance which leads to non-uniform localised heating. This causes deformation of the tip face of the electrode cap which affects the quality of the resulting welds. It is important therefore to ensure that all surfaces are free from such impurities prior to welding. The life time of the electrode caps are also reduced by the use of zinc coatings on the steel to inhibit corrosion [3]. In addition to electrode cap wear indicating poor welds, expulsion during the welding cycle, more commonly known as splash, is a key sign that the welds produced are of an inadequate quality. The presence of splash during welding also promotes electrode cap wear. Given that the cost for carrying out destructive tests is high, there is an incentive to develop non destructive testing methods that are less dependent on skilled operators, as a substantial knowledge of the technique is required to carry out the tests. Current ultrasonic spot weld inspection techniques provide valuable information about weld quality non-destructively, but this is performed manually, offline and after welding has been carried out, and is once again only practicable on random samples. 2. SPOT WELD QUALITY When determining weld quality it is necessary to monitor more than one attribute. These attributes are usually visible after a weld is created or can be made visible by performing destructive tests on the welded joint by using the peel test, the chisel test or even by cross sectioning the weld. Non destructive methods, for example ultrasonic inspection can also be used. Establishing conditions which give an acceptable weld and checking that they are being maintained during a production run is one of the first methods used to help retain weld quality during fabrication. Another simple approach would be to make regular measurements of the electrode tip Destructive testing Of the many different quality control tests that can be carried out on a spot weld, the main tests are the destructive peel test and the chisel test. These tests are conducted on randomly 253

3 chosen welds obtained from the production line. If a poor weld is discovered, the batch to which it belongs is assumed to be faulty and is rejected [4] Peel test A welded joint is peeled apart until the sheets are separated revealing a weld nugget which is then measured to determine whether or not the exposed nugget is of the required standard [5]. A weld nugget s criteria can be easily classified by the use of Equation (2) taken from the British Standard BS14373:2007 [6]. This equation states that the minimum diameter, d, of the revealed nugget after the peel test, should be greater than 3.5 t and no more than 5 t, where t is the thickness of the thinnest sheet within the stack up. If the resultant nugget diameter complies with this equation, the nugget is said to be good, if not then the weld is presumed a failure. The measurements for the weld nugget diameter can then be used to create weld growth curves from which weldability lobes can be formed. These are used to evaluate the welding process. (2) Chisel test When performing the chisel test a tapered chisel is placed in between the welded sheets taking care not to touch the actual weld. The chisel is then forced into the space by means of a hammer, until the joint separates. This results in a pulled weld nugget. This type of testing method is generally used when the peel test is impractical, for example, when welding using thick gauge steel which makes it virtually impossible to use the peel test Influences on weld quality Microstructure of a weld During welding, the micro-structure of the steel in and surrounding the weld becomes altered due to the rapid annealing from the electrodes. As a result of the vast change in heat penetration throughout the welding cycle, the grain structure of steel becomes altered, which can quickly change the strength and reliability of the material. The micro-structure of this steel before and after welding therefore plays an important part in spot weld quality [7]. The metallurgy of the welded joint can be categorized into two major regions, the fusion zone and the heat affected zone (HAZ). In the fusion zone the solidification completely changes the micro-structure of the steel. The HAZ, however, represents the regions that are in close proximity to the weld, where the heat input is insufficient to melt the steel but sufficient to alter the micro-structure [8]. This area is also frequently susceptible to the development of micro-structural defects due to the often weakened mechanical properties and metallurgic processes involved during welding. If the HAZ spreads to the electrodes the probability of electrode failure becomes greatly increased. The micro-structure of a weld is revealed by taking a cross-section of the weld nugget, polishing it to a mirror finish and using Nital (a chemical solution of alcohol and nitric acid) to chemically etch the polished weld nugget. This chemical etching of the weld reveals, amongst other things, the constituent zones of the weld which appear by the intensity of the etching or different colorations. These differences show the structure generated by the welding operation [9]. 254

4 Here the weld nugget width can be established along with the HAZ, the indentation from the electrodes, and the penetration, as shown in Figure 2. This micro-structure is used to help determine weld quality Expulsion Fig. 3: Micro-structure of a Weld Expulsion is the unwanted formation of small metal particles that are expelled from the welding area during the welding process. These small particles, that are visible as hot sparks or splash, are usually thrown into the air, which then solidify to form small balls or filaments but can also remain loosely attached to the weld area, as shown in Figure 3. Loosely Attached Filament Fig. 4: Filaments on the Surface of a Weld At a particular current level the weld diameter will reach what is regarded as the minimum acceptable size (according to the British Standard) [6]. If the current is increased beyond this critical level, the size of the nugget will increase until a point is reached when the electrodes can no longer contain the molten metal. This is when splash occurs. The expulsion causes the electrodes to collapse further into the metal which results in a thinner weld, potentially containing discontinuities, which can spread with vibration and lead to weld failure. Part of the expelled metal is often from the electrode itself, which results in a reduced electrode life. Sheet surface contamination can also lead to splash due to the dissimilar thermal conductivities of the steel and the surface contamination. The zinc coating used for 255

5 galvanisation has a lower melting point (419 C) than steel (1370 C), so this is expelled from the surface before the steel begins to melt. SORPAS was used to simulate welds that produced splash [10]. Figures 4 and 5 show how the splash changes from being expelled from within the weld to being produced from the electrode cap-sheet interface, indicating that part of this expulsion is from the electrode cap itself. Splash Splash Fig. 5: Splash Occurring from within the Weld Fig. 6: Splash Occurring from the Electrode Tip- Sheet Interface Electrode cap wear As the zinc begins to melt, the electrode force pushes some of this molten zinc to the edge of the mechanical contact area until solid-solid contact is re-established at the sheets. This displaced zinc effectively increases the electrical contact area resulting in a lower contact resistance, which means that more heat is generated within the tip which leads to a decrease in the welding current density and subsequently, in substandard welds [11]. The current level is increased accordingly in order to help counteract this problem. The zinc from the coating also alloys with the metal of the electrode caps which causes non-uniform pitting of the electrode tip, as shown in Figure 6, which eventually degrades the edges of the tip face [12]. Fig. 7: Electrode Tip Degradation Figures 7 and 8 show scanning electron microscope (SEM) results, where images of both a new and used electrode tip have been captured showing this degradation of the tip face. Electrode caps can be redressed to re-establish the correct profile (until the cap needs replacing) but this requires the welding to cease while it is performed. This changing tip profile has a profound effect on weld integrity. 256

6 Fig. 8: SEM Image of a New Electrode Cap with 1000 Magnification Fig. 9: SEM Image of Used Electrode Cap with 1000 Magnification The effect of alloying in used electrode tips makes it slightly harder than the copper of the new tip. This may seem like an improvement, however, the resistance of the electrode caps also increases leading to more heat being generated at the tips [13]. This effectively causes annealing of the electrodes making the resulting brass softer. Brass is made softer by annealing and becomes harder when it is being worked, making it eventually become brittle. Heated brass crumbles easily if it is bent while it is hot, and this occurs due to the high pressures being used and the repeated mechanical impacting at the interface [14]. 3. CURRENT NDE TECHNIQUES Many NDE methods are used to evaluate weld quality. NDE can be performed using various methods including, acoustic emission, eddy current, x-ray analysis, magnetic particle inspection and ultrasonic inspection to name a few. Ultrasonic pulse-echo NDE is a well established method for ensuring the integrity of welds as it can identify whether or not a weld contains cracks or voids. It can also check for incomplete penetration and other weld defects that are not noticeable upon visual inspection. This type of weld testing has been made compulsory in many welding standards and procedures. It is also the most widely used method within the automotive industry due to the recent advances in ultrasonic equipment and transducer design allowing spot weld testing to be performed quickly and with greater accuracy. It is a method of characterizing the thickness or internal structure of a test piece through the use of high frequency sound waves. The frequencies used for testing using ultrasonics are typically in the range of 0.5MHz to 20MHz which would mean that the resulting wavelength, in steel, would be in mm [15]. The use of frequencies below this range would mean that the interaction of the waves with internal flaws would become uncertain. The transmit and receive, or through transmission method, is an ultrasonic testing method where one transmitting transducer and one receiving transducer are placed on opposite sides of the object to be tested. An ultrasonic signal is then passed through the object. A reduction compared to the expected amplitude of the received signal would indicate the presence of a defect. As both a transmitter and receiver are required to perform the transmit receive testing method, this type of ultrasonic NDE inspection is rarely used within the automotive industry as access to both sides of the body-in-white structure is often impossible. However, as this paper proposes an online ultrasonic testing system where inspection is carried out simultaneous to welding, the ultrasonic transmitter and receiver can be located on the electrode arms of the spot welding machine. 257

7 4. RESULTS 4.1. Results on experimental setup SORPAS was used to simulate welds in order to find the correct welding parameters prior to real welding, to reduce steel waste [10]. IF180 steel was then used to carry out a set of welds using the parameters chosen from simulations. A current of 7kA and a pressure setting of 3.5bar were the chosen settings. An experiment was performed offline as proof of principle, using the setup shown in Figures 9 and 10, showing the actual electrode arms and spot welding electrode caps. A vice provided the force required to hold the electrode caps in close contact with the steel coupons. In order to produce a set of spot welds that resemble the forming weld nugget, 50 spot welds were carried out, with increasing weld times measured in cycles of 20ms. The produced welds were opened using the peel test and the resultant weld nuggets were measured. The average nugget diameters for each cycle are shown in Table 1, which shows that as the weld time increases, the size of the resulting weld nugget increases accordingly. Although the nugget diameter is larger at 9 and 10 cycles, the weld will contain cracks and voids due to the presence of splash during welding. Function Generator Spectrum Analyzer Welded Steel Transmitter Receiver Amplifier Electrode Arms Fig. 10: Ultrasonic Experimental Setup Transmitter Electrode Caps Receiver Water Connection Electrode Arms Vice to Provide Force Fig. 11: Close up of the Electrode Arms A simulation using the same parameters and cycle times was completed using SORPAS. Figure 11 shows the predicted weld nugget diameters when using 8 cycles welding time, where at 7kA the predicted weld nugget diameter is 3.8mm, which is very similar to the real weld results shown in bold, in Table

8 Weld Nugget Diameter (mm) Each of the welded steel coupons was placed in between the two electrode caps and held securely using the vice. A function generator (TTi TG MHz DDS) was used to send a sinusoidal voltage to the transmitting transducer. The resulting electrical signal was amplified using a Mini Circuits ZFL-500LN amplifier, the output of which was monitored using a spectrum analyser (Hewlett Packard 8594E, 9 khz-2.9 GHz). The results from this test, as shown in Figure 12, reveal that, as predicted, the lower the welding time, the lower the amplitude of the received signal. This is because of the air gaps that are present between the steel coupons, and as the weld time increases these air gaps reduce in size as the metal begins to melt, resulting in a greater amplitude. However, as the weld time increases above 8 cycles, splash begins to occur and therefore the received signal amplitude decreases due to the discontinuities that are formed when the molten metal is expelled from the forming weld nugget. Table 1: Weld Nugget Diameter for 1-10 Cycles Welded with 7kA and 3.5bar Weld Time (Cycles) Average Nugget Diameter (mm) Splash 1 Stuck No 2 Stuck No 3 Stuck No 4 Stuck No 5 Stuck No No No No Yes Yes Cycles mm Current (ka) Fig. 12: SORPAS Simulation for IF180 with 8 Cycles Weld Time and 3.5bar 259

9 Amplitude ( V) Cycle Cycles 5 Cycles 7 Cycles 9 Cycles Frequency (MHz) Fig. 13: Ultrasonic Results for IF180, Welded with 7kA and 3.5bar 5. CONCLUSION AND FUTURE WORK The effects of zinc coatings, expulsion and electrode cap wear on spot weld quality have been discussed. To help sustain spot weld quality once electrode cap wear begins, it is necessary to increase the current level (16). The use of SORPAS has led to a decrease in both metal and energy wastage, as it can be used to predict a range of welding currents for a particular pressure, which produce a good weld. This avoids unnecessary experimental tests using parameters which result in excess undersized or splash welds. A novel non destructive ultrasonic method has also been investigated. This transmitted ultrasonic waves through the spot welding electrode arms. Results from an experimental setup showed that as the weld forms, the received signal will increase in amplitude. Upon the production of discontinuities, the signal strength will then begin to decrease due to the change in acoustic impedance between the steel and the discontinuity from which the transmitted signal will be reflected. Two ultrasonic transducers are being developed that will have a lower resonant frequency than the transducers used to achieve the results presented. There is no necessity to have high frequency transducers when using the transmit receive technique as overlapping of the transmitted signal and the subsequent echoes does not occur. The final system also needs to be tested on an industrial spot weld machine to see whether these results can be replicated. 6. REFERENCES [1] Blitz J., Ultrasonics- Methods and Applications, Butterworths, (1971). [2] Athi N., Cullen J.D., Al-Jader M., Wylie S.R., Al-Shamma a A.I., Shaw A, Hyde M., Experimental and Theoretical Investigations to the Effects of Zinc Coatings and Splash on Electrode Cap Wear, JOM, (Online Publication Complete) doi: /j.measurement

10 [3] Kimchi M., Gugel M.D., and White C.L., Mechanisms of Electrode Wear during Resistance Spot Welding Hot-Dipped Galvanized Steel, Cooperative Research Program Summary Report SR0009, EWI, Columbus, Ohio, (2000). [4] Krieger R.J., Wenk S.A., McMaster R.C., Non destructive test methods for inspection of welded joints, Welding Journal, 33, , (1954). [5] Cullen J.D., Athi N., Al-Jader M., Johnson P., Al-Shamma a A.I., Shaw A., El-Rasheed A.M.A., Multisensor Fusion for Online Monitoring of the Quality of Spot Welding in Automotive Industry, Measurement 41 (4) , (2008). [6] BS EN ISO 14373:2007."Resistance Welding: Procedure for Spot Welding of Uncoated and Coated Low Carbon Steels". British Standards. (2007). [7] Shi G., Westgate S.A., "Resistance Spot Welding of High Strength Steels". JOM, (2003). [8] Honeycombe R.W.K., Bhadeshia H.K.D.H., Steels- Microstructure and Properties, Butterworth- Heinemann Ltd, Cambridge (1995). [9] Granjon H., Fundamentals of Welding Metallurgy, Abington Publishing, (1991). [10] SORPAS- Simulation and optimization of resistance projection and spot welding processes. Denmark : SWANTEC Software and Engineering ApS, (2008). [11] Zhang H., Senkara J., Resistance Welding- Fundamentals and Applications. CRC Press, Taylor and Francis Group, London, (2006). [12] Zhang X.Q., Chen G.L., Zhang Y.S., "Characteristics of Electrode Wear in Resistance Spot Welding of Dual-Phase Steels", Materials and Design, 29, (2008). [13] Parker J.D., Williams N.T., Holliday R.J., "Mechanisms of Electrode Degradation when Spot Welding Coated Steels", STWJ, 3, 65-74, (1998). [14] Davies G., Materials for Automobile Bodies Butterworth-Heinemann, (2003). [15] Olympus- High Speed Video, Remote Visual Inspection, NDT, Non Destructive Ultrasonic Test Equipment. Olympus Corporation. [Online] Olympus, (2008), [16] Holliday R.J., Parker J.D., Williams N.T., "Electrode Deformation when Spot Welding Coated Steels", Welding in the World, 35, , (1995). 261