A. M. Chertov

A one-dimensional numerical model of acoustic wave propagation in a multilayered structure of a resistance spot weld

A one-dimensional model of acoustic wave propagation in a multilayered structure of a spot weld is developed. The inhomogeneity of the material properties due to the thermal inhomogeneity is included in the equation of motion. The model enables us to deal with arbitrary spatial distributions of Lame constants and density. The model allows analysis of travel time, multiple reflections, and interference in a given geometry. Use of this model could provide information to help predict behavior of the waves in the transmission (reflection) mode at different plate thicknesses and welding settings.

Inverse Problem Solution to Find Real‐Time Temperature Distribution Inside the Spot Weld Medium Using Ultrasound Time of Flight Methods

The scope of this paper is to find temperature distributions in the spot weld at different stages of welding using ultrasonic through transmitted waves. The finite difference modeling provides the temperature distributions in the spot weld. For the given welding schedule the delay of the signal passing through the heated area is unique for every temperature distribution. Based on the experimentally obtained TOF curve it is possible to determine the temperature at any point of the weld.

Grain Size Measurement of Copper Spot Welding Caps Via Ultrasonic Attenuation and Scattering Experiments

During ultrasonic testing of resistance spot welds in real time, the probe sends the sound waves through the thickness of the copper electrode cap into the materials being welded. Characteristics of the reflected waves from the weld interfaces allow a reliable decision to be made on the quality of the joint. Transmission of high frequency sound waves through the relatively thick copper welding cap cause the signal to be greatly attenuated due to grain scattering. For this reason, close monitoring of the copper cap properties prior to installation is essential for adequate performance. Finding copper alloys with a small average grain size is essential in order to minimize the attenuating effects. The conducted backscatter and attenuation experiments indicate correlation between the ultrasonically measured parameters and the optically found copper grain size. This correlation suggests that the attenuation or backscatter technique could be used alone in order to validate the proper copper alloy to be used in spot weld probes. Using nondestructive testing techniques for this purpose greatly reduces the time and cost involved compared to optical techniques.

REAL‐TIME ULTRASONIC EXPULSION DETECTION AND INDENTATION MEASUREMENT IN RESISTANCE SPOT WELDS

The thickness of a resistance spot weld resulting from electrode indentation is an important quality control parameter for estimating the final nugget diameter and identifying the occurrence of expulsion. An ultrasound transducer has been installed in a welding electrode allowing for evaluation of a spot weld during welding, using echo pulses. In such a system, the unknown increase in temperature during welding increases time of flight through the weld, while geometric contraction of the plates decreases it. These opposing effects often give false representations of the true weld thickness. This paper proposes a novel method of separating these effects. This is done using signal and image processing of successive A‐scans to identify the solid‐liquid nugget interface, where the melting temperature of the material is known. This paper demonstrates how the final weld thickness can be determined very accurately at the point in time when the nugget completely solidifies. It can also be shown that excessive ind...

Enhancing real-time ultrasound signatures of molten nugget growth for quality evaluation of resistance spot welds

This paper presents an effective means of enhancing weak acoustic reflections in real-time ultrasound signatures of a spot weld, by analyzing the propagation of acoustic waves through the electrodes and heated weld region. A new method of processing the ultrasound data is presented that removes undesired acoustical reflections in the electrode cap and enhances weak reflections at the solid-liquid interface. To facilitate this, frequency and amplitude attenuation resulting from the weld medium and acoustical interfaces of the layered weld structure are examined. Sources of echoes that destructively interfere with the desired reflections are also identified and removed by optimal filter design. Finally, basic image processing techniques are applied to a B-scan representation of the weld to improve visualization of the liquid nugget during cooling. This additionally permits the accurate measurement of the final weld thickness; an important quality control parameter.

Blind component separation for highly corrupted ultrasonic signals in real-time spot weld inspection

This paper introduces an efficient technique to analyze ultrasonic signals obtained in the process of nondestructive testing of resistance spot welds. The ultrasonic transducer, integrated inside a water-cooled welding electrode, sends multiple impulses into the welded metal sheets in reflection mode. The harsh environment surrounding the inspection system generates multiple sources of noise. Bubbles in the cooling water stream, temperature variations around transducer, electromagnetic noise from power electronics, transformers and servo motors corrupt the received A-scans. The method describes how to separate signal components and extract desired features from corrupted A-scans to distinguish the reflections from multiple interfaces of spot weld structure. As welding progresses, the technique is capable of monitoring the dynamics of the key parameters of spot weld growth and the presence of the liquid nugget within welded sheets. Several experiments were carried out using the welding system and robots available at the Institute for Diagnostic Imaging Research as well as the data received from leading auto makers in the North America and Europe.

New developments for in situ ultrasonic measurement of transient temperature distributions at the tip of a copper resistance spot weld electrode

This paper presents the new development of a joint thermal simulation and ultrasonic time of flight (TOF) measurement technique for the In situ measurement of transient temperature distributions. A coupled finite element model, using conductive media DC and heat transfer by conduction, was used to simulate the expected transient temperature distributions within the copper electrode during welding. Simulated temperature distributions were then used to model acoustic wave propagation through the geometry. This paper shows how the inverse problem solution of determining the actual temperature distribution and electrode tip temperature can be performed by using measured dynamic time of flight data in conjunction with finite element results of expected wave propagation.

IN‐LINE ULTRASONIC INVESTIGATION OF SPOT WELD QUALITY USING MULTI‐TRANSDUCER SET‐UP

This article presents one of the most recent developments in real‐time ultrasonic quality control methods for resistance spot welds. Currently existing in‐line systems use single‐element transducer to characterize the spot weld quality. The new set‐up is based on 8 ultrasonic transducers placed in one line above the center of the spot weld. The solution presented in this paper makes another step forward towards improvement of accuracy of measurements using array of ultrasonic transducers.

REAL TIME ULTRASONIC ALUMINUM SPOT WELD MONITORING SYSTEM

Aluminum alloys pose several properties that make them one of the most popular engineering materials: they have excellent corrosion resistance, and high weight-to-strength ratio. Resistance spot welding of aluminum alloys is widely used today but oxide film and aluminum thermal and electrical properties make spot welding a difficult task. Electrode degradation due to pitting, alloying and mushrooming decreases the weld quality and adjustment of parameters like current and force is required. To realize these adjustments and ensure weld quality, a tool to measure weld quality in real time is required. In this paper, a real time ultrasonic non-destructive evaluation system for aluminum spot welds is presented. The system is able to monitor nugget growth while the spot weld is being made. This is achieved by interpreting the echoes of an ultrasound transducer located in one of the welding electrodes. The transducer receives and transmits an ultrasound signal at different times during the welding cycle. Valuable information of the weld quality is embedded in this signal. The system is able to determine the weld nugget diameter by measuring the delays of the ultrasound signals received during the complete welding cycle. The article presents the system performance on aluminum alloy AA6022.

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

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.