Key words:

spot welds, ultrasonic quality control.

Investigations of spot welds quality
based on ultrasonic techniques

A. Ambroziak^a*, M. Korzeniowski^a, P. Kustroń^a

^a Institute of Production Engineering and Automation of Wroclaw University of Technology

ul. Łukasiewicza 3/5 50-371 Wrocław,

*E-mail address: andrzej.ambroziak@pwr.wroc.pl

This article contains some information about the main methods of ultrasonic quality control of spot welds. Resistant welding process is very sensitive for lots of factors like: electric power excursion, electrodes and material surface’s condition, electrical by-pass, etc. It usually causes worse quality of the connection. To prevent this factor’s influence, many methods are being investigated. Recently, many of the promising methods are based on ultrasonic waves. That’s why we would like to present advantages and disadvantages of two systems.

First, OFF-LINE method can give us information about typical unconformities of spot welds but only after the process. This method is very popular nowadays especially in thin-walled materials. It lets to check the quality of even a hundred percents of spot welds without destroying them.

Second ON-LINE method is based on measuring selected parameters of ultrasonic wave, which flow across the spot-joins during welding process. It cause that we can check quality of spot weld during its arising. Diagrams of ultrasonic transmission shows that ultrasonic method can give many, important information about welding process, like: size of welding nuggets, it’s arising and crystallization moment but first of all, current switching-off moment. All abilities of this method can be used to find weld quality, controlling welding process and prevent many dangerous factors in the time of weld arising.

1. Introduction

The resistance spot welding is the most popular method of joining steel sheets.
The connection arises by passage the current and action of welding force. Heating of joining parts is an effect of heat generation on electrical resistance of welding circuit.

The resistance spot welding is the process of welding by which the joining parts are pressured by welding electrodes which conduct welding current into the spot weld [1]. Connecting 2 or 3 parts of sheets is possible by the resistance spot welding. During this process one or more welding joints can be obtained. It depends of applied welding machines. The resistance welding process is quick, efficient. This is the reasons why the spot welding is the most popular kind of joining in automotive industry (manufacturing bodies, chairs and many others). The spot welds are mainly performed by robots. Because of the fact that welding machines are unreliable, spot welds can have some defects.

Unconformities which can appear in spot welds causes the spot welds can have less strength and can lead into total destruction of manufacturing parts.

The typical unconformities of spot welds are [2][3]:

· cold weld,

· small-diameter of spot weld,

· bad shape of the nugget,

· cracks inside/around the nugget,

· deep indentation of welding electrodes in sheets.

Fig.1. Typical flaws in spot welding joints [3].

In automotive industry the elements of cars bodies are joined by resistance welding. The best quality of welding joints is very important. On-line systems aren’t developed enough, so at the present it is impossible to control all spot welds.

The non-destructive method which can be applied to test the quality of spot welds is ultrasonic off-line method. Ultrasonic systems are in common use from ages, but testing of thin plates generates a lot of problems. The upper range frequency of typical ultrasonic transducers used to flaws detection is 10MHz. It limits the thickness of the tested plates to 3mm so to test thin-plate spot welds applying the higher frequency is required. The range of plate-thickness in automotive industry is from 0.75 to 1.5 mm. It implicates the frequencies of ultrasounds ca. 20 MHz [5], [6].

2. OFF-LINE ultrasonic tests

There are two principal ultrasonic methods used to detect unconformities inside spot weld. The first one is the pulse-echo method the second one is the flow method. To test quality of spot welds in automotive industry the pulse-echo method was used.

This method implicates applying one ultrasonic head worked alternately:
as a transmitter and as receiver.

It consists of ultrasonic transducer which generates longitudinal vibrations. Ultrasonic beam flows across the delaying path which is ended by an elastic membrane.
The homogeneity of spot weld determines the back-wall reflection. The multiplied reflections are observed at the flaw detector. In case of internal unconformities (e.g. gas pores) the signal observed at flaw detector is an effect of reflection from the unconformities. It’s possible to determine distance from unconformities to ultrasonic head.

To determine the quality of spot welds mostly following factors have to be taken into account:

- numbers of back-wall reflections,

- attenuation of ultrasonic wave (in dB),

- numbers of reflections from flaws,

- envelope of echoes.

Dimensions of the defects, measured by ultrasonic tests, can diverge from their real size. It comes from different reflecting ability of variety reflectors. A shape, diameter and structure of the defects and ultrasonic waves’ parameters are main factors which determine their reflecting ability. Moreover, a diameter of the ultrasonic transducer and a distance between ultrasonic head and the defect are very important.

The best reflecting abilities have these planar discontinuities which are oriented perpendicularly to the wave’s flaw direction.

The experimental spot-welds joints were checked by using the echo method.
The results of the ultrasonic tests were verified by destructive tests (metallography).

Metallographic picture	Ultrasonic diagram
Good spot-weld
Incomplete fused joint

Fig.2. Results of spot-weld inspection [15]

Figure 2 shows two different cases: good and incomplete fused joint. In the first case, the result of ultrasonic test reveals good quality of the spot-weld. First diagram shows short sequence and high dumped consecutive pulses. It testifies about good quality of spot-join. Metallographic test confirms that fact.

The second case describes incomplete fused joint. Second diagram shows long and low dumped sequence of pulses. It proves the lack of the nugget.

3. ON-LINE ULTRASONIC QUALTIY CONTROL OF SPOT WELDS

Popularity of spot welding process and application of spot welds in responsible constructions have caused the need of controlling the welding quality while the process runs. The aim is to minimize the quantity of faulty spot welds and control the process.

There are plenty of methods, which allow for quality control of produced joints.
They are mainly base on measuring:

· current, voltage and dynamic resistance [10], [11],

· transmission and velocity of ultrasonic waves[12][13],

· thermal expansion [16],

· infrared radiation [16],

· acoustic emission [16].

These methods have a range of advantages and disadvantages. In case of controlling electrodes movement during the welding, acoustic emission, infrared radiation or ultrasonic method it is necessary to apply very complicated measuring apparatus.

Method of measuring dynamic resistance becomes more and more popular [10], [11] However each of above mentioned methods allows to receive only specific information about the welding process.

Systems described below, bases on measuring the amplitude of ultrasonic waves.

There is a few technological solutions elaborated by different concerns, but its functioning principles are similar [18] [19].

On basis of researches conducted by Welding Establishment of Wrocław University of Technology it has been claimed that, there’s strict correlation between amplitude of ultrasonic wave measured while welding process and quality of produced joints.
In application that was used for measuring, ultrasonic heads were placed in proper construction.

a)b)

Rys. 3. Construction of electrodes

a) scheme , b) picture.

On base of ultrasonic wave transmission curve – measured during the welding process, we can classify given spot weld to estimated quality category. The ultrasonic wave transmission curve gives us also a lot of information about the process that occurs in arising weld joint. Those are eg. forming of liquid nugget, its growing and becoming a solid phase.

Researches that have been made so far now predicate mainly on measuring the only one ultrasonic wave parameter – it is transmission of the wave flowing through arising spot weld. That’s why it is justifiable to pay attention to other parameters such as velocity and wave passing time from transmitting head to receiving head, during the welding process.

The velocity of ultrasonic wave is typical property of medium in which it spreads. Its value is different for solids, liquids or gases, even for different kinds of metals. Moreover the relevant factors that determine the velocity of sound spreading in given mediums are their density and Young’s module.

During the resistance spot welding process the changes of above mentioned parameters follow, as an effect of substantial changes of material temperature. Especially Young E module, besides frequently occurs a change of condensation state (e.g. becoming of liquid spot weld nugget).

Due to this, in compound of arising spot weld, substantial ultrasonic wave velocity changes will follow, and the same crucial changes of passing time between transmitting head and receiving head too.

Fig.4. Diagrams of ultrasonic transmission, received for
(1mm thickness) mild steel and various welding current.

As well as from perspective of produced spot weld quality and from controlling of this process, the most important area of transmission diagram is current switch-off moment. According to observed and registered diagrams, there have been claimed that it illustrates the spot weld quality in most precisely way.

Figure 4 shows three registered tracks of wave crossing through spot weld while the jointing process. We may notice that with the welding current growth, also rises the value of ultrasonic wave transmission.

Fig.5. Time of flight diagrams, received for
(1mm thickness) mild steel and various welding current.

Figure 5 illustrate the results of measuring Time of Flight - TOF[ns] of ultrasonic wave through subsequent mediums, these are: liquid coolant, material of electrode and joined materials. As a result of changes these mediums’ temperature the wave spreading group velocity also changes. Of course the time in which the wave reaches receiving head. Due to this, the direct time measuring has been made actually, and is quoted in nanoseconds - on figure 5.

Registered diagrams (fig.4 and 5) reflect the spot welds produced with different values of welding current. In this way have been obtained joints in different quality categories (beginning with sticking – with welding current I_z = 6,1kA, through to small spot weld nugget – class C – with welding current I_z = 7,8kA, on perfect one spot weld in A class ending – welding current I_z = 8,6kA).

4. CONCLUSION

On the ground of presented results, it was found that in spot welds of thin walled elements:

· Ultrasonic tests can be apply to prove their quality

· Examination requires using special ultrasonic heads equipped in liquid intermediate medium to improve a contact of ultrasonic head with spot weld surface,

· Application of ultrasonic waves during the spot welding process provides a lot of important information about arising spot weld and run of the welding process.

· Ultrasonic quality control is very promising method but its applying should be verified by metallography.

5. REFERENCES

[1] A. Klimpel: Spawanie zgrzewanie i cięcie metali, WNT Warszawa1999.

[2] H. Papkala: Zgrzewanie oporowe metali, WiHK „KaBe” s.c. Krosno, 2003.

[3] A. Klimpel: Kontrola i zapewnienie jakości w spawalnictwie. WPŚ Gliwice 1998

[4] W. Roye: Die Ultrachall Priifung von Komponenten und Fugeverbindungen

im Automobilbaum, DGZfP Berichtband 79-CD, 1999.

[5] R. Rosenberg: Ultraschallprufung und Dieckenmessung rund ums Automobilbau, Panametrics Hofheim-Wallau 1999.

[6] Krautkramer Company Journal „Echo 36” - State of art in ultrasonic testing of spot welds – NDTnet 1998 April, Vol. 3 No.4.

[7] A. Lewinska-Romicka: Badania nieniszczące, podstawy defektoskopii, WNT, Warszawa, 2001.

[8] Ultrachallprufung von Widerstandspunktschweifiyerbindungen im Karosseriebau der Volkswagen Sachsen GmbH, DGZfP Berichtband 79-CD, 1999.

[9] C. Kohler: Die Einfuhrung der Priifung von Widerstandsgeschweifiten Punkten mittels Ultraschall in der Automobilindustrie – Uberblick der Entwicklungen und Activaten der lezten Jahre, DGZfP Berichtband 79-CD, 1999.

[10] J. Kozaczyński, Z. Mikno, P. Stodolny: System do kontroli jakości połączeń zgrzewanych w oparciu o sieci neuronowe, Seminarium Instytutu Spawalnictwa, Gliwice, 2004.

[11] A. Straube, A. Torzewski, B. Winien: PQS^weld – System kontroli dla zgrzewania punktowego i garbowego na bazie fuzzy logic. Podstawy, możliwości i zastosowania praktyczne, Seminarium Instytutu Spawalnictwa, Gliwice, 2004.

[12] A. Ambroziak, Z. Koralewicz, M. Korzeniowski, P. Kustroń: Kontrola jakości powstających zgrzein metodą ultradźwiękową, Prace Naukowe Instytutu Technologii Maszyn i Automatyzacji PWr., Zeszyt nr 73.

[13] A. Ambroziak, L. Krynicki, Z. Koralewicz: Określenie przydatności badań ultradźwiękowych do oceny jakości połączeń zgrzewanych punktowo, Przegląd Spawalnictwa Nr 7-8/2000.

[14] A. Ambroziak, Z. Koralewicz, M. Korzeniowski, P. Kustroń: Badania ultradźwiękowe zgrzein punktowych, Spajanie metali i tworzyw w praktyce,
Nr 1/2005.

[15] A. Ambroziak, A. Kisiel, M. Korzeniowski: Zastosowanie badań ultradźwiękowych do oceny zgrzein w cienkościennych elementach stalowych, Przegląd Spawalnictwa 5-7/2004.

[16] S. Piech: Kontrola adaptacyjna w procesie zgrzewania oporowego punktowego, Przegląd Spawalnictwa, nr 5/1983.

[17] E. Talarczyk: Podstawy techniki ultradźwięków, Wydawnictwo Politechniki Wrocławskiej, 1990.

[18] VOGT Werkstoffprüfsysteme: Inline ultrasonic testing system for process control during resistance spot welding, www.vogt-ndt.de.

[19] Bosh Rexroth: Automatic ultrasonic tests – non destructive during the welding process, Materiały konferencyjne, Essen, 2003.