NDE of Mechanical Joints of Dissimilar Materials

Raguvarun K1, Vineeth. C1, Krishnan Balasubramaniam1and Prabhu Rajagopal1a)

Santhosh S2, Satish V.Kailas2, G. Padmanabham3

1Centre for NDE, Indian Institute of Technology Madras, Chennai 600036, Tamilnadu, India 2Indian Institute of Science, Bangalore 560012, Karnataka, India

3International Advanced Research Centre for Powder Metallurgy and New Materials

Hyderabad 50005, Telangana, India

a) prajagopal@iitm.ac.in

Abstract. Advanced mechanical joining methods are of increasing interest to the automotive industry, in view of the growing demand for dissimilar metal joints in new designs, driven by a push towards vehicles with a smaller energy footprint. Segments of lightweight structures made of materials such as Aluminum (Al) alloys are thus required to be joined to high strength steel. However, conventional joining procedures such as spot-welding are challenged in such cases, by issues such as differences in melting point and thermal conductivity: mechanical joining methods are attractive, as they are not influenced by thermal effects. Moreover, mechanical joining methods do not require pre-drilling of parent materials and also reduce material wastage.

The objective of this paper is to demonstrate the utility of NDT methods in bringing physical insight into advanced mechanical joining processes and providing a positive feedback channel to improve the quality of the joints produced. The mechanical joining procedures considered are Clinching and Self-pierced Riveting, and Steel-Steel, Al-Al, and Steel-Al joints are studied. Two NDT methods namely, Ultrasonic C-Scan Testing and X-ray Computed Tomography (CT) are used to inspect the joint samples produced. Coupons of joint samples in Al-Steel and Steel-Al configurations are examined using the GE V|tome|x s CT machine, with energy in the range 120-140 kv and C-scan is performed using MHz immersion probes. The studies reveal severe cracks on the rivet skirt of the samples when self-pierced rivets are made from Al into Steel. This happens due to the excessive flaring of rivet or in the higher hardness rivets which have lower ductility. Guidelines for improving the process and directions for further work are discussed.

Keywords: Self-Piercing Rivets, Clinching, Ultrasonic Testing, X-ray Computed Tomography

INTRODUCTION

In automotive industry there is a demand for low energy consuming vehicles. This can be overcome by lightweight materials such as Aluminum alloys. These alloys will be used with high strength steel to make joint. Such dissimilar materials cannot be welded by spot welding process due to difference in melting point and thermal conductivity. This can be overcome by advanced mechanical joints like self-pierced and clinched rivets [1]. Self-pierced riveting is a cold joining process used to join two or more overlapping metal sheets by pushing a rivet without making holes [2] and Clinching is a Plastic deformation process joined by hemming with a punch and die[3]. The various steps in clinching is positioning, pushing, upsetting and retraction [4]. In self-

National Seminar & Exhibition on Non-Destructive Evaluation, NDE 2014, Pune, December 4-6, 2014 (NDE-India 2014)

Vol.20 No.6 (June 2015) - The e-Journal of Nondestructive Testing - ISSN 1435-4934www.ndt.net/?id=17889

National Seminar & Exhibition on Non-Destructive Evaluation, NDE 2014, Pune, December 4-6, 2014 (NDE-India 2014)

Vol.20 No.6 (June 2015) - The e-Journal of Nondestructive Testing - ISSN 1435-4934www.ndt.net/?id=17889

pierced and clinched rivets there is no influence of thermal effects. The advantages of these modern joints over traditional joints are we can join two dissimilar materials without pre-drilling the material and it requires low energy, it eliminates material wastage and it achieves high strength and increased fatigue properties [5].

The purpose of this work is to study the quality of a rivets made by self-piercing and clinching process. Non Destructive Testing (NDT) is used for determination of material flaws/ defects so as to not affect the physical and mechanical properties of the sample. Two different NDT methods are used for testing, namely: X Ray Tomography and Ultrasonic C scan. X Ray Tomography method generates a 3 Dimensional (3D) view of the sample in the form of a gray scale image due to the differential penetration of X rays through the sample, while Ultrasonic C scan generates a plan view of the sample based on the data obtained by the reflection of ultrasonic waves from different interfaces in the material.

X Ray Tomography has been performed by the Phoenix V|tome|xs equipment supplied by General Electric (GE) [6]. The inspection has been done using a 180 kv Nano focus X ray tube, which has a resolution of ½ µand Ultrasonic Testing has been performed by C Scan immersion test machine supplied by Trotix [7].

(a) (b)

Figure.1Photograph Showing (a) Al- Steel SPR Joint and (b) Steel-Al Clinched Steel-Al Joint

PROBLEM STUDIED

Photographs of SPR and Clinching joints are shown in Fig.1. Self-pierced Al-Steel joint which is having the thickness 1.2 mm and 3 mm with a punching speed of 0.1 mm/sec. The Clinching joint made by Steel-Al which has a thickness of 2mm and 1.2mm and punching speed of 0.1mm/sec are examined using X- Ray Tomography and Ultrasonic C- scan Testing. 

METHODOLOGY

3D X-Ray Computed Tomography

In this method, the scan was performed on the entire region of the sample using a Nano focus tube. The X- Ray machine made by General Electric- Measurement and Control Solutions has been employed to perform CT scan [6]. It is fitted with Micro focus tube and Nano focus tube. The capacity of micro focus tube is 240 Kv and the resolution is 10 µ. The capacity of Nanofocus tube is 180 Kv and the resolution is 0.5 µ. Phoenix datosx software is used for data acquisition and reconstruction [8], Volume graphics studiomax is used for analysis [9].

HighClinchingthe resolu1000ms. Ultrason

Ultrasonito detect done in s(cross sec(LabViewsample (W(Width vpulse frowith stepInstrumenprocessinillustrated

Ultrasoniscan macand a gaidamping sample w

Figure.

h resolution g joints.The ution is40 µThe scannin

nic C Scan T

ic C scan is and evaluat

such a way ction) and siw) and reproWidth vs Le

vs Thicknessm every poi

pper motors nts) Data A

ng. Photograd in Fig.2

ic Immersionchine that coin of 30 dB of 10 Ohm

was mounted

.2 Photograp

X- Ray tomenergy whic

µm. The aveng time was

Testing

one of the wte the type, lthat the ful

ignals (A scaoduced in theength), the cos) while A scint along the

for enablinAcquisition aph of the X

n C scan waould translate

and dampinwere given

d inside a ple

(a)

ph of (a) X- R

mography wch is requireeraging rate 30 minutes a

widely used location andl volume ofans) from eae form of anorrespondingcan at everye scan (Ampng traverse Card (DAQ

X- Ray Tomo

as performee in the X,Yng of 10 Oh

n for Clinchiexi glass tank

Ray Tomogr

was performeed to perform

is 4 and thand 360 ima

methods in d size of deff the sampleach point is rn image. Theg B scan givy point givesplitude vs Tiof the prob

Q) is used ography mac

d on the SPY and Z direhm were giving joint. Thk filled with

raphy Machi

ed on the enm the scan ishe time requages were acq

Non Destrufects/ flaws ie is scannedrecorded by

e C scan imaves the crosss the signal ime). The sc

be along diffor data ac

chine and U

PR and Clincections. A 15ven for SPR he resolutionwater (to ac

ine and (b) U

ntire regions 130 Kv and

uired to acququired

ctive Testinin a given s

d along a parthe data acq

age gives the sectional viobtained frocanning macfferent axes.cquisition anltrasonic C

ching rivetsu5 MHz focujoint and ga

n of the scanct as a coupla

(b)

Ultrasonic T

n of the SPRd 90 µA [10uire one ima

g (NDT) in ample. Testirticular thicquisition softe plan view oiew of the saom the Ultrachine is equ An NI(Natnd digital sscan machin

using a 3- as probe wasain of 26 dBn is 0.1 mmant)

esting Mach

R and 0] and age is

order ing is kness

ftware of the ample asonic ipped tional signal ne are

axis C s used B and

m. The

hine

RESULTS AND DISCUSSION

Results Obtained from X- Ray Tomography for Al-Steel SPR Joint

Scan was performed on the entire region of the sample. The 3D view of the SPR joint is illustrated in Fig. 3 and the top and front view of the specimen is illustrated in Fig.4

Figure.3Three Dimensional View of Al-Steel SPR joint Obtained by CT Scan

(a) (b)

Figure.4 (a) Top View and (b) Front View of CT Scan Image

In Computed Tomography Severe crack was detected in the rivet skirt for Al-Steel joint. The reason for crack formation could be excessive flaring of rivet skirt.

Results Obtained from Ultrasonic C Scan for Al- Steel SPR Joint

The Ultrasonic C Scan Test was performed on the joint in button up position. The images were analyzed by Labview and it is illustrated in Fig.5

(a)

(b) (c)

Figure.5 (a) C Scan (b) A Scan and (c) B Scan Images of the Al-Steel SPR Sample

Results Obtained from X- Ray Tomography for Steel-Al Clinching Joint

Scan was performed on the entire region of the sample. The 3D view of the Steel-Al Clinching joint is illustrated in Figure. 6 and the top and front view of the specimen is illustrated in Fig. 7

Figure.6 Three Dimensional View of Steel -Al Clinching joint Obtained by CT Scan

(a) (b)

Figure.7 (a) Top View and (b) Front View of CT Scan Image

The results obtained from CT scan shows the quality of the interlock region. The neck thickness of the joint describes the strength of the interlock.

Results Obtained from Ultrasonic C Scan for Steel-Al Clinching Joint

The Ultrasonic C Scan Test was performed on the joint in button up position. The images were analyzed by Labview and it is illustrated in Fig.8

(a)

(b) (c)

Figure.8 (a) C Scan (b) A Scan and (c) B Scan Images of the Steel-Al Clinching Joint

The Ultrasonic C scan test indicates some cracks when the gating lies between 2 and 3.5. The corresponding A scan and B scan results are also shown.

CONCLUSION

NDE was carried out on two dissimilar joints namely Al-Steel and Steel-Al made by Self-Piercing and Clinching rivets by 3D X- Ray Tomography and Ultrasonic C scan. Studies revealed that severe cracks were detected in the rivet skirt of the Al-Steel joint made by Self-Piercing rivets. Due to the excessive flaring of rivet the cracks are formed. The rivet has sufficient strength to penetrate the upper sheet and also causes flaring due to ductility. In Steel-Al Clinching joint the neck thickness was identified by Computed Tomography and some cracks are also detected by Ultrasonic C scan test. The neck thickness determines the strength of the interlock. Further research is currently looking at developing an online process monitoring approach to maintain quality control in the joining process. REFERENCES [1]Chan-Joo Lee, Jae-Young Kim, Sang-Kon Lee, Dae-Cheol Ko , and Byung-Min Kim, “Parametric Study on Mechanical Clinching Process for joining Aluminium Alloy and High- Strength Steel Sheets”, J. Mech. Science. and Tech, Vol.24, pp. 123-126, 2010.

[2] Paul Briskham, Nicholas Blundell, Li Han, Richard Hewitt, and Ken Young, “Comparison of Self-Pierce Riveting, Resistance Spot Welding and Spot Friction Joining for Aluminium Automotive Sheet”, International Automotive Research Centre, University of Warwick, UK, 2006.

[3] Ken-Ichiro Mori, Niels Bay, Livan Fratini, Fabrizio Micari, and A. Erman Tekkaya, “Joining by Plastic Deformation”, CIRP Annals- Manufacturing Technology, 2013.

[4] Stephan Busse, Marion Merklein, Karl Roll, Michael Ruther, and Michael Zurn, “Development of a Mechanical Joining Process for Automotive-Body-In-White Production”, Daimler AG and University of Erlangen, Int. J. Mater. Form,Vol. 3, pp. 1059-1062, 2010.

[5] Xiacong He, Ian Pearson, and Ken Young,“Self-Pierce Riveting for Sheet Materials: State of Art”, J. Mater. Proces. Tech, Vol. 99, pp. 27-36, 2008.

[6] Information on http://www.gesensinginspection.comaccessed as on 01.11.2014

[7] Information on http://www.trotix.com accessed on 01.11.2014

[8] Information on http://www.phoenixxray.comaccessed as on 01.11.2014

[9] General Electric (GE), Industrial Radiography and Image Forming Techniques, 2009. http://www.ge-mcs.com

[10] ASTM E 1570- 11, Standard Practice for Tomographic Examination.