Non-Destructive Evaluation of Dissimilar Aluminum Alloys (AA1100 & AA2014) Welded Using Friction Stir Welding P.Balaji, V.Kalyanavalli, D.Sastikumar, S.Muthukumaran1 and S.Venukumar1 Physics Department, National Institute of Technology, Tiruchirappalli, India Metallurgical and Materials Engineering Department,1 National Institute of Technology, Tiruchirappalli, India Email: [email protected]
ABSTRACT: Friction Stir Welding is a solid state welding process for joining materials like aluminum, magnesium, zinc and copper alloys which are employed in aerospace, rail, automobile and marine industries. Generally AA2014 and AA1100 are welded using conventional welding technique in marine industries for galvanizing purpose. The drawbacks in the conventional welding process in aluminum is the presence of a oxide layer, high thermal conductivity, high coefficient of thermal expansion, solidification shrinkage and above all the high solubility of hydrogen and other gases in molten state. These lead to
More Info at Open Access Database www.ndt.net/?id=15213 use of vacuum setup in the conventional welding process which is costly. But in FSW, the welding process is done by solid state welding of the material and not by melting the materials, thus the above said problems are solved. In this work, FSW joints have been made between AA1100 and AA2014 of aluminum plates dimension (140 x 70 x 5) mm using different combinations of process parameters. The non destructive analyses have been done to check the weld quality using ultrasonic C-scan and radiography. The defects that are mostly present in the friction stir welding process are disbanding, tunnel defect and kissing- bond, are analyzed using ultrasonic C-scan technique.
Key Words: Friction Stir Welding; ultrasonic A, B & C-scan; . INTRODUCTION: Aluminum (Al) alloy is a light weight material and has many industrial application. But they are not easily weldable by conventional fusion welding techniques because the quality of the welded joint is deteriorated due to the presence of porosity, hot cracking and distortion [3]. To overcome this, The Welding Institute (TWI), United Kingdom in December 1991, has invented Friction Stir Welding (FSW) as a solid-state joining technique, and it was initially applied to aluminum alloys [1]. FSW has shown great results for joining metals together without significantly altering the original properties of the parent metals. FSW is a solid-state joining process which is applicable to all metals and plastics. The popularity of this welding procedure is due to the fact that unlike conventional welding, it does not create brittle bonds caused by the Localized heating of metal. A major weld quality aspect in question is how to be able to detect the internal defect formed in the weld and a Non-Destructive Testing (NDT) technique is most appropriate in this regard. Non-Destructive Testing rose from the necessity to detect flaws within components to avoid detrimental and repetitive failures. NDT is a wide group of analysis techniques used in science and industry to evaluate the characteristics of a material or component without causing damage to it. The techniques used are non-invasive. NDT does not permanently alter the article being inspected; they are highly valuable techniques that can save both money and time.
Many different types of NDT methods exist, the most commonly used testing methods on FSW of AA2014 and AA-1100 aluminium alloy being ultrasonic testing and eddy current testing. The techniques successfully employed include the x-ray detection and ultrasonic C-scan. The Ultrasonic C-scan testing was found to do well at detecting porosity and tiny voids in the joints, while the liquid penetrating fluid inspection was suitable for revealing the Hidden root crack-like flaws. However, there is no published literature on non-destructive testing on friction stir welds of dissimilar materials. Non-destructive testing for defects deduction is the most effective method of determining the integrity of a friction stir weld, hence research studies into non-destructive techniques of dissimilar joints between aluminium and aluminium alloys will ultimately lead to achieving optimized setting to produce quality welds and increase material performance in this regard. The aim of this paper therefore, is to present results of non-destructive testing techniques on dissimilar friction stir welds of aluminum AA1100 and AA2014 alloys.
II. EXPERIMENTAL :
A.SAMPLE PREPARATION:
The Friction Stir welds consisting of aluminum alloy AA-1100 and AA-2014-T6 were successfully produced using modified vertical milling machine at the department of metallorgy, National Institute of Technology, Tiruchirappalli. The chemical composition of the weld is shown in table-I. The dimension of the weld coupon was (140mm x 70mm x 5mm) and butt joint configuration was considered in this research. The plates were grounded in grinding machine and cleaned with acetone to degrease before the welding procedure. The composition of the aluminum alloy was shown in Table-I.
Table-I: Composition of the aluminum alloy AA 1100 & AA 2014:
Elements Cu Mg Mn Fe Si Cr Zn Ti AA1100 0.05 0.05 0.05 0.4 0.25 - 0.07 0.05 AA2014 4.2 1.1 0.12 0.3 0.58 0.1 - -
The Friction Stir Welds were produced by varying the rotational speeds between 710 and 1200 rpm and the feed rate between 20 and 50 mm/minute. Rotational speeds of 710, 900, and 1200 rpm were employed while 20, 40 and 50 mm/min were the feed rates considered representing the low, medium and high settings respectively. The tool geometry also has been varied. The tool employed was cylindrical 10mm diameter pin with the depth of 4 mm and another tool pin of 10 mm diameter tapered to 8 mm diameter with 4 mm depth. The AA1100 was placed in tool advancing side and AA2014 was placed in tool retreating side. A weld length of 110 mm was produced for every setting. Visual testing and liquid penetrate testing were carried out in the welded sample for surface cracks. The aluminum alloy and their quality of welded sample were shown in table-II.
Table-II: Weld Matrix And Samplle Designation:
Specimen Rotational Feed Rate Remarks of sample in visual Designation Tool Speed (mm/min) testing and LPT (rpm) A-1-C Cylindrical 710 20 Defective pin A-2-T Tapered pin 710 20 Comparatively less defective
B-1-C Cylindrical 900 40 Less defective pin B-2-T Tapered pin 900 40 Defect less
C-1-C Cylindrical 1200 50 Distortion in sample pin C-2-T Tapered pin 1200 50 Less distortion in sample
Image of the samples welded using cylindrical pin & tapered pin:
Fig-1 Image of the welded samples:
B. IMMERSION ULTRASONIC TESTING:
Immersion Ultrasonic C scan testing had been carried out using a probe with frequency of 5 MHz, focal length of 2” and water acting as a couplant. The sound wave velocity of aluminium is 6300 m/s. sound velocity in water 1500 m/s. Water path distance was 50.8 mm.The acqUT software had been used to load the requires data for the scan. The sample was scanned with resolution of 0.1 mm along the joining as well as across the joining. The scanning was done along y-axis for a length of 130 mm and indexed along x- axis for the length of 45 mm. The first signal amplitude from the ultrasonic test method shows the front wall echo whereas the second signal amplitude shows the back wall echo. Detection and localization of defect were done by the A, B and C scan.
RESULTS AND DISCUSSION:
C-scan ultrasonic studies were carried out on the FSW joint using a 5 MHz point focused immersion transducer, focused at the weld region. A reference standard was created for AA 1100 and AA 2014 to interpret the signals coming from the defect. Out of six samples two samples were taken for the study. Each and every point in the samples was scanned and interpreted. Reference sample was taken for predicting the nature and kind of different defect in A-2-T and B-2-T samples.
Result of varies echo coming from different artificial defects in reference standard:
Fig-2 C-scan image of the reference sample
Thus using reference sample it was easy to understand how the ultrasonic signal respond to the different defects oriented at different location was studied. The C-scan image of the sample A-2-T was taken and it had been analyzed for the defect using exqUT software. With the combined aid of A-Scan it was easy to point out the orientation of defects in the sample.
Result of the A-2-T sample (710 rpm& 20 mm/min.):
Fig-3 C-scan image of A-2-T sample welded at 710 rpm with feed rate of 20 mm/min.
Result of B-2-T (900 rpm & 40 mm/min):
Fig-4 C-scan image of B-2-T sample welded at 900 rpm with feed rate of 40 mm/min.
The complete interpretation of the weld region of the sample B-2-T was done using C-Scan with the combined aid of A-scan and B-scan it has been concluded that B-2-T sample is free from defects. Thus from the above study it is clear that C-scan can be effectively used for assessing the bond integrity of FSW joints in a Non-destructive way. The detailed analysis also can pinpoint the de-bond location and help in the accept/reject criteria for such joints.
CONCLUSIONS:
In the present study, ultrasonic C-scan evaluation was carried out on Aluminium alloy dissimilar metal joint welded using FSW. The study revealed that there were disbonds, tunnel defect,cavity and kissing-bond present in the joint. Visual testing and liquid penrtrant testing were carried out on the welded sample. Out of six samples four has been pointed as defective samples visualy. The sample A-2-T and B-2-T had been taken liquid penetrant testing for pin pointing the surface cracks. The A-2-T is found to have surface defects and B-2-T is free from defects. Furter the sample A-2-T and B-2-T had been taken ultrasonic C- scan. The sample A-2-T has different internal defects. The shape and orientation of the defect had been studied using C-scan. With A-scan it had been studied how the defects responds to the ultrasonic signal. Detailed analysis showed that the sample welded at the speed of 900 rpm with feed rate of 40 mm/min feed rate was free from defects. References: 1. E.D. Nicholas and W.M. Thomas, A Review of Friction Processes for Aerospace Applications, Int. J. Mater. Prod. Technol., 1998, 13(1–2), p 45–55
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