Contents
Chapter 2 Literature Review
2.6 Defect identification in FSW process
The FSW process is found to be advantageous in many aspects compared to other fusion welding processes (Mishra and Ma, 2005; Gibson et al., 2014). Defects occurred in fusion welding processes such as porosity, lack of penetration, spattering of electrode material and defects related to solidification of joining material is out of question in FSW process. This makes the process free from defects commonly occurred in fusion welding processes. However, the FSW process suffers from defects occurred due to improper or sub-optimal selection of influencing process parameters.
Evaluation of welded samples for identification of defects can be achieved in terms of destructive as well as non-destructive techniques. Destructive testing methods include bend test, tensile test, fatigue test etc. These methods destroy the welded components that impede the use of the components. The non-destructive testing methods include visual testing, dye penetration testing, radiographic testing, eddy current testing,
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ultrasonic testing etc. (Gibson et al., 2014). In these techniques the welded components do not lose its integrity and it can be effectively used if passed the test. The limitations with these testing methods are high initial investment and skilled operator for the processes.
Investigation of welding defects in AA5456 aluminum alloy friction stir welds was carried out by Chen et al. (2006). The defects were detected using mechanical sectioning method and observing the samples under optical microscope. Void defect and kissing bond defect were reported to occur in the friction stir welded samples by Cao et al. (2009) in magnesium alloy. Mechanical sectioning method was used for detection of defects in the welds followed by optical microscopy. Defects in friction stir welded magnesium alloy were also investigated by Zhang et al. (2006). They also reported formation of void and kissing bond defect in the welded samples. Mechanical sectioning method followed by optical microscopic observation for defect detection. Investigation of defect formation in friction stir welded 6013 aluminum alloy was carried out by Zhao et al. (2014). Tunnel defect and kissing bond are the two defect types reported to occur during the process. Mechanical sectioning method with optical microscopic technique was used for identification of defects. Liu et al. (2012) investigated the effect of tool rotational speed and welding speed on formation of void defect in FSW process. High tool rotational speed with low welding speed was reported to be the cause of void defects. In this study the defects were characterized by mechanical sectioning method.
Voids and excessive flash were reported to occur in FSW of AA7075 aluminum alloy by Rezaei et al. (2011). They also reported that high tool rotational speed with low welding speed might be one of the reasons for voids in the welded samples. Macrostructure features of the joints obtained through mechanical sectioning of the joints were used for defect identification. Characteristics of kissing bond defect in friction stir welded AA1050 aluminum alloy were investigated by Sato et al. (2005). Improper heat generation due to low combination of tool rotational speed and welding speed were reported to be the possible cause of formation of kissing bond defect. Microscopic technique was used for identification of defects in the welded samples.
X-ray radiography and infrared thermography techniques were used by Saravanan et al. (2014) for identification of defects in friction stir welded joints. A methodology has been proposed based on radiography and thermography for identification of internal defects in FSW process. The study claimed that with the
Literature Review
developed method voids and micro pores can be detected with high accuracy. Signal features computed from vertical force signals acquired during FSW process were presented as indicator for identification of surface level defects by Kumari et al. (2016) and Kumar (2015). However, the method had not been tested for identification of internal defects in FSW process which is more challenging. Vertical force signal features based identification of worm hole defect in FSW process was also presented by Boldsaikhan et al. (2011). The authors concluded that with the inclusion of defect the frequency spectrum of the signals shows notable deviation than the spectrum for defect free welding cases. Effect of tool rotational speed and welding speed on defect formation of friction stir welded AA2014 aluminum alloy was investigated by Rajamanickam et al.
(2009). The welded samples are tested using X-ray radiography for identification of defect. Rosado et al. (2010) proposed a non destructive testing method for identification of defects in FSW process. The authors developed a new type of eddy current probe for identification of internal defects in FSW process. The investigation reported that with the proposed methodology detection of defects with characteristics size of 60 μm would be possible. A methodology based on transient eddy current for defect identification in FSW process was presented by Smith (2005). Eddy current based conductivity measurements were performed for the estimation of flaws within the welded samples. Shen et al. (2011) summarized the possible use of available non destructive methods for identification of defects in FSW process. Besides a series of optical metallographic inspection the research work considered X-ray detection, ultrasonic C-scan technique, ultrasonic phased array inspection and fluorescent penetrating fluid inspection technique were tested for identification of defects. Effect of tool rotational speed, welding speed and plunging force on defect formation of FSW of die casting aluminum alloy was investigated by Kim et al. (2006). Three defect types were reported in the investigation. Mass of flash due to excess heat input, cavity or groove like defect due to insufficient heat input and cavity produced by abnormal stirring. X-ray radiography technique was used for identification of internal defects occurred during FSW process.
The above discussion on review of available literature for possible defects in FSW process and its detection leads to the impression that improper selection or sub- optimal range of process parameters are mainly responsible for defect generation. The welding parameters should be selected within the suitable welding window of materials for sound defect free welds using FSW process. The researchers concentrated mostly on
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macrographs of welds for identification of defects using mechanical sectioning methods.
However, few researchers concentrated on testing and developing non destructive methods for defect identification in FSW process although the effort is less. The tested or developed methodologies are limited for implementation in FSW process at every scale and needs further development of methods for reliable identification of defects in welded samples. One of the possible reasons that most of the researchers attempted conventional practices for identification of defect is the cost associated with the available non destructive testing equipment and further precise knowledge base for interpretation of results. Real time information contained in process signals can be extracted through suitable processing technique and features can be developed for defect identification in FSW process.