Mohammad W. Dewan

Impact of Friction Stir Welding (FSW) Process Parameters on Thermal Modeling and Heat Generation of Aluminum Alloy Joints

Friction stir welding (FSW) is a solid-state joining process, where joint properties largely depend on the amount of heat generation during the welding process. The objective of this paper was to develop a numerical thermomechanical model for FSW of aluminum–copper alloy AA2219 and analyze heat generation during the welding process. The thermomechanical model has been developed utilizing ANSYS® APDL. The model was verified by comparing simulated temperature profile of three different weld schedules (i.e., different combinations of weld parameters in real weld situations) from simulation with experimental results. Furthermore, the verified model was used to analyze the effect of different weld parameters on heat generation. Among all the weld parameters, the effect of rotational speed on heat generation is the highest.

Effect of surface treatment and nanoclay on thermal and mechanical performances of jute fabric/biopol ‘green’ composites

The effects of surface modification of jute fibers and nanoclay on jute–biopol green composites are evaluated by the thermal and interlaminar shear strength (ILSS) characterizations. Four subsequent chemical treatments including detergent washing, dewaxing, alkali treatment, and acetic acid treatment were performed to facilitate better bonding between the fiber and matrix. The scanning electron microscopy and Fourier transform infrared spectroscopy study confirmed improved fiber surfaces for better adhesion with matrix after final treatment. Enhanced thermal performance and tensile properties were obtained due to chemical treatments. Montmorillonite K10 nanoclay (2–4 wt.%) was dispersed into a biodegradable polymer, biopol, using solution intercalation technique and magnetic stirring. Nanoclay-infused biopol resulted in 7% improvement in the degree of crystallinity over the neat biopol. Jute fiber-reinforced biopol biocomposites with and without nanoclay were produced using treated and untreated jute fibers by the compression molding process. Treated jute fiber-reinforced biopol composites (TJBC) without nanoclay showed 5% and 9% increases in decomposition temperature and storage modulus, respectively, and 19% decrease in coefficient of thermal expansion compared to untreated jute fiber-reinforced biopol composites (UTJBC). The respective values were 5%, 100%, and 45% for 4% nanoclay-infused TJBC compared to UTJBC without nanoclay. ILSS evaluated by the short-beam shear tests, improved by 20% in the TJBC compared to the UTJBC. Incorporation of 4 wt.% nanoclay in TJBC further improved the ILSS by 22% compared to that of TJBC without nanoclay.

Phased Array Ultrasonic Testing for Post-Weld and OnLine Detection of Friction Stir Welding Defects

ABSTRACT Nondestructive evaluation (NDE) techniques of phased array ultrasonic testing (PAUT) and digital X-ray radiography were employed on friction stir (FS)-welded Aluminum Alloy (AA)-2219-T87 specimens. PAUT intricacies required for scanning of FS-welded specimens with a 10-MHz 32-element transducer are discussed. The time corrected gain (TCG) calibration is required for scanning with an increase in index offset to compensate for decrease in A-Scan signal peak amplitude. Calibration techniques to find small defects with appropriate size tolerances are also established. The NDE technique of digital X-ray radiography is compared to PAUT, where it was found that a calibrated PAUT system is able to discover defects less than 0.2 mm where X-ray radiography could not. Incomplete penetration (IP), wormhole (WH), surface cavity (SC), and internal void (IV) defects are analyzed. Furthermore, an online PAUT system for FSW has been developed and successfully tested. The work provided herein will provide a gateway for an ultimate goal of an automated PAUT online sensing system.

Influence of Weld Defects and Postweld Heat Treatment of Gas Tungsten Arc-Welded AA-6061-T651 Aluminum Alloy

Welding defects and the reduction of mechanical performances are the foremost problems for fusion welded aluminum alloys joints. The influences of weld defects and post-weld heat treatment (PWHT) on tensile properties of gas tungsten arc (GTA) welded aluminum alloy AA-6061-T651 joints are investigated in this current study. All welded specimens are non-destructively inspected with phased array ultrasonic testing (PAUT) to classify weld defect and measure the projected defects area-ratio (AR). Ultimate tensile strength (UTS) decreased linearly with the increase of the size of weld defect but tensile toughness behaved non-linearly with defect size. Depending on defect size, defective samples’ joint efficiency (JE) varied from 35 to 48% of base metal’s UTS. Defect-free as-welded (AW) specimens observed to have 53% and 34% JE based on UTS and yield strength (YS) of base metal, respectively. PWHT was applied on defect-free welded specimens to improve tensile properties by precipitation hardening, microstructures refining, and removal of post-weld residual stresses. Solution treatment (at 540 °C) followed by varying levels of artificial age-hardening time was investigated to obtain optimum tensile properties. For GTA welded AA-6061-T651, peak aging time was 5

Post-weld residual stresses and heat treatments of Gas Tungsten Arc Welded aluminum alloy AA6061-T651

Heat-treatable AA-6061-T651 Aluminum alloys (Al-Mg-Si) have found considerable importance in structural and aerospace applications for their high strength to weight ratio and improved corrosion resistance properties. Intrinsic weld defects, post-weld residual stresses, and microstructural changes are the key factors for performance reductions and failures of welded structures. Gas-Tungsten-Arc-Welding (TIG/GTAW) was carried out on AA-6061-T651 plates with Argon/Helium (50/50) as the shielding gases. Non-destructive Phased-Array-Ultrasonic-Testing (PAUT) was applied for the detection and characterization of weld defects and mechanical performances. Ultrasonic technique was used for the evaluation of post-weld residual stresses in welded components. The approach is based on the acoustoelastic effect, in which ultrasonic wave propagation speed corresponds to the magnitude of stresses present within the materials. To verify the PAUTs residual stress results, a semi-destructive hole-drilling technique was use...

Effects of Residual Stresses and the Post Weld Heat Treatments of TIG Welded Aluminum Alloy AA6061-T651

Heat treatable AA-6061 T651 Aluminum alloys (Al-Mg-Si) have found considerable importance in various structural applications for their high strength to weight ratio and corrosion resistance properties. Weld defects, residual stresses, and microstructural changes are the key factors for the performance reduction as well as failure of welded structures. Tungsten inert gas (TIG/GTAW) welding was carried out on AA-6061 T651 Aluminum Alloy plates using Argon/Helium (50/50) as the shielding gas. Non-destructive phased array ultrasonic testing (PAUT) was applied for the detection and characterization of weld defects and characterization of the mechanical performances. In this study, ultrasonic technique was also used for the evaluation of post-weld residual stresses in welded components. The approach is based on the acoustoelastic effect, in which ultrasonic wave propagation speed is related to the magnitude of stresses present in the materials. To verify the estimated residual stresses by ultrasonic testing, hole-drilling technique was carried out and observed analogous results. The effects of post weld heat treatment (PWHT) on the residual stresses, grain size, micro hardness, and tensile properties were also studied. The grain size and micro hardness were studied through Heyn’s method and Vickers hardness test, respectively. Lower residual stresses were observed in post-weld heat-treated specimens, which also experienced from microstructure and micro hardness studies. The PWHT also resulted enhanced tensile properties for the redistribution of microstructures and residual stresses.Copyright

A Fully Coupled Thermomechanical Model of Friction Stir Welding (FSW) and Numerical Studies on Process Parameters of Lightweight Aluminum Alloy Joints

This paper presents a new thermomechanical model of friction stir welding which is capable of simulating the three major steps of friction stir welding (FSW) process, i.e., plunge, dwell, and travel stages. A rate-dependent Johnson–Cook constitutive model is chosen to capture elasto-plastic work deformations during FSW. Two different weld schedules (i.e., plunge rate, rotational speed, and weld speed) are validated by comparing simulated temperature profiles with experimental results. Based on this model, the influences of various welding parameters on temperatures and energy generation during the welding process are investigated. Numerical results show that maximum temperature in FSW process increases with the decrease in plunge rate, and the frictional energy increases almost linearly with respect to time for different rotational speeds. Furthermore, low rotational speeds cause inadequate temperature distribution due to low frictional and plastic dissipation energy which eventually results in weld defects. When both the weld speed and rotational speed are increased, the contribution of plastic dissipation energy increases significantly and improved weld quality can be expected.

Effects of Rotating-Bending and Torsional Fatigue Loads on Gas Tungsten Arc (GTA) Welded AISI 1018 Low Carbon Steel Joints

Most welded structures are subjected to multiaxial fatigue load and majority of the fatigue failure initiated from weld joints. Therefore, it is important to evaluate multiaxial fatigue behavior of commonly used welded materials. In the current investigations, the influence of rotating bending fatigue load along with torsional pulsed load was appraised for most commonly used AISI 1018 low carbon steel. A rotating-bending-torsional fatigue testing unit was designed and manufactured for biaxial fatigue test of welded and un-welded specimens. For welded specimens, Gas Tungsten Arc welding (also known as Tungsten-Inert-Gas (TIG) welding) was carried out on 19.05 mm diameter round bar of AISI 1018 steel using ER70-S2 filler metal. For rational comparison, only defects-free specimens were carefully chosen and tested. After welding, uniaxial tensile test was conducted to understand the fatigue loading criteria during rotating-bending fatigue test. Due to TIG welding, tensile strength was decreased considerably about 18% as compared to base metal. Rotating-bending (RB) and rotating-bending-torsional (RBT) fatigue tests were conducted to obtain a systematic understanding of biaxial fatigue behavior. RB fatigue life of welded specimens reduced compared to base metal as a result of complex thermal cycle during welding process and microstructural changes. Under combined loading conditions (RBT), base metal specimens did not exhibit significant difference on the fatigue behavior. However, for the welded specimens, the fatigue strength was reduced by about 12.8%. Moreover microstructural characterization and fracture surface analysis were performed to understand the fracture behavior of the tested specimens.Copyright

Effect of Weld Defects on Tensile Properties of Lightweight Materials and Correlations With Phased Array Ultrasonic Nondestructive Evaluation

Fusion welding of Aluminum and its alloys is a great challenge for the structural integrity of lightweight material structures. One of the major shortcomings of Aluminum alloy welding is the inherent existence of defects in the welded area. In the current study, tests have been conducted on tungsten inert gas (TIG) welded AA6061-T651 aluminum alloy to determine the effects of defect sizes and its distribution on fracture strength. The information will be used to establish weld acceptance/rejection criteria. After welding, all specimens were non-destructively inspected with phased array ultrasonic and measured the projected area of the defects. Tensile testing was performed on inspected specimens containing different weld defects: such as, porosity, lack of fusion, and incomplete penetration. Tensile tested samples were cut along the cross section and inspected with Optical Microscope (OM) to measure actual defect sizes. Tensile properties were correlated with phased array ultrasonic testing (PAUT) results and through microscopic evaluations. Generally, good agreement was found between PAUT and microscopic defect sizing. The tensile strength and toughness decreased with the increase of defect sizes. Small voids (area ratio <0.04) does not have significant effect on the reduction of tensile strength and toughness values. Once defective “area ratio (cross sectional area of the defect) / (total specimen cross sectional area)” reached a certain critical value (say, 0.05), both strength and toughness values decline sharply. After that critical value both the tensile strength and toughness values decreases linearly with the increase of defect area ratio.© 2014 ASME