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تعداد صفحات این فایل: ۲۶ صفحه

بخشی از مقاله انگلیسیعنوان انگلیسی:A novel method for improving fatigue life of friction stir spot welded joints using localized plasticity~~en~~

Abstract

This study introduces a new method for enhancing the life and strength of friction stir spot welded (FSSW) joints using a localized plasticity process. The aim of this investigation is to evaluate the effect of cold expansion on the improvement of fatigue life of FSSW joints in aluminum alloy 7075-T6 plates through conducting experimental and numerical procedures. In the experimental section, the fatigue tests were carried out using constant amplitude load control servo-hydraulic fatigue testing machine on the as-welded and cold expanded FSSW joints with cold expansion level of 3.3%. In the numerical part, three-dimensional finite element models were implemented to calculate stress and strain distributions in two batches of specimens using two multi-axial fatigue criteria: Smith–Watson–Topper and Fatemi–۷۷۷Socie. The obtained experimental and numerical S–N data revealed that the cold expansion method could improve the fatigue life of FSSW joints in all load ranges so that in high cycle regimes, it could improve the fatigue life up to 6 times. Although the results revealed that there is a relatively good agreement between estimated fatigue lives and experimental data in reasonable fatigue life regime, considering the fatigue crack growth life can lead to better estimation.

۱ Introduction

Friction Stir Spot Welding (FSSW) technology is being targeted at modern automotive and aerospace industries in order to resolve the significant issues such as: energy saving, environmental preservation, fuel economy improvement, and performance. One of the efficient procedures is through replacing conventional iron based alloys with light and high strength materials such as aluminum, magnesium and reinforced polymer composites in fabrication of vehicles, as reported by Blawert et al. [1], and Davies [2]. These mentioned alloys are difficult to fusion weld, and their mechanical properties are drastically attenuated by fusion welding process, as reported by Cavaliere et al. [3]. The resistance spot welding (RSW) with some privileges like high tolerance to poor part fit up compared with fusion welding techniques is commonly employed for joining steel sheets; however, this method leads to poor mechanical performance [4, 5]. Generally, friction stir welding (FSW) is a revolutionary joining method which was introduced by TWI in 1991[6] with various advantages such as good retention of baseline mechanical properties, little distortion, and low residual stresses in comparison with traditional welding techniques. German factory GKSS [7] has recently applied friction welding process for spot welding, called friction stir spot welding (FSSW). In this new generated solidstate joining process, the specimens are connected to each other based on friction heating on the facing surfaces of predesigned special tool and sheets. The stirring process plays an important role in producing a plastically deformed zone around the pin. This new invented technology has been adopted by Mazda Motor [8] in mass production of hood and rear door of the sport model Mazda RX8 In FSSW the whole process of welding occurred under the melting point of welded alloys. That’s why it is appropriate for low melting point materials such as Al and Mg. Due to overwhelming development in application of this welding technique, many researchers have been allured to study the several aspects of this technology. The main issues are microstructural and mechanical characteristics of the joints, optimization of welding process parameters, capability in joining dissimilar materials, strength of the joints which are influenced by deviations in process parameters, and fatigue life and failure modes of spot friction welded joints. Most of the previous studies investigated the spot friction welds between similar and dissimilar aluminum alloys. Smith et al. [9] proposed a review on the advantages of using FSSW in transportation industry. Merzoug et al. [10] experimentally determined the optimal parameters of FSSW process in order to enhance the mechanical properties of Al 6065T5 welded joints. Da Silva et al. [11] investigated the effect of joining parameters on the mechanical properties and micro structural features; and described the material flow of dissimilar intermixing during FSSW into different aluminum alloys. Rodrigues et al. [12] studied the influence of welding parameters such as conical and flat shoulders, on the microstructure and mechanical characteristics of joints to get non defective welds. Most of the previous studies have focused on determining the influence of welding parameters on the microstructures, failure modes of SFW joints under quasistatic loading conditions. In another study by Lin et al. [13], the microstructures and failure modes of spot friction welds in lapshear specimens of aluminum 6111T4 sheets were studied using optical micrographs. Wang and Chen [14] predicted fatigue lives of FSSW in AL 6061T4 lap shear specimens under cyclic loading conditions based on Paris law and local stress intensity factors as functions of the kink length. Rash Ahmadi et al. [15] investigated the fatigue life of single friction stir spot welds by performing finite element analysis and strainbased modified Morrow’s damage equation considering hardness distribution, assuming the cyclic material constants and mechanical strength of different zones around the FSSW are proportional to the base material hardness value. A new technique has been recently proposed to improve the strength of friction welded joints which results in enhancement of static load capacity and fatigue life. In this method, a number of actions are used such as adding some particle reinforcement during the friction welding process, refilling the joints by using a special designed doubleacting tool to remove the created cavity in welded zone, and applying thermal annealing to friction welded structures. Some of the relevant studies have been done by Miller et al. [16]. They studied addition of metal particles during the spot friction welding process to create a localized Metal Matrix Composite (MMC) for the improvement of lap shear strength in AL 6111T4 alloys; and experimental procedures revealed that the MMC reinforcement improved the lap shear strength of Spot Friction Welded (SFW) joints by about 25%. Uematsu et al. [17] investigated the effects of refilling probe hole and increased effective cross sectional area of the nugget on tensile and fatigue behavior of FSSW joints. Although experimental results revealed that tensile strength of refilled joints was higher than the aswelded joints, the fatigue strength of the refilled joints was almost the same or in some cases lower than the joints with probe hole. In parallel with our mentioned goal, a process has been proposed to improve the fatigue life of FSSW joint in this research. This process generates compressive residual stresses at the periphery of the joints via passing a designed concave pin with larger size than the created hole in the center of the weld. The main objective of the present study is to evaluate the performance of the proposed method through investigating its effects on fatigue behavior of FSSW joints of aluminum 7075T6 and therefore some reliable strain based multi-axial fatigue criteria have been selected to predict fatigue lives of specimens based on finite element analysis.

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