Elsevier

Journal of Alloys and Compounds

Volume 726, 5 December 2017, Pages 1262-1273
Journal of Alloys and Compounds

Effect of multi-pass friction stir processing on the microstructure, mechanical and wear properties of AA5083/ZrO2 nanocomposites

https://doi.org/10.1016/j.jallcom.2017.08.084Get rights and content

Highlights

  • Multi-pass friction stir processing from 1 to 8 pass results in refinement of microstructure.

  • Further processed samples show remarkably higher amount of tensile strength.

  • Worn surface of the base metal is indicative of occurrence of adhesive wear.

  • Fractography indicates presence of dimples on the fracture surface of both the base material.

Abstract

In this study, multi-pass friction stir processing using zirconia nanoparticles was performed on AA5083 sheets to fabricate surface nanocomposites. Effect of using different number of passes on the microstructure, microhardness, tensile and wear properties of the specimens was investigated. Therefore, it was found that iteration of FSP consistently improves tensile properties and microhardness of the materials. This was mainly related to microstructural modification through FSP, including better dispersion of powders, finer grain size and minimum clustered particles. EBSD and TEM analysis indicated a large number of high angle grain boundaries and occurrence of continuous dynamic recrystallization (CDRX) in the 8-pass FSP'ed nanocomposite. Wear test also showed that wear rate of the further processed specimens was distinctly lower than their counterparts. Moreover, fracture surface of the eight-pass processed material revealed ductile fracture consisting mostly of dimples and voids, similar to the base material.

Introduction

Friction stir processing (FSP) is a relatively novel method based on the concept of friction stir welding (FSW) for selective or localized microstructure refinement of aluminum-based alloys, in order to enhance their mechanical and physical properties [1], [2]. FSP possess many advantages over conventional routes of processing including lack of casting defects, uniform distribution of alloying elements and reinforcements (in case of composites), phase homogenization, etc [3], [4]. It also has been successfully used for achieving fine grained structure parts, metal matrix composites (MMCs) as well as surface (layer) composites [5], [6].

In this process, a rotating tool comprising a shoulder and pin is plunged into a single piece of material, providing plastic flow by frictional heat and stirring, thereby refining microstructure of the material, inducing superplasticity through grain refinement and/or incorporation and dispersion of reinforcements in metals and alloys in order to manufacture composites [7], [8], [9], [10].

FSP has found numerous applications for fabricating aluminum based metal matrix composites (Al-MMCs) used in car and aircraft components, in which strength to weight ratio is the main criterion. Also, surface metal matrix composites (SMMCs) are manufactured using FSP. The latter is mainly carried out using two distinct methods, one being adding reinforcement to a solvent and applying the resulting product to the surface of plates. While the other involves cutting one or more grooves along the surface of plates, which are then filled by reinforcing particles, and performing FSP along the groove to produce a surface composite [11].

Some studies have reported fabrication of aluminum matrix composites and surface composites on aluminum alloys using FSP. HI Kurt investigated effect of FSP parameters on hybrid ratio on the ultimate tensile strength (UTS) of aluminum (Al) matrix (5083) hybrid composites. He found that UTS was significantly increased with increasing CNT, tool rotational and traverse speeds [3].

SHAHRAKI et al. produced AA5083/ZrO2 nanocomposite layer using FSP and reported uniform distribution of ZrO2 particles and improvement in tensile and hardness properties [12].

Yuvaraj et al. have studied wear characteristics of Al5083 surface hybrid nano-composites produced using FSP. They reported better mechanical and wear resistance properties of the composite relative to the base alloy [4]. Adem Kurt et al. used FSP for incorporation of SiC particles into the commercially pure aluminum to form particulate surface layers. They found that increase in rotational and traverse speed leads to a more uniform distribution of SiC particles. Also they reported higher bending strength and three times improvement in hardness of the surface composites compared to that of base aluminum [5].

S.A. Alidokht et al. investigated microstructure and tribological performance of A356 Al alloy based hybrid composite produced by FSP. They observed higher wear resistance of the processed samples relative to that of the as-cast alloy, which was related to formation of MoS2 layer on the top of the worn surface [13]. Mehdi Zohoor et al. have performed FSP on 5083Al alloy using micro and nano-sized Cu particles in order to investigate the microstructural and mechanical properties. They found that the optimum distribution of reinforcing particles was achieved using four FSP passes. Also, they found that composites with nanosized Cu particles show higher YTS, UTS, elongation and hardness values relative to the as-received samples [14].

Dolatkhah et al. investigated effects of FSP parameters on microstructural and mechanical properties of Al5052/SiC metal matrix composite. They found that change of tool rotational direction between FSP passes, increase in number of passes and decrease of SiC particles size enhance hardness and wear properties [6].

It is to be noted that with respect to multi-pass FSP, relatively few studies have been reported in the literature. For example, by carrying out multiple FSP of an as-cast AZ91 magnesium alloy, Asadi et al. [15] have reported achieving finer grain size and improvement of hardness, strength, elongation and wear resistance. Also, Sharifitabar et al. [16] have reported that increase in the number of FSP passes yields higher tensile and yield strength, but regarding elongation, while its value was higher in all the FSP'ed materials than the base material, it was decreased after 4-pass friction stir processing.

Barmouz and Givi [8] have used multi-pass FSP for fabricating Cu/SiC composites and observed improvement in dispersion of SiC particles and reduction of grain size through increase in the FSP passes. Nonetheless, El-Rayes and El-Danaf [17] have found that increasing number of passes for friction stir processing of aluminum alloy 6082 results in increase in the grain size of the stir zone (SZ) and also reduce in the UTS of the SZ due to accumulation of heat and dissolution of the hardening phase. On the other hand, Shafiei-Zarghani et al. [18] applied multi-pass FSP to fabricate Al/Al2O3 nanocomposites and reported more uniform distribution of alumina particles, higher hardness and wear resistance through increase in the number of FSP passes.

Recently, Chen et al. have investigated Influence of multi-pass friction stir processing on the microstructure and mechanical properties of Al-5083 alloy. They found that the grain size was increased with increasing the rotational speed. Also, application of multiple process passes did not change the microstructure and mechanical properties of the stir zone significantly. Additionally, they observed occurrence of abnormal grain growth (AGG) throughout the SZ using low rotational speed. However, high rotational speed and increasing number of passes was found to be beneficial for controlling AGG [19].

In this study, we aim to investigate effect of number of FSP passes for fabrication of AA5083/ZrO2 surface nanocomposites on their microstructure, mechanical and wear properties.

Section snippets

Experimental procedure

Commercial AA5083-H111 plates with dimensions of 5 × 120 × 100 mm was used as starting material. Chemical composition of the base materials is given in Table 1. Also, zirconia powder with the mean size of 110 nm and purity of 99.5% obtained from US Research nanomaterials Inc. was used as the reinforcing material. Firstly, a groove was machined out of the base materials with depth and width of 2 mm and 1 mm, respectively. The FSP tool was made of H-13 hardened steel consisting of a shoulder

Microstructure

Optical micrographs of the base material and fabricated nanocomposites are shown in Fig. 3. Based on Fig. 3b to f, three characteristic zones of friction stir processing are clearly distinguishable, namely heat affected zone (HAZ), thermo-mechanically affected zone (TMAZ) and stir zone (SZ). Also, fine equiaxed grains as a result of dynamic recrystallization are observed in all samples except the base material, which is due to frictional heat and intense plastic deformation during FSP [17], [23]

Conclusions

In the present study, effect of friction stir processing with different number of passes on the properties of AA5083-zirconia nanocomposites was investigated. Based on the current work, the following conclusions can be drawn:

  • 1

    Multi-pass friction stir processing using 2,4,6 and 8 passes consistently improves tensile properties of the AA5083 sheets, in addition to hardness due to constant refinement of microstructure and distribution of reinforcement particles, as was evident from the

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