Effect of tool rotational speed on microstructure and microhardness of AA6082/TiC surface composites using friction stir processing

A.Thangarasu 1,a, N.Murugan 2,b, I.Dinaharan3,c, S.J.Vijay4,d

1 Department of Mechanical Engineering, Sri Ramakrishna Institute of Technology, Coimbatore,

Tamil Nadu, India 2 Department of Mechanical Engineering, Coimbatore Institute of Technology, Coimbatore,

Tamil Nadu, India 3 Department of Mechanical Engineering, Coimbatore Institute of Technology, Coimbatore,

Tamil Nadu, India 4 School of Mechanical Sciences, Karunya University, Coimbatore,Tamil Nadu, India

aathangarasu@gmail.com, bmurugan@cit.edu.in, cdinaweld2009@gmail.com, dvijayjoseph@karunya.edu,

Key words: Friction stir processing, Aluminum alloy, Titanium Carbide, Microstructure.

Abstract. Friction stir processing (FSP) is as a novel modifying technique to synthesize surface

composites. An attempt has been made to synthesis AA6082/TiC surface composite using FSP and

to analyze the effect of tool rotational speed on microstructure and microhardness of the same. The

tool rotational speed was varied from 800 rpm to 1600 rpm in steps of 400 rpm. The traverse speed,

axial force, groove width and tool pin profile were kept constant. Scanning electron microscopy was

employed to study the microstructure of the fabricated surface composites. The results indicated

that the tool rotational speed significantly influenced the area of the surface composite and

distribution of TiC particles. Higher rotational speed provided homogenous distribution of TiC

particles while lower rotational speed caused poor distribution of TiC particles in the surface

composite. The effect of the tool rotational speed on microhardness is also reported in this paper.

Introduction

Nowadays , one of the biggest challenges to automobiles and aircraft fabrication industries

are finding ways to develop new light weight materials with higher strength to weight ratios.

Aluminum matrix composites (AMCs) are outstanding candidates for such applications owning to

the high ductility of the matrix and the high strength of hard reinforcing phases [1]. However, a

uniform distribution of reinforcements in AMCs is critical and a difficult task. Also the

incorporation of hard, non - deformable ceramic particles into the aluminium matrix results in loss

of ductility and toughness of AMCs. The lifespan of the component is governed by the properties of

the component surfaces. The inner matrix will retain its ductility and toughness, if the surface of the

components alone is modified by reinforcing with ceramic particles. Surface composite is the term

used to denote such modified surfaces [2]. Recently, much attention has been paid to a new surface

modification concept named friction stir processing (FSP). Mishra et al. [3] developed FSP solid

state processing technique, based on the principles of friction stir welding (FSW). One of the

methods to fabricate surface composites using FSP solid state technique is to make a groove of

required size, compact with reinforcement particles, plunge the non-consumable rotating tool and

traverse along the groove [4]. Matrix material’s plastic deformation and its interfacial friction

generate sufficient heat to plasticize the material and the deep stirring action of the tool pin

distributes the ceramic particle to the plasticized material. This solid state technique has been

effectively carried out by several investigators to fabricate surface composite on aluminum,

magnesium, steel and titanium alloys [5]. Mahmoud et al. [6] produced AA1050/SiC surface

composite using FSP technique and reported the formation of defects at higher tool rotational

speeds. Lim et al. [7] fabricated AA6111/CNT surface composite using FSP and found improved

distribution of carbon nanotubes at increased tool rotational speeds. Kurt et al. [8] fabricated

AA1050/SiC surface composite by FSP and reported that increased tool rotational speed affected

Applied Mechanics and Materials Vols. 592-594 (2014) pp 234-239© (2014) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMM.592-594.234

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the thickness of the surface composite, grain size, dispersion of the precipitates and reinforcing

particles. Asadi et al. [9] fabricated AZ91/SiC surface composite using FSP and reported an

increase in grain size and a decrease in hardness when tool rotational speed is increased. In the

present work, an attempt is made to fabricate AA6082/TiC surface composite using FSP technique

and study the effect of tool rotational speed on the microstructure and microhardness of the

composites.

Experimental procedure

The specimens used in this work were a 10 mm thickness AA6082 plates with 100 mm

length and 50 mm width. A groove of width 0.8 mm and depth 5 mm was contrived along the centre

line of the plate using wire cut EDM and compacted with TiC powder (size ~2 µm). In order to

prevent sprinkling of the TiC particles, the upper side of the groove was closed with a pinless FSP

tool. The material of the tool used in this work was (HCHCr.) with shoulder diameter of 22 mm.

The tool pin used in FSP is a standard M6 x 1 (pitch length of 1 mm, diameter of 6 mm) with pin

length of 5.5 mm (Fig.1a). An indigenously built FSW machine was used to carry out the FSP.

Finally, the tool with pin was plunged into the aluminium plate to produce the composite. The

traverse speed (60 mm/min) and axial force (10 kN) were kept constant. The tool rotational speed

was varied from 800 to 1600 rpm in steps of 400 rpm.

Specimens of 10 mm thickness prepared by cutting the friction stir processed plates at its

centre perpendicular to the processing direction, mechanically polished as per standard

metallographic procedure, etched with Keller’s reagent and observed using a metallurgical

microscope and scanning electron microscope. The digital image of the macrostructure of the

etched specimen was captured using a digital optical scanner. The microhardness was measured

using a microhardness tester at 500 g load applied for 15 seconds at various locations in the surface

composite and its average was calculated.

Fig.1. (a) FSP Tool (b) Typical crown appearance of friction stir processed AA6082/TiC

composites.

Results and discussion

Optical macrograph of crown appearance of friction stir processed aluminium with TiC

particles is shown in Fig. 1(b). It can be seen from the figure that the crown appearance of stir zone

was smooth, completely sound and there were no depressions or prominences. Semicircular features

similar to those formed during the conventional milling process are visible. The rubbing action of

the tool shoulder on the aluminium plate forms such features. Several trial experiments were

conducted initially to finalize a set of optimized process parameters to produce a defect free crown.

It is essential to achieve a smooth crown appearance because surface defects in the crown lead to

internal defects in the surface composite.

Applied Mechanics and Materials Vols. 592-594 235

Macrostructure of AA6082/TiC surface composites

The variation of macrostructure when tool rotational speed is increased from 800 rpm to

1600 rpm is presented in Fig. 2 for a constant traverse speed and axial force. It is evident from the

figure that the tool rotational speed greatly affects the area of friction stir processed zone that

contains the surface composite. The area of the AMC was computed to be 39 mm2 at 800 rpm and

57 mm2 at 1600 rpm. The area of the surface composite increases as the rotational speed is

increased. The area of the surface composite was measured using an image analyzing software.

Frictional heat is generated as a result of rubbing of the tool shoulder on the aluminum matrix. The

quantity of frictional heat generated is dependent upon the tool rotational speed [10]. When the tool

rotational speed increases up to 1600 rpm, the frictional heat generated increases. The amount of

plasticized aluminium is subsequently increased and material flow enhances with increase of tool

rotational speed. The actual volume fraction of TiC particles is reduced when the area of the surface

composite was increased, because the same amount of TiC particles packed in the groove is to be

distributed to more amount of plasticized aluminum. Typical FSW defects including tunnels, pin

holes or wormhole etc., are not found in the macrostructure of the surface AMCs. This indicates

that the selected set of process parameters is appropriate to produce sound AMCs.

Fig.2. Macrostructure of the friction stir zone containing AA6082/TiC surface composite at tool

rotational speed: (a) 800 rpm (b) 1200 rpm and (c) 1600 rpm.

Microstructure of AA6082/TiC surface composites

The effect of the tool rotational speed on the microstructure of AA6082/TiC surface composite is

shown in Fig. 3. The onion ring patterns are observed in the specimens with 1600 rpm

(Fig.3a). This band is due to material flow from the warmer zone, in the top to cooler zone, in the

bottom. The formation of the onion ring and the heat input is closely related. When the heat input is

not enough, the flow does not occur, because of the high flow resistance. The onion ring could not

be observed for the sample of low rotational speed. On the other hand, when the heat input is

sufficient for the flow, a clear onion ring could be observed. Therefore, it is proposed that the onion

ring is formed by the flow induced by the shoulder [11].

Figure 3 shows the SEM micrographs of AA6082/TiC surface composites at different

rotational speeds. The grains are coarsened as the tool rotational speed is increased from 800 rpm to

1600 rpm. The increase in the tool rotational speed generates higher frictional heat which leads to

coarsening of grains. The SEM micrographs clearly reveal the distribution of TiC particles in the

aluminium matrix. The distribution is not uniform at 800 rpm owing to the presence of TiC clusters

at several places. As it is seen, with further increase of tool rotational speed, the cluster size is

reduced and the reinforcement distribution is improved, due to increase in heat input and material

flow in the stir zone. The average spacing between TiC particles increased when the tool rotational

speed is increased. Apart from frictional heat generation, the tool rotational speed does two more

functions. Tool rotation stirs the plasticized materials as well as impacts material flow behavior

across the friction stir processed zone.

236 Dynamics of Machines and Mechanisms, Industrial Research

Fig.3. (a) Onion ring structure of higher rotational speed specimen. SEM micrograph of

AA6082/TiC composite at tool rotational speed: (b) 800 rpm (c) 1200 rpm and (d) 1600 rpm.

The formation of clusters at 800 rpm can be attributed to insufficient stirring action and inadequate

material flow from the advancing side to retreading side. The TiC particles that were preplaced in

the groove did not mix with the plasticized aluminum properly. Hence, clusters are formed. When

the tool rotational speed increases, the amount of stirring and material flow are increased. The

friction stir processing zone where the surface composite is formed is subjected to high plastic

strain. The increase in inter particle spacing can be attributed to drop in the actual volume fraction.

It is evident from micrographs shown in Fig. 3 that the tool rotational speed is a vital process

parameter which ominously dictates the distribution of TiC particles. The interface between TiC

particles and the aluminium matrix appears (Fig.3b-d) to be clean and is not surrounded by any

voids or reaction products. Lee at al. [12] analyzed the various reaction products at the interface of

TiC and aluminum matrix in Al/TiC AMCs prepared by infiltration casting route. No porosity or

reaction products can be seen in the micrographs of the AA6082/TiC surface composite around TiC

particles that confirms the presence of a clear interface. A clean interface increases the load bearing

capacity of the AMC and also provides good bonding between TiC particles and aluminium matrix.

Microhardness of AA6082/TiC surface composites

Microhardness of AA6082/TiC AMCs at various tool rotational speeds is presented in Fig.

4. When the tool rotational speed was increased, the microhardness decreased. The microhardness

was found to be 147 HV at 800 rpm and 105 HV at 1600 rpm. The microstructure coarsening in the

stir is more evident due to the fairly more amount of heat generation when higher tool rotation

speed is used. Therefore hardness in the stir zone decreases with increasing rotation speed. On the

other hand, a rotational speed of 800 rpm was too slow to reduce enough heat flow and deformation

and to produce a suitable distribution of TiC particles in the aluminium matrix, related to low heat

input and therefore less grain growth, these samples have the highest hardness value (HV).

Applied Mechanics and Materials Vols. 592-594 237

Fig. 4. Microhardness of AA6082/TiC surface composites at different rotational speeds.

Further the distribution and the inter particle spacing significantly affects the hardness of

the composite [13]. The microhardness is inversely proportional to the inter particle spacing. As

tool rotational speed is increased, the inter particle spacing increases. The increase in surface area

and decrease in actual volume fraction increases inter particle spacing when tool rotational speed is

increased. Therefore, the microhardness of the composite drops.

Summary

In the present work, AA6082/TiC surface composites were successfully synthesized using

the FSP and the effect of tool rotational speed on microstructure and microhardness of the surface

composite. The area of the surface composite, distribution of TiC particles and microhardness were

significantly influenced by tool rotational speed. The area of the surface composite increased when

tool rotational speed was increased. Lower tool rotational speed resulted in the formation of TiC

particle clusters and vice versa. The microhardness of the surface composite decreased when tool

rotational speed was increased. The microhardness was found to be 147 HV at 800 rpm and 105 HV

at 1600 rpm.

Acknowledgements The authors are grateful to the Management and Department of Mechanical Engineering,

Coimbatore Institute of Technology, Coimbatore, India for extending the facilities to carry out this

investigation. The authors also acknowledge the financial support rendered by the Naval Research

Board, DRDO, Govt. of India.

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