Abstract: process named as Friction Stir Welding. Friction stir

Abstract:Comparedwith unreinforced metals, metal matrices reinforced with bio-compatible ceramicphases are exhibiting osteoconduction, good wear resistance, high compressiveand tensile strengths and good toughness, which make them promising materialsfor bio medical applications. Although, reinforced ceramic phases on metalmatrix composites are giving desired properties than the unreinforced metals,it is more advantageous to have ceramic phases uniformly distributed on surfaceof metal matrix composite. So, this can be achieved by a novel technique calledfriction stir processing (FSP) which is a root of solid state welding processnamed as Friction Stir Welding. Friction stir processing (FSP), adopted toprepare surface composite with nano-hydroxyapatite (nHAP) as reinforcement andmagnesium alloy AZ31 as substrate. FSP was carried out for total six differentparameters i.

e. three parameters with varying traverse speed at constant rotationalspeed, other three parameters with varying rotational speed at constanttraverse speed and a comparison of mechanical properties was made betweenprocessed samples and human bone. The FSP’ed AZ31-nHAP surface compositeobtained twice the tensile strength of human femur bone (250.54 MPa), Impacttoughness gained nearly thrice the toughness of human femur bone (2.43 MPa) andFracture toughness obtained nearly four times the fracture toughness of humanfemur bone (20.

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52 MPa-m1/2). Scanning Electron Microscope (SEM) wasused to study the topology and composition of all the processed samples and avery good dispersion of nHAP powder as a white bulky cloud matter on theAZ31-nHAP surface composite was found for the constant traverse speed (20mm/min) condition at rotational speed N=1800 rpm, which strengthens the futureusage of this surface composite as an implant to fractured human bone.Introduction:   In today’s world, magnesium and its alloysare plunging deep into various fields of engineering and science with itsremarkable properties such as low density, good strength, and corrosionresistance. The study of magnesium and its alloys as degradableimplants is one of the promising research topics in the area of biomaterials.Prime interest behind adapting the magnesium for biomaterial application istheir load bearing capacity and good mechanical properties near to the naturalhuman bone.Magnesium alloys are mixtures of magnesium with other metals viz., aluminium,zinc, manganese, silicon, copper, zirconium and rare earth metals.

Magnesiumalloys have Hexagonal lattice structure which makes the alloy complicated, todeform plastically. So, magnesium alloys are casted easily when compared withaluminium, copper and steel etc. Magnesium alloys are two types i.e., castalloys and wrought alloys. In this research work, wrought alloy AZ31 is usedfor evaluating its mechanical properties which is friction stir processed withnHAP as reinforcement.

Friction stir processing is a novel technique, which isoffshoot of friction stir welding, invented by The Welding Institute (TWI) in1991.                            Fig. 1 AZ31 before and after Processing                                        Fig. 2 FrictionStir Processing                                                                                                                             Friction stirprocessing (FSP) 1 is a solid-state process in which a speciallydesigned rotating cylindrical tool, consisting of a pin and a shoulder, isplunged into the sheet. The tool is then traversed in the desired direction asshown in Fig. 2. The rubbing of the rotating shoulder generates heat whichsoftens the material (below the melting temperature of the sheet) and with themechanical stirring caused by the pin, the material within the processed zoneundergoes intense plastic deformation yielding a dynamically recrystallizedfine grain structure. A Shallow groove of 1 mm width and 2 mm depth wasproduced on surface of AZ31 alloy and groove was filled with nHAP powder asshown in Fig.

1. On the other hand, Hydroxyapatite, a calcium phosphatemineral 2 which resembles natural bone mineral has emerged as a promising bioceramic material due to its excellent bio-compatibility and bone formingability. By applying Hydroxyapatite coating on surface of implant materials,will improve bio-activity and osseointegration. Bio-activity wasinvestigated by immersing specimens in super saturated simulated body fluid(SBF×5) kept in a water bath at a temperature of 37­ oC for 72 h. Wettabilityof samples were also investigated by measuring contact angles, which says theimmersion of nHAP particles in AZ31 alloy. Corrosion behaviour 3-5 wasalso studied by immersing samples in SBF 5× at 37 oC at 1, 2, and 3days to measure weight loss. In this way corrosion rates of sample were studiedby calculating weight loss for 3 days.

Experimental Procedure:     A. Materialprocurement and Processing            Commercially available Magnesiumalloy (Exclusive Magnesium, Hyderabad, India) AZ31B plate (2.87% Al, 0.72% Zn,0.3% Mn, Remaining being Mg) of size 375 × 240 × 12 mm which is hot rolled istaken.

The FSP tool is made of H13 tool steel with shoulder diameter 15 mm andtapered pin with diameter varying from 5 mm to 3 mm over 2.7 mm of length. Thetotal plate was cut into 10 pieces by using Electron Discharge Machining (EDM),out of which six are for tensile testing and four for impact toughness test.

Six different processing parameters 6 are taken to study the changes inmechanical properties and are optimized to achieve defect free processedsamples.                                   Fig. 3 FSP Toolwithout Pin                                          Fig. 4 FSP Tool with Pin            The Non consumable FSP tool with andwithout pin was made by turning the H13 steel on Lathe machine as shown in Fig3, Fig. 4. A Shallow groove of 1 mm width and 2 mm depth was produced onsurface of AZ31 alloy and groove was filled with nHAP powder. NanoHydroxyapatite powder (Nano Wings Private Limited) containing Nanoparticles of strip like geometry whose thickness in between 50-80 nm, widthis 10 µm, length in between 20-40 µm was considered. The next step wasplunging the rotating tool by the pin into the sheet for stirring the alloy andrequired surface composite 10 was prepared.

For tension test, threeparameters at constant rotational speed with varying traverse speed, and threeparameters at constant traverse speed with varying rotational speeds were takenas shown in Table 1. Similarly for Impact test, three parameters at constantrotational speed with varying traverse speed and three parameters at constanttraverse speed with varying rotational speed were taken as shown in Table 1. Incommon, the work piece was applied with a load of 5 KN and the processed samplewas named as FSP’ed AZ31-nHAP surface composite.Table 1 Friction stir processing conditions Parametric Conditions Variables At Constant Rotational Speed, N = 1200 rpm Vx1  = 25 mm/min Vx2   = 32 mm/min Vx3  = 45 mm/min At Constant Traverse Speed,  Vx = 20 mm/min N1  = 1200 rpm N2  = 1400 rpm N3  = 1800 rpm     B. Tensile Test            The Friction stir processed samplesare now tested for obtaining mechanical properties such as tensile strength andimpact toughness. The first six samples were cut into dumbbell shape Fig.

5according to ASTM B557M sub-specimen size. Tensile test was done onUniversal testing machine with capacity of 100 KN and toughness was done oncharpy impact test machine. Fig. 5 ASTM Tensile Specimen    C. Impact Test            Theremaining samples were cut into cubical shape at friction stir processed regionaccording to IS 1757 (1988), a specimen of 10 × 5 × 55 mmdimension as shown in Fig. 6 in order to find impact toughness of composite. Fig. 6 Impact Test Specimen            Afterthe testing of samples, the topology and composition of all the processedsamples were studied under Scanning Electron Microscope (SEM- Zeiss).

Results and Discussion:            Sixtensile specimens which were friction stirred at different parameters aretested for tensile strength by universal testing machine. Each sample is madein to dumbbell shape according to ASTM B557M and allowed for tensiletest. The results are tabulated as follows in Table 2.ScanningElectron Microscope:            AScanning electron microscope (SEM) is a type of electronmicroscope that produces the images of a sample by scanning the surface ofmetal with a focused beam of electrons. The electrons interact with atomsin the sample, producing various signals that contain information about thesample’s surface topography and the composition. Scanning Electron microscope images for the processed alloys at differentparameters are studied and are as follows: Conclusions:             In thispresent work, Mechanical properties of AZ31-nHAP surface composite fabricatedby Friction stir processing by using six different parameters was evaluatedsuccessfully and a comparison with human bone was made.

The conclusions drawnare as follows:At Constant Traverse speed, Vx=20mm/min; N=1800 rpm, Best result was obtained as·        Higher tensilestrength, ?U1 = 254MPa which is twice the tensile strength of human femur bone (124 MPa)·        Higher impacttoughness, U1 = 2.43 MPa which is thrice impact toughness of humanfemur bone (0.94 MPa) ·        Higher fracturetoughness, KIC1 = 20.52 MPa-m1/2which is four times the fracture toughness of human femur bone (5.1MPa-m1/2)            Wecan also observe that SEM images for conditions, Vx­=20 mm/min;N=1800 rpm showing us very good uniform dispersion and Vx=25 mm/min;N=1200 rpm, showing good dispersion. Hence, it is concluded that the conditionof constant traverse speed with varying rotational speeds can affect more whencompared with constant rotational speed condition.

Hence, AZ31-nHAP surfacecomposite can be a replacement to human femur bone with its good mechanicalproperties.