Abstract: process named as Friction Stir Welding. Friction stir

Abstract:
Compared
with unreinforced metals, metal matrices reinforced with bio-compatible ceramic
phases are exhibiting osteoconduction, good wear resistance, high compressive
and tensile strengths and good toughness, which make them promising materials
for bio medical applications. Although, reinforced ceramic phases on metal
matrix composites are giving desired properties than the unreinforced metals,
it is more advantageous to have ceramic phases uniformly distributed on surface
of metal matrix composite. So, this can be achieved by a novel technique called
friction stir processing (FSP) which is a root of solid state welding process
named as Friction Stir Welding. Friction stir processing (FSP), adopted to
prepare surface composite with nano-hydroxyapatite (nHAP) as reinforcement and
magnesium alloy AZ31 as substrate. FSP was carried out for total six different
parameters i.e. three parameters with varying traverse speed at constant rotational
speed, other three parameters with varying rotational speed at constant
traverse speed and a comparison of mechanical properties was made between
processed samples and human bone. The FSP’ed AZ31-nHAP surface composite
obtained twice the tensile strength of human femur bone (250.54 MPa), Impact
toughness gained nearly thrice the toughness of human femur bone (2.43 MPa) and
Fracture toughness obtained nearly four times the fracture toughness of human
femur bone (20.52 MPa-m1/2). Scanning Electron Microscope (SEM) was
used to study the topology and composition of all the processed samples and a
very good dispersion of nHAP powder as a white bulky cloud matter on the
AZ31-nHAP surface composite was found for the constant traverse speed (20
mm/min) condition at rotational speed N=1800 rpm, which strengthens the future
usage of this surface composite as an implant to fractured human bone.

Introduction:

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   In today’s world, magnesium and its alloys
are plunging deep into various fields of engineering and science with its
remarkable properties such as low density, good strength, and corrosion
resistance. The study of magnesium and its alloys as degradable
implants is one of the promising research topics in the area of biomaterials.
Prime interest behind adapting the magnesium for biomaterial application is
their load bearing capacity and good mechanical properties near to the natural
human bone.
Magnesium alloys are mixtures of magnesium with other metals viz., aluminium,
zinc, manganese, silicon, copper, zirconium and rare earth metals. Magnesium
alloys have Hexagonal lattice structure which makes the alloy complicated, to
deform plastically. So, magnesium alloys are casted easily when compared with
aluminium, copper and steel etc. Magnesium alloys are two types i.e., cast
alloys and wrought alloys. In this research work, wrought alloy AZ31 is used
for evaluating its mechanical properties which is friction stir processed with
nHAP as reinforcement. Friction stir processing is a novel technique, which is
offshoot of friction stir welding, invented by The Welding Institute (TWI) in
1991.

                     

     Fig. 1 AZ31 before and after Processing                                        Fig. 2 Friction
Stir Processing

                                                                                                     

                        Friction stir
processing (FSP) 1 is a solid-state process in which a specially
designed rotating cylindrical tool, consisting of a pin and a shoulder, is
plunged into the sheet. The tool is then traversed in the desired direction as
shown in Fig. 2. The rubbing of the rotating shoulder generates heat which
softens the material (below the melting temperature of the sheet) and with the
mechanical stirring caused by the pin, the material within the processed zone
undergoes intense plastic deformation yielding a dynamically recrystallized
fine grain structure. A Shallow groove of 1 mm width and 2 mm depth was
produced on surface of AZ31 alloy and groove was filled with nHAP powder as
shown in Fig. 1. On the other hand, Hydroxyapatite, a calcium phosphate
mineral 2 which resembles natural bone mineral has emerged as a promising bio
ceramic material due to its excellent bio-compatibility and bone forming
ability. By applying Hydroxyapatite coating on surface of implant materials,
will improve bio-activity and osseointegration. Bio-activity was
investigated 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. Wettability
of samples were also investigated by measuring contact angles, which says the
immersion of nHAP particles in AZ31 alloy. Corrosion behaviour 3-5 was
also studied by immersing samples in SBF 5× at 37 oC at 1, 2, and 3
days to measure weight loss. In this way corrosion rates of sample were studied
by calculating weight loss for 3 days.

Experimental Procedure:

     A. Material
procurement and Processing

            Commercially available Magnesium
alloy (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 is
taken. The FSP tool is made of H13 tool steel with shoulder diameter 15 mm and
tapered pin with diameter varying from 5 mm to 3 mm over 2.7 mm of length. The
total 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 in
mechanical properties and are optimized to achieve defect free processed
samples.

                   

                Fig. 3 FSP Tool
without Pin                                          Fig. 4 FSP Tool with Pin

            The Non consumable FSP tool with and
without pin was made by turning the H13 steel on Lathe machine as shown in Fig
3, Fig. 4. A Shallow groove of 1 mm width and 2 mm depth was produced on
surface of AZ31 alloy and groove was filled with nHAP powder. Nano
Hydroxyapatite powder (Nano Wings Private Limited) containing Nano
particles of strip like geometry whose thickness in between 50-80 nm, width
is 10 µm, length in between 20-40 µm was considered. The next step was
plunging the rotating tool by the pin into the sheet for stirring the alloy and
required surface composite 10 was prepared. For tension test, three
parameters at constant rotational speed with varying traverse speed, and three
parameters at constant traverse speed with varying rotational speeds were taken
as shown in Table 1. Similarly for Impact test, three parameters at constant
rotational speed with varying traverse speed and three parameters at constant
traverse speed with varying rotational speed were taken as shown in Table 1. In
common, the work piece was applied with a load of 5 KN and the processed sample
was 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 samples
are now tested for obtaining mechanical properties such as tensile strength and
impact toughness. The first six samples were cut into dumbbell shape Fig. 5
according to ASTM B557M sub-specimen size. Tensile test was done on
Universal testing machine with capacity of 100 KN and toughness was done on
charpy impact test machine.

 

Fig. 5 ASTM Tensile Specimen

 

   C. Impact Test

            The
remaining samples were cut into cubical shape at friction stir processed region
according to IS 1757 (1988), a specimen of 10 × 5 × 55 mm
dimension as shown in Fig. 6 in order to find impact toughness of composite.

Fig. 6 Impact Test Specimen

            After
the testing of samples, the topology and composition of all the processed
samples were studied under Scanning Electron Microscope (SEM- Zeiss).

Results and Discussion:

            Six
tensile specimens which were friction stirred at different parameters are
tested for tensile strength by universal testing machine. Each sample is made
in to dumbbell shape according to ASTM B557M and allowed for tensile
test. The results are tabulated as follows in Table 2.

Scanning
Electron Microscope:

            A
Scanning electron microscope (SEM) is a type of electron
microscope that produces the images of a sample by scanning the surface of
metal with a focused beam of electrons. The electrons interact with atoms
in the sample, producing various signals that contain information about the
sample’s surface topography and the composition. Scanning Electron microscope images for the processed alloys at different
parameters are studied and are as follows:

Conclusions:

            In this
present work, Mechanical properties of AZ31-nHAP surface composite fabricated
by Friction stir processing by using six different parameters was evaluated
successfully and a comparison with human bone was made. The conclusions drawn
are as follows:

At Constant Traverse speed, Vx=20
mm/min; N=1800 rpm, Best result was obtained as

·        
Higher tensile
strength, ?U1 = 254
MPa which is twice the tensile strength of human femur bone (124 MPa)

·        
Higher impact
toughness, U1 = 2.43 MPa which is thrice impact toughness of human
femur bone (0.94 MPa)

·        
Higher fracture
toughness, KIC1 = 20.52 MPa-m1/2
which is four times the fracture toughness of human femur bone (5.1
MPa-m1/2)

            We
can 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 condition
of constant traverse speed with varying rotational speeds can affect more when
compared with constant rotational speed condition. Hence, AZ31-nHAP surface
composite can be a replacement to human femur bone with its good mechanical
properties.