RETRACTED: Workability Behavior of Aluminium Hybrid Composites (P/M)

Workability is a degree of the amount of deformation that a powder metallurgy bits and pieces can survive prior to fracture occurred in the forming or upsetting processes. Ductile fracture is the most general mode of breakdown in bulk forming process. The formability is a complicated happening, dependent upon the method as well as the material parameters. An investigational research work was performed for the kind of the working behavior of Al–SiC-Y2O3 hybrid composite under triaxial stress state circumstance. Upsetting of Al–SiCY2O3 powder metallurgy compacts with various aspect ratios and initial preform densities were carried out and the working behavior of the powder compacts at various state conditions was computed. In the powder metallurgy technique cold pressing can be used for compaction of the reinforcement of SiC and Y2O3 with Aluminum hybrid composites (Al+SiC+Y2O3). The bottle green compacts can be sintered and workability characteristics can be studied. In the hybrid composites, SiC content has been different from 0% to 20% with different particle sizes namely 60 and 80μm. Y2O3 nanoparticles of size 30-50nm and reinforced with 1wt% , 2wt% and 3wt%. The experimental Results can be analyzed for workability and frictional stress state conditions during upsetting as a role of relative density. The formability stress index values obtained for various addition of SiC and Y2O3.


Introduction
Powder metallurgy (P/M) is a extremely developed method of manufacturing accurate metal parts. Over the last even decades, the technology has matured from making self-lubricating bearings for autos to complex carrier gear set in automobile transmissions and high strength powder-forged connecting rods in engines. P/M technology includes the basic steps: mixing of elemental powders, compaction and sintering or heating the shape in a controlled atmosphere furnace to bond the particles together metallurgically. In the development of new aerospace alloys based on aluminium, titanium and super alloys, P/M technique plays a predominant role [1]. P/M products are widely used from the automotive industry through to aerospace, ordnance, power tool, electronics, business machines, household appliances, garden equipment and much more. This success is due to the advantages of the process which offers over other metal forming technologies such as forging and extrusion; advantages in material utilization, shape complexity and nearest shape dimensional control among others. These, in turn, yield benefits in lower costs and greater versatility [2] Workability is concerned with the extent to which a material can be deformed in a specific metalworking process without the initiation of cracks. The ductile fracture of the component is the most common mode of failure in any metalworking process. Workability is the complex technological concept that depends upon the ductility of the material and the details of the process parameters. It is the term used to the shaping of materials on various bulk deformation processes like forging, extrusion and rolling, to evaluate the capacity of a material to withstand the induced internal stresses of forming before the splitting of material occurs [3]. Therefore, a complete description of the workability of a material is specified by its flow stress which depends on processing variables such as strain, strain rate, preheat temperature, die temperature, its failure behavior and the metallurgical transformation that characterize the alloy system to which it belongs.
Mechanical, metallurgical and machinability studies on Al-SiC P/M composites have been done by various authors [4][5][6][7][8][9][10][11][12]. When the size of the SiC reinforcement of the composite is fine, the degree of cyclic hardening becomes high [13]. The composite materials have a monotonic increase in relative density with pressure [4]. The cyclic stress response of MMCs has strong dependence on the percentage of the reinforcement.
The ductile failure of the Aluminum matrix has been studied for the nucleation, growth and coalescence of voids under tensile load [5]. Studies have revealed that the Al-SiC composite gives better tensile fatigue performance compared to monolithic alloy [14]. Compression deformation test on Al-SiC composite has been carried out at elevated temperature and it has been found that the SiC added Aluminum powder metallurgy composite gives better formability compared to pure Aluminum and proved thro FEM technique [15].
Workability criterion of P/M compacts have been discussed by Abdel-Rahman and El-Sheikh [16], investigating the effect of relative density on the criteria forming limit of P/M compacts during upsetting also. They have proposed the criteria called formability stress index (b) for describing the effect of the mean stress and the effective stress with the help of two theories, proposed by Kuhn-Downey and Whang-Kobayashi.
One of the chief characteristics of the plastic deformation of metals is the fact that the shear stress required to produce slip intermittently increases with increasing shear strain [28]. The strain hardening is a phenomena of slip caused by the previous plastic deformation which in turn is caused by dislocations interacting with each other. Narayanasamy et al. [29,30] performed the work on the strain hardening behavior of the powder metallurgy composites. They evaluated the work hardening characterirstics of sintered Al-Al2O3 [31,33] and Al-Fe [29,33,34] composites preform under uniaxial, plane and triaxial stress state conditions.
In this paper, a complete investigation on the workability criteria of Aluminum and Al-SiC-Y 2 O 3 powder preforms was made during cold upsetting. Powder metallurgy preforms with various percentages and aspect ratios were discussed for studying the behavior of workability during cold upsetting under triaxial stress state condition. Literature dealing with the effect of various percentage on the workability under triaxial stress state condition on Al-Glass composite is not available. The present investigation is an attempt to evaluate the effect of the percentage on the workability parameter under triaxial stress state condition.  true strain in the axial direction ε 0 true strain in the hoop direction ε r true strain in the radial direction β formability Stress Index ρ o initial preform density of the perform (g/cc) ρ f density of the preform after deformation(g/cc) ρ th theoretical density of the fully dense material (g/cc) σ θ/reff stress ratio parameter ε m true mean strain h o /D o aspect ratio

Experimental Details
A. Compact preparation:Atomized aluminium powder of 100µm was procured from M/s. Metal Powder Company Ltd., Madurai, Tamil Nadu, India and analyzed for its purity. The same was found to be 99.7% and insoluble impurities to be 3%. The characterization of aluminium powder was studied by determining the flow rate, apparent density and particle size distribution and these details are provided in Table I.
Powder mix corresponding to Al-various percentage of SiC (different particle sizes namely 60 and 80µm) and Yttrium oxide (particle size of 30-50nm) were blended for all combinations on a pot mill to obtain a homogeneous powder blend. The weight percentage of SiC varied from 0 to 20 % with an interval of 4 % and weight percentage of yttrium oxide varied from 1%,2% and 3%. Cylindrical cold green compacts of various percent of hybrid composite of the above said blend were prepared using a suitable die assembly set. The die set assembly was made out of High Carbon High Chromium Steel ( Figure 1). Zinc-stearate was used as lubricant and applied on the inner surface of the die, outer surface of the punch and the top surface of the butt so as to avoid sticking of powder to these surfaces. A known weight of powder was taken and poured in to the die with its butt inserted at the bottom and then the punch was introduced from the top of the die. Green compacts (cylindrical in shape) of the powder blend was prepared on a 1.0 MN capacity hydraulic press using suitable punch and die assembly as shown in Figure 2. The compacting pressure applied was 520MPa, which was maintained for all composition of Al-SiC-Y 2 O 3 Hybrid composites.

Modern Achievements and Developments in Manufacturing and Industry
After compaction, the ejection of green compact was done by removing the butt and placing the die on two parallel blocks of the same height leaving a hole right in the centre of the free space between the two blocks and then applying pressure by means of hydraulic press. The damage to the compact was avoided by given that cotton under the compact. Thus when the punch started moving down through the die cavity, the compact came down in between the space provided by arranging the supporting blocks. The ejection load of the lower order suggested that there was very little sticking of the powder with the die and the punch surfaces. The free surfaces of the green compacts were first coated before sintering with an indigenously developed ceramic mixture [20] and dried under room-temperature condition for a period of 10 hours. Then, second coating was applied at a direction of 90º to the direction of first coating and was allowed to dry for a further period of 10 hours under the same condition as stated above.
B. Sintering: Sintering of powder sequentially involves the establishment and growth of bonds between the particles of powder at their areas of contact and migration of the grain boundaries formed at the bonds. This resulted in spheroidization of the pores between the particles, and the elimination of small pores. As the sintering temperature increases, porosity decreases and shrinkage increases [35]. Here, in the present case, the shrinkage was less because the sintering temperature (605ºC) was lower. Bonds formed between the particles during sintering and the number of particle bonds increase as the temperature increases. The ceramic-coated green compacts were sintered in an electric muffle furnace in the temperature range of 605ºC for a sintering time of 120 minutes and allowed to get cooled to room temperature in the furnace itself. Energy dispersive X-ray spectroscopy (EDAX) analysis: Figure 3 shows the presence of Silicon carbide, Aluminum and yttria particles in the provided sample. These EDAX facilities were availed of from Central Electrochemical Research Institute, Karaikudi, Tamil Nadu, India, using SEM model HITACHI S-3000H. Higher magnification was used for the present study.

D. Deformation test:
Initial diameter (Do), initial height (ho) and the initial perform relative density (ρ o ) of the specimen were measured and recorded. Each compact was subjected to the incremental compressive loads of 0.01 MN and the upsetting was carried between two flat, mirror finished open dies on a hydraulic press of 1.0 MN capacity. The deformation was carried out until the appearance of first visible crack was noticed on the free surface. After each interval of loading, dimensional changes in the specimen such as height after deformation (h f ), top contact diameter (D TC ), bottom contact diameter (D BC ), bulged diameter (D B ) and density of the preform (ρ f ) were measured. The schematic diagram showing the various parameters measured before and after deformation is provided in Fig. 4. Using the Archimedes principle, the density of upset performs was also determined after every loading interval. The deformation tests were continued until the fracture occurred at outer surface of the specimen.  The true effective stress can be determined from the following expression in terms of cylindrical coordinates as explained elsewhere [36] σ² eff = {[ σ² z + σ² θ + σ² r -R² (σ z σ θ + σ θ σ r + σ z σ r )] / [2R² -1]} Since σ r = σ θ for cylindrical axisymmetric upsetting operation, the true effective stress is determined as follows: σ eff = {[ σ² z + 2σ² θ -R² (σ z σ θ + σ² θ + σ z σ θ )] / [2R² -1]} ½

B. Formability stress index:
As an evidence of experimental examination implying the importance of the spherical component of the stress state on fracture, a Vujovic and Shabaik [37] proposed a parameter called a Formability Stress Index 'β' is given by, β = [ 3σ θ / σ eff ] This index determines the fracture limit as explained in Ref. [16].

Result and Discussion
A. Workability behavior of stresses on Al-SiC-Y2O3 hybrid composite Perform Fig. 5a-5d has been plotted between various triaxial stresses namely the true axial stress (σ z ), the true hoop stress (σ θ ), the true effective stress (σ eff ), the true mean stress (σ m ) and true axial strain (ε z ) for Aluminum contains various percentages of SiC and Y 2 O 3 .

Advanced Materials Research Vols. 984-985
It is observed that the true axial stress (σ z ) increases with increasing amount of SiC added in the Al-SiC-Y 2 O 3 composite. From these figures it is understood that the true axial stress (σ z ) and other stresses namely the true hoop stress (σ θ ), the true effective stress (σ eff ), and the true mean stress (σ m ), are affected by the amount of SiC content and Y 2 O 3 content. The same behavior has been observed when stresses namely the true hoop stress (σ θ ), the true effective stress (σ eff ) and the true mean stress (σ m ), are plotted against the true axial strain (ε z ). As SiC content increases the porosity level decreases and the relative density increases for the same compacting pressure. This may be one of the reasons for the increasing stresses for higher amount of SiC content. Further, with the increase of SiC content, the SiC particulates impede the motion of dislodgment and hence the stress required for further deformation increases. Due to these reasons all stresses increase with increasing amount of Silicon carbide. Fig.6a-6b has been plotted between the relative density and the formability stress index for the upsetting of Al-SiC-Y 2 O 3 powder compacts for the various aspect ratios and two different particle size of SiC under triaxial stress state condition. It is observed that the formability stress index varies with aspect ratio, particle size of SiC and various percentages of SiC. For the lower aspect ratio and higher initial preform density, the stress formability stress index takes a very high value. For compacts of higher aspect ratio and lower initial fractional preform density, the stress formability index value moves closer to the minimum value. Table II provides the maximum value of the formability index (b) for various SiC percentage addition tested in the Al-SiC-Y 2 O 3 composites. It is observed that as the SiC content increases, the formability stress index (β) also increases to a very high value because of better densification due to greater amount of load transfer rate to the Aluminum matrix by SiC percentage. The value of maximum formability stress index (β) for various Yttria content in composites are tabulated in Table III. The reason may be attributed to the fact that the pore size decreases with increasing addition of Yttria as that higher amount of load may be transferred by the Yttria to the Aluminum matrix. Here, better densification can be achieved for the higher addition of Yttria. The reason is that as the Yttria content increases, the relative density also increases and porosity decreases.

B. Workability behavior of formability stress index(β) on Al-SiC-Y2O3 hybrid composite Perform
The higher addition of Yttria in Aluminum matrix reduces the pore size. Smaller is the pore size, higher is the mean stress value. This may be the reason that the formability stress index (β) Modern Achievements and Developments in Manufacturing and Industry increases with increasing amount of Yttria content. This result is in good agreement with the findings of Narayanasamy et al. as described elsewhere [38].

C. Workability behavior of stress ratio parameter on Al-SiC-Y2O3 hybrid composite Perform
However, when the stress ratio parameter (σ m /σ eff ) is plotted against the relative density (R) for Aluminum containing five different SiC(two different particle size) percentages, two different aspect ratios and three different yttria percentages as in Fig. 7a-7b.
As the SiC content increases, the negative slope value also increases because Al-SiC-Y 2 O 3 composite exhibits fine pores compared to pure Aluminum. Because of this, the stress ratio parameter (σ z /σ m ) decreases with increasing SiC content. The reason is the true axial stress (σ z ) that is compressive in nature. The values of maximum stress ratio parameter (σ m /σ eff ) for variation of SiC contents in composites are tabulated in Table IV, and the maximum values of stress ratio parameter (σ m /σ eff ) for variation of yttria contents in composites are tabulated in Table V.

Conclusions
The following conclusions can be drawn from the above results and discussions. The addition of higher SiC content as reinforcement in the Aluminum matrix increases the strength property, because of reduction in pore size or increase in relative density. In general, the formability stress index (β) increases with increasing addition of SiC because the pore size decreases and the geometric work hardening would be lesser.
The formability stress index (β) increases with increasing addition of yttria because of better densification and decrease in pore size.
80µm size SiC added composites show higher formability stress index value (β) compared to other particle size added composite because of better densification due to better load transfer rate of SiC to the aluminium matrix compared to smaller particle size.
The stress ratio parameter (σ m /σ eff ) is found to be higher for Al-SiC-Y 2 O 3 composites compared to pure aluminium because of better densification.
The stress ratio parameter (σ z /σ m ) decreases in the case of Al-SiC-Y 2 O 3 composites compared to pure aluminium because of fine pore size associated with high hydrostatic stress (σ m ).
For performs with higher percentage addition of SiC,or yttria the initiation of crack exhibits at lower fracture strain.