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Abstract

Experimental investigation has been carried out to evaluate the effect of titanium carbide (TiC), molybdenum carbide (Mo₂C), iron carbide (Fe₃C) and tungsten carbide (WC) addition on the composite aluminium preforms. The hot upsetting of the composite aluminium preforms with various carbide contents, namely, Al-4% TiC, Al-4% WC, Al-4% Fe₃C and Al-4% Mo₂C, and different aspect ratios, namely, 0.4 and 0.6, was carried out and the workability behaviour of the same was determined. The influence of carbide addition in the aluminium composite and initial preform geometry on the relative density (R), stress ratio parameters, $σ_{θ} / σ_{eff}$ , $σ_{m} / σ_{eff}$ and $σ_{z} / σ_{eff}$ , and formability stress index was studied.

Keywords

Metal matrix composites plastic deformation porosity powder processing

Introduction

Powder metallurgy (P/M) manufacturing route is used for mass production of precision engineering materials as its ability to produce complex parts with close tolerances and with maximum material utilization. Aluminium metal matrix composites (MMCs) are used for wide variety of industrial applications due to its unique properties such as low specific density, high strength, low thermal expansion and good wear resistance and are economically viable.^1–5 Ductile aluminium matrix reinforced with stronger and stiffer carbides provides a combination of properties of the metallic material and ceramic reinforcement components.⁶ Titanium carbide (TiC) and tungsten carbide (WC)–based composites are currently used in high-strength application where improved strength, wear resistance and corrosion resistance are required^7,8 and aluminium reinforced with WC prepared via warm accumulative roll bonding process showed improved mechanical properties.⁹ One of the simplest secondary processes used by many researchers^10–12 is open die forging of cylindrical billets. The residual porosity left in the part causes the failure (visible cracks appearing on the free surface) in the compacts during open die forging and, hence the workability of the materials needs to be studied.

The workability or formability of the P/M material plays a major role in determining whether the P/M material will be formed successfully or fracture initiates in the forming process. Workability is a measure of the extent of deformation that a material can withstand due to the induced internal stresses of forming prior to fracture and is not only dependent on the material but also on several forming parameters such as stress and strain rates, friction and temperature.^13–15 Many researchers^16–20 have studied hot formability of aluminium MMC where the constitutive equations related to flow stress, temperature, strain rate and flow strain. Kuhn and Downey²¹ proposed a plasticity theory relating yield stress and deformed density by studying the deformation behaviour and the plasticity theory of some sintered P/M materials via open die forging.

A plasticity theory proposed by Shima and Oyane²² and Green²³ has been utilized to study workability and deformation behaviour considering numerous spherical cracks and voids, stress in the direction of compression and relative density. Narayanasamy et al.^24–26 studied the fracture criterion of porous materials under plane stress state, uniaxial stress state and triaxial stress state conditions relating hoop stress, mean stress, effective stress and the several stress ratio parameters. One of the most important parameters in the study of workability characteristics is the formability stress index proposed by Abdel-Rahman and El-Sheikh.¹⁴ It describes the effect of hydrostatic stress and the effective stress on the P/M compacts. Vujovic and Shabaik²⁷ and Doraivelu et al.²⁸ proposed a new yield criterion for porous material validating it using experiments and simulations. Ko et al.²⁹ studied the microstructure and hot workability of SiC_p/AA 2024 composite and reported that dynamic recrystallization was responsible for the hot restoration of the composites. Furthermore, they reported upon increasing the SiC_p volume fraction, the flow stress increased and the failure strain decreased.

Narayanasamy et al.³⁰ studied hot forging of P/M sintered high-strength 4% TiC composite steel preforms under different stress state conditions proposing a new geometrical shape factor and exponential relationship between the relative density ratio and hoop strain. It was reported that a straight line relationship was established between relative density against new geometrical shape factor and respective stress ratio parameters. Rajeshkannan¹² studied workability of sintered copper alloy preforms during cold upsetting using different curve fitting techniques and reported that decreasing aspect ratio facilitated deformation and improved the formability stress index; however, it limited height strain to fracture. Similar study was carried out by Narayanasamy et al.³¹ via hot forging of 4% TiC composite steel preforms and similar results were reported. Taha et al.¹¹ presented some experimental data on workability of aluminium particulate–reinforced MMCs prepared by stir-casting, squeeze-casting and P/M techniques. They reported that the workability of aluminium SiC- and Al₂O₃-reinforced MMCs is affected positively by the following: applying intermediate heat treatment, decreasing particulate volume fraction, decreasing particulate size and reinforcing with SiC in a wrought alloy matrix rather than Al₂O₃.

The residual porosity left in the P/M parts after the primary P/M process is a major drawback and these residual porosities are the main cause of failure during the secondary process and, hence workability studies are important in the preform geometry design and die constraint designs. Thus, the present investigation is aimed to establish the workability limit under triaxial stress state condition of P/M preforms of Al-4% TiC, Al-4% iron carbide (Fe₃C), Al-4% WC and Al-4% molybdenum carbide (Mo₂C) (weight percentage) experimentally and to establish the technical relationship that exists between the characteristics of axial stress, hoop stress, hydrostatic stress, effective stress and formability stress index with respect to true height strain and densification.

Experimental details

Materials and characteristics

Aluminium powder of less than or equal to 150 µm in size (diameter) and respective carbide powders, namely, TiC, WC, Mo₂C and Fe₃C, of less than or equal to 50 µm in size (diameter) was used in this experiment. The basic characterization of elemental aluminium powder such as flow rate, apparent density, compressibility and sieve analysis has been carried out using standard methods of testing. The characteristics are given in Tables 1 and 2, respectively.

Table 1.

Sieve size analysis of aluminium powder.

Sieve size (µm)	250	+200	+150	+100	+75	+45	−45
Retention in sieve (weight %)	0.2	0.3	16.3	55.3	9.5	7.9	10.5

Table 2.

Characterization of aluminium powder and its blends.

Property	Al	Al-4% WC	Al-4% TiC	Al-4% Fe₃C	Al-4% Mo₂C
Apparent density (g/cm³)	1.091	1.345	1.186	1.308	1.325
Flow rate (s/50 g) by hall flow meter	87.306	79.647	85.202	80.559	80.481
Compressibility (g/cm³) at pressure of 130 ± 10MPa	2.356	2.113	2.280	2.235	2.210

Powder blending and compaction

The required mass of aluminium and respective carbide powders was accurately weighed and mixed to obtain Al-4% TiC, Al-4% WC, Al-4% Fe₃C and Al-4% Mo₂C in a ball milling machine with powder mixed to stainless steel balls (10 mm inner diameter) with a ratio of 1:1 by weight. Air tight containers were used here to avoid oxidation of aluminium powders. The ball mill was operated for 10 h at 200 r/min to get a homogenized mixture. The apparent density was measured at 1-h interval to ensure homogeneous mixture was obtained. Towards the end of blending process, a consistent apparent density ensured homogeneous mix. The blended powders were then compacted using 100 ton capacity hydraulic press into cylindrical billets of aspect ratio (height-to-diameter ratio) of 0.4 and 0.6. The respective compacting pressures were obtained from the compressibility curve prepared for each material so as to obtain an initial theoretical density of 0.86 ± 0.01. Initial theoretical density is the initial density of the preform divided by 100% density of the preform (no pores).

Ceramic coating, drying and sintering

Immediately after compaction, an indigenously developed ceramic coating was applied on the compacts. This coating was allowed to dry for a period of 12 h at normal atmospheric conditions. Recoating was employed to the preforms in the direction 90° to that of the earlier coating. Again the compacts were allowed to dry for a period of 12 h. The coating was applied to avoid oxidation of compacts during the sintering process. The ceramic-coated compacts were sintered in an electric muffle furnace at a temperature of 220 °C for 30 min (drying process) and then at the temperature of 594 °C for further 60 min.

Hot deformation and measurements

Hot upsetting of the sintered preforms followed immediately after the sintering process at a temperature of 594 °C to the different levels of height strain. The forging operation was carried out with no lubricant. Dimensional measurements such as deformed height (h_f) and deformed diameters, namely, contact diameter at the top surface (D_c₁), contact diameter at the bottom surface (D_c₂) and bulged diameter (D_b), were carried out after every step of deformation. The density measurements of the forged specimens were carried out using Archimedes principle. Experimental data were used to calculate the stress ratio parameters, namely ( $σ_{θ} / σ_{eff}$ ), ( $σ_{m} / σ_{eff}$ ) and ( $σ_{z} / σ_{eff}$ ), axial strain, true diameter strain, percent relative density, actual bulged length and formability stress index.

Theoretical analysis

Using the mathematical expressions, the various upsetting parameters that influence workability characteristics of the selected composite were determined and presented here. The state of stress in a homogeneous compression process is as follows: according to Abdel-Rahman and El-Sheikh¹⁴

σ_{z} = \frac{load}{contact surface area} = - σ_{eff}, σ_{r} = σ_{θ} = 0

(1)

σ_{m} = (\frac{σ_{z}}{3}) = - (\frac{σ_{eff}}{3})

(2)

and the expression for the axial strain can be written as follows

ε_{z} = - ε_{eff} = \ln (\frac{h_{f}}{h_{o}})

(3)

and true hoop strain is

ε_{θ} = ε_{r} = \ln (\frac{D_{f}}{D_{o}})

(4)

where h_o is the initial height of the preform, h_f the forged height of the preform, D_f the contact diameter after deformation of the preform and D_o the initial diameter of the preform.

According to Narayanasamy et al.,^13,32 the hoop strain under plane stress state which includes the forged bulged diameter (D_b) and forged contact diameter (D_c) can be expressed as follows

ε_{θ} = \ln [\frac{2 D_{b}^{2} + D_{c}^{2}}{3 D_{o}^{2}}]

(5)

Plastic deformation of P/M materials is effected by the residual pores and the analysis of such materials requires an appropriate yield criterion which should take the pore effect into account. Many researchers over the years have analysed several different yield criteria for sintered powder materials and a typical theorem is that the plastic deformation occurs when the elasticity strain energy reaches a critical value.^33–35 The formulation can be written as

A J'_{2} + {BJ}_{1}^{2} = Y^{2} = δ Y_{0}^{2}

(6)

where A, B and δ are yield criterion parameters and are functions of relative density, J₁ is the first invariant of the stress tensor, $J'_{2}$ is the second invariant of the stress deviator and Y₀ and Y are yield strength of a solid and partially dense material having relative density R, respectively.³⁵ The parameters J₁ and $J'_{2}$ in the cylindrical coordinate system where the axis represents radial, circular and axial direction can be expressed as follows

J'_{2} = \frac{1}{6} [{(σ_{r} - σ_{θ})}^{2} + {(σ_{θ} - σ_{z})}^{2} + {(σ_{z} - σ_{r})}^{2}]

(7)

J_{1} = σ_{r} + σ_{θ} + σ_{z}

(8)

Here, for axisymmetric forging, $σ_{r} = σ_{θ}$ , $J'_{2}$ and $J_{1}^{2}$ can be written as

J'_{2} = \frac{1}{6} (2 σ_{θ}^{2} + 2 σ_{z}^{2} - 4 σ_{θ} σ_{z})

(9)

J_{1}^{2} = 4 σ_{θ}^{2} + σ_{z}^{2} + 4 σ_{θ} σ_{z}

(10)

Substituting equations (9) and (10) into equation (6) gives

\frac{A}{6} (2 σ_{θ}^{2} + 2 σ_{z}^{2} - 4 σ_{θ} σ_{z}) + B (4 σ_{θ}^{2} + σ_{z}^{2} + 4 σ_{θ} σ_{z}) = δ Y_{0}^{2}

(11)

Qin and Hua³³ have investigated and compared several yield criterion parameters based on plastic Poisson’s ratio, relative density and flow stress of the matrix material by previous researchers. The following yield criteria parameters are chosen in this research as $A = 2 + R^{2}$ , $B = (1 - R^{2}) / 3$ and $δ = 2 R^{2} - 1$ . Equation (11) can now be written as

Y_{0} = σ_{eff} = {[\frac{(σ_{z}^{2} + 2 σ_{θ}^{2} - R^{2} (σ_{θ}^{2} + 2 σ_{θ} σ_{z}))}{2 R^{2} - 1}]}^{0.5}

(12)

Equation (12) gives the expression for effective stress in terms of cylindrical coordinates and can be expressed in another form as

\frac{σ_{eff}}{σ_{z}} = {[\frac{1 + 2 {(σ_{θ} / σ_{z})}^{2} - R^{2} (2 (σ_{θ} / σ_{z}) + {(σ_{θ} / σ_{z})}^{2})}{2 R^{2} - 1}]}^{0.5}

(13)

According to Narayanasamy et al.,²⁵ the state of stress in a triaxial stress condition is given by

α = \frac{d ε_{θ}}{d ε_{z}} = \frac{(2 + R^{2}) σ_{θ} - R^{2} (σ_{z} + 2 σ_{θ})}{(2 + R^{2}) σ_{z} - R^{2} (σ_{z} + 2 σ_{θ})}

(14)

Using equation (14) for the values of Poisson’s ratio ( $α$ ), relative density (R) and axial stress ( $σ_{z}$ ), the hoop stress ( $σ_{θ}$ ) under triaxial stress state condition can be determined as follows

σ_{θ} = [\frac{2 α + R^{2}}{2 - R^{2} + 2 R^{2} α}] σ_{z}

(15)

where $α = d ε_{θ} / d ε_{z}$

Furthermore, rearranging equation (15)

\frac{σ_{θ}}{σ_{z}} = [\frac{2 α + R^{2}}{2 - R^{2} + 2 R^{2} α}]

(16)

Under triaxial stress state cylindrical coordinates, the hydrostatic stress can be written as follows assuming $σ_{θ} = σ_{r}$

σ_{m} = \frac{σ_{r} + σ_{θ} + σ_{z}}{3} = \frac{2 σ_{θ} + σ_{z}}{3}

(17)

Furthermore, rearranging equation (17)

\frac{σ_{m}}{σ_{z}} = \frac{1}{3} (1 + \frac{2 σ_{θ}}{σ_{z}})

(18)

The stress formability factor under triaxial stress state condition is given as

β = 3 [\frac{(σ_{m} / σ_{z})}{(σ_{eff} / σ_{z})}]

(19)

The stress formability factor as expressed in equation (19) is used to describe the effect of mean stress and the effective stress on the forming limit of P/M compacts in upsetting.

Results and discussion

A plot has been presented as shown in Figure 1 between axial strain and relative density for two different initial aspect ratios (height-to-diameter ratio of preforms), namely, 0.40 and 0.60, these plots being drawn for initial relative density of 86%. Relative density is the instantaneous density of the preform divided by 100% density of the preform (no pores).

Figure 1.

Relationship between relative density and axial strain during hot deformation

As seen in Figure 1, the relative density increases up to 0.45 axial strain, and thereafter relative density is almost constant till 0.60 axial strain. Then the relative density slightly lowers towards the final stages of deformation. The slight dip in relative density is more evident in Al-4% WC composite. Lower aspect ratio preforms showed better densification in comparison to higher aspect ratio due to the presence of lower pore bed height in the lower aspect ratio preforms. TiC-reinforced aluminium showed better densification rate and better final density achieved followed by Fe₃C-reinforced aluminium and then Mo₂C-reinforced aluminium. WC-reinforced aluminium had the lowest densification rate and final density achieved. However, an inverse relationship exists between densification and fracture strain for the respective composites.

Furthermore, a plot has been presented as shown in Figure 2 between diametrical strain and axial strain for two different initial aspect ratios, namely, 0.40 and 0.60, these plots being drawn for initial relative density of 86%. As seen in Figure 2, the lateral deformation is highest in WC-reinforced aluminium composite followed by Mo₂C, then Fe₃C and then TiC composites. This means the effective closer of pores is higher in TiC-reinforced aluminium; therefore, it has higher density than any other composite tested.

Figure 2.

Variation in the diameter strain with respect to the axial strain during hot deformation.

Table 3 shows the equations obtained using polynomial best fit graphs with correlation values close to 1.0 for R versus ε_z. It can be seen that for zero height strain, a constant value of approximately 0.86 is obtained. This constant will change accordingly if the initial relative density is varied as the constant represents the initial relative density. Furthermore, the first-order value in Table 3 is found to be positive, meaning it is contributing to the densification linearly. Also, it is seen that the ε_z coefficient increases as the aspect ratio decreases, projecting that decreasing aspect ratio promotes densification. The second-order coefficient is found to be negative, and hence, its contribution is negative to densification and this negative contribution is more in lower aspect ratio preforms. However, the effect is less as the coefficient values are small in a range of 0–0.3 and is multiplied to the square of axial strain.

Table 3.

Polynomial curve fitting result –R versus ε_z.

Al-4TiC	0.4	$R = - 0.2922 ε_{z}^{2} + 0.2834 ε_{z} + 0.8650$	$R_{c}^{2} = 0.9958$
	0.6	$R = - 0.2395 ε_{z}^{2} + 0.2579 ε_{z} + 0.8578$	$R_{c}^{2} = 0.9905$
Al-4Mo₂C	0.4	$R = - 0.1966 ε_{z}^{2} + 0.1725 ε_{z} + 0.8602$	$R_{c}^{2} = 0.9970$
	0.6	$R = - 0.0941 ε_{z}^{2} + 0.1006 ε_{z} + 0.8643$	$R_{c}^{2} = 0.9974$
Al-4Fe₃C	0.4	$R = - 0.1617 ε_{z}^{2} + 0.1843 ε_{z} + 0.8535$	$R_{c}^{2} = 0.9908$
	0.6	$R = - 0.1447 ε_{z}^{2} + 0.1697 ε_{z} + 0.8539$	$R_{c}^{2} = 0.9930$
Al-4WC	0.4	$R = - 0.1294 ε_{z}^{2} + 0.1218 ε_{z} + 0.8538$	$R_{c}^{2} = 0.9935$
	0.6	$R = - 0.0859 ε_{z}^{2} + 0.0861 ε_{z} + 0.8588$	$R_{c}^{2} = 0.9957$

A plot has been presented as shown in Figure 3 between stress ratio, $σ_{θ} / σ_{eff}$ , and relative density, R, for two different initial aspect ratios, namely, 0.40 and 0.60, these plots being drawn for initial relative density of 86%. Furthermore, similar plots have been plotted for stress ratios $σ_{m} / σ_{eff}$ and $σ_{z} / σ_{eff}$ presented in Figures 4 and 5. From these plots, it can be seen the effect of aspect ratio and composition is nil on the characteristics behaviour. The respective stress ratio parameters increase as the relative density increases. The stress ratio behaviour can be found in two stages against densification, one from the start of densification till 0.912 relative density and the other from 0.912 relative density till the end of deformation. The slope is found to increase from stage 1 to stage 2 as high load is required to further deform the specimen as very little pores are left towards the end of deformation. Against densification, the highest stress ratio value was achieved by TiC content composite followed by Fe₃C composite, Mo₂C composite and the lowest was WC-containing composite. Furthermore, the highest stress ratio parameter was obtained for smaller aspect ratio preform, which is true for all stress ratio preforms. This indicates that WC-containing composite and higher aspect ratio preform have further chance to be deformed provided the preform is free of cracks. The lateral deformation at the contact surface is lowest in TiC composite as seen in Figure 2 for any given height strain, indicating the bulging phenomenon is highest in TiC composite compared to Fe₃C, Mo₂C, and WC composites. The same can be seen in Table 4. Hence, the hoop stress is found to be highest in TiC composite compared to other composites. The same is true for the hydrostatic stress ( $σ_{m} / σ_{eff}$ ) found to be highest in TiC composite.

Figure 3.

Relationship between stress ratio ( $σ_{θ} / σ_{eff}$ ) and relative density during hot deformation.

Figure 4.

Relationship between stress ratio ( $σ_{m} / σ_{eff}$ ) and relative density during hot deformation.

Figure 5.

Relationship between stress ratio ( $σ_{z} / σ_{eff}$ ) and relative density during hot deformation.

Table 4.

Actual bulged length for the respective composites at 0.40 height strain.

Composite		Bulge length (mm)
Al-4% TiC	0.4	1.052
	0.6	1.460
Al-4% Fe₃C	0.4	1.037
	0.6	1.394
Al-4% Mo₂C	0.4	0.980
	0.6	1.355
Al-4% WC	0.4	1.020
	0.6	1.305

A plot has been presented as shown in Figure 6 between stress ratio, $σ_{θ} / σ_{eff}$ , and axial strain for two different initial aspect ratios, namely, 0.40 and 0.60, these plots being drawn for initial relative density of 86%. Furthermore, similar plots have been plotted for stress ratios $σ_{m} / σ_{eff}$ and $σ_{z} / σ_{eff}$ presented in Figures 7 and 8. The effect of aspect ratio on the stress ratios, $σ_{θ} / σ_{eff}$ , $σ_{m} / σ_{eff}$ and $σ_{z} / σ_{eff}$ is literally nil when plotted against axial strain except for TiC-containing composite. The stress ratio parameters rise to a maximum value as the deformation starts and then settle for the steady-state stress for the rest of the deformation. However, all the stress ratios slightly lower towards the end mainly in Al-4% Mo₂C and Al-4% WC. For any given height strain, the hoop, mean and axial stress obtained are highest for TiC-containing compacts, followed by Fe₃C, Mo₂C and lowest for WC-containing compacts. This means WC-containing compacts can be further deformed provided it is free from defects. The TiC particulates impede the motion of dislocations more than Fe₃C, Mo₂C and WC particulates, and hence, the stress required for further plastic deformation for TiC composite is more than other composite. Due to this reason, the stress ratios, $σ_{θ} / σ_{eff}$ , $σ_{m} / σ_{eff}$ and $σ_{z} / σ_{eff}$ , are higher for TiC composite for any given true height strain. Furthermore, the polynomial curve fitting results with correlation values close to 1.0 for σ_θ/σ_eff versus ε_z are given in Table 5. It is found that the first-order values decrease while the second-order values increase with the increasing aspect ratio. Also, the first-order values that increase the hoop stress linearly are found to be higher in TiC composite compared to other composites.

Figure 6.

Relationship between stress ratio ( $σ_{θ} / σ_{eff}$ ) and axial strain during hot deformation.

Figure 7.

Relationship between stress ratio ( $σ_{m} / σ_{eff}$ ) and axial strain during hot deformation.

Figure 8.

Relationship between stress ratio ( $σ_{z} / σ_{eff}$ ) and axial strain during hot deformation.

Table 5.

Polynomial curve fitting result –σ_θ/σ_eff versus ε_z.

Al-4TiC	0.4	$σ_{θ} / σ_{eff} = - 2.3226 ε_{z}^{2} + 2.3946 ε_{z} + 0.7555$	$R_{c}^{2} = 0.9989$
	0.6	$σ_{θ} / σ_{eff} = - 2.209 ε_{z}^{2} + 2.3554 ε_{z} + 0.5968$	$R_{c}^{2} = 0.9922$
Al-4Mo₂C	0.4	$σ_{θ} / σ_{eff} = - 1.183 ε_{z}^{2} + 1.0494 ε_{z} + 0.7852$	$R_{c}^{2} = 0.9991$
	0.6	$σ_{θ} / σ_{eff} = - 0.5314 ε_{z}^{2} + 0.5667 ε_{z} + 0.8191$	$R_{c}^{2} = 0.9951$
Al-4Fe₃C	0.4	$σ_{θ} / σ_{eff} = - 1.138 ε_{z}^{2} + 1.3437 ε_{z} + 0.6908$	$R_{c}^{2} = 0.9949$
	0.6	$σ_{θ} / σ_{eff} = - 1.0076 ε_{z}^{2} + 1.1931 ε_{z} + 0.7070$	$R_{c}^{2} = 0.9920$
Al-4WC	0.4	$σ_{θ} / σ_{eff} = - 0.9698 ε_{z}^{2} + 0.9169 ε_{z} + 0.7061$	$R_{c}^{2} = 0.9950$
	0.6	$σ_{θ} / σ_{eff} = - 0.7433 ε_{z}^{2} + 0.7681 ε_{z} + 0.7089$	$R_{c}^{2} = 0.9961$

A plot has been presented as shown in Figure 9 between formability index, $β$ , and axial strain for two different initial aspect ratios, namely, 0.40 and 0.60, these plots being drawn for initial relative density of 86%. The formability of the TiC-containing compacts was found to be higher followed by Fe₃C and Mo₂C composites. WC-containing compacts showed lowest formability ratio for any given axial strain. For any given axial strain, the relative density is found to be highest in TiC composite (Figure 1), meaning the porosity level is low and hence the reason for high formability index for TiC composite. The true hydrostatic stress ( $σ_{m} / σ_{eff}$ ) is higher in TiC composite than the true effective stress compared to Fe₃C, Mo₂C and WC composites (Figure 7) and is the reason for high formability in TiC composite. The compositions are calculated using weight percentage and TiC particulate being the lowest weight followed by Fe₃C, Mo₂C and WC (Table 2). This means the amount of smaller and fine pores present in Al-4TiC composite is more than other composites. The effective closure of pores is more in TiC composite during hot upsetting increasing the densification (Figure 1), and hence, for the same reason, the formability ratio is higher in TiC composite.

Figure 9.

Relationship between formability stress ratio and axial strain during hot deformation.

The polynomial curve fitting results with correlation values close to 1.0 for β versus ε_z are given in Table 6. Second-order and third-order polynomial fits are used to obtain the correlation values near unity; however, the constant, first- and second-order values are upmost important. A constant formability stress value was obtained for zero axial strain. This constant value is approximated to be 2.0 ± 0.3. It is found that the first-order values increase while the second-order values and constant values decrease with the increasing aspect ratio, and hence, the formability of the material decreases with increasing aspect ratio. The first-order values are found to be positive while the second-order values are found to be negative revealing that at the initial stages of deformation for low enhancement in axial strain the formability index is profound.

Table 6.

Polynomial curve fitting result –β versus ε_z.

Al-4TiC	0.4	$β = - 7.1569 ε_{z}^{2} + 7.2373 ε_{z} + 2.2798$	$R_{c}^{2} = 0.9979$
	0.6	$β = - 7.5964 ε_{z}^{2} + 7.4891 ε_{z} + 1.7654$	$R_{c}^{2} = 0.9971$
Al-4Mo₂C	0.4	$β = - 3.4469 ε_{z}^{2} + 2.9723 ε_{z} + 2.3094$	$R_{c}^{2} = 0.9965$
	0.6	$β = + 3.8739 ε_{z}^{3} - 7.7228 ε_{z}^{2} + 4.3776 ε_{z} + 2.1973$	$R_{c}^{2} = 0.9943$
Al-4Fe₃C	0.4	$β = - 3.3318 ε_{z}^{2} + 3.8454 ε_{z} + 2.1501$	$R_{c}^{2} = 0.9928$
	0.6	$β = + 4.6303 ε_{z}^{3} - 9.5042 ε_{z}^{2} + 6.0699 ε_{z} + 1.9044$	$R_{c}^{2} = 0.9913$
Al-4WC	0.4	$β = + 3.1557 ε_{z}^{3} - 7.1485 ε_{z}^{2} + 4.5426 ε_{z} + 1.8638$	$R_{c}^{2} = 0.9931$
	0.6	$β = + 9.71 ε_{z}^{3} - 15.516 ε_{z}^{2} + 7.4421 ε_{z} + 1.6901$	$R_{c}^{2} = 0.9963$

Furthermore, a plot has been presented as shown in Figure 10 between formability index, $β$ , and relative density for two different initial aspect ratios, namely, 0.40 and 0.60, these plots being drawn for initial relative density of 86%. The effect of aspect ratio and composition showed nil effect on the formability behaviour against relative density; however, the final formability ratio achieved against relative density and axial strain is also important. In view of this, a graph of formability index at fracture and axial strain at fracture is plotted as shown in Figure 11. The axial strain at fracture is found to be higher for higher aspect ratio preforms in comparison to lower aspect ratio preforms. There is an inverse relationship between fracture strain and fracture formability stress index as shown in Figure 11. This effect is more in lower aspect ratio preforms. WC-containing compacts showed higher fracture strain followed by Mo₂C composite, then Fe₃C composite and lowest fracture strain for TiC composites. Also, the number of carbide particles is higher in Al-4% TiC composite compared to other composites prepared, meaning the amount of pores is higher in Al-4% TiC leading to lower fracture strain.

Figure 10.

Relationship between formability stress ratio and relative density during hot deformation.

Figure 11.

Relationship between fracture strain ( $ε_{z}^{f}$ ) and formability stress index at fracture ( $β^{f}$ ) during hot deformation.

Furthermore, to understand the deformation behaviour of Al-4% TiC, Al-4% Fe₃C, Al-4% Mo₂C and Al-4% WC, the microstructure view of 100× magnification is shown in Figures 12 and 13, respectively. Particularly, the view was selected one at the centre (Figure 12) and other one at the extreme diametric side of each of the preforms (Figure 13) in order to view the presence of porosities. The upsetting axis is horizontal in Figures 12 and 13. The grain boundaries are clear and straight. As seen in Figure 12, less number of pores is found in TiC and Fe₃C when compared to Mo₂C and WC composites; however, more equi-axed grains are found in Mo₂C and WC composites compared to TiC and Fe₃C composites. The reduction in pores from undeformed specimen (green compact) to medium deformed and full deformed (at fracture) can be seen in Figure 13. The grains are equi-axed form in the green compact (Figure 13(a)), and then during deformation process, the grains elongate in the direction perpendicular to the compression direction as seen in Figure 13(c). The pores at the centre of the specimen are found to be spherical shape (Figure 12) while at the diametric ends are mainly elongated (Figure 13).

Figure 12.

Optical micrographs of various sintered aluminium composites with 50% deformation at the centre.

Figure 13.

Optical micrographs of Al-4% TiC at the diametrical end: (a) undeformed, (b) medium deformed and (c) fully deformed.

Conclusion

The design of preform shape and die are very important such that the final part produced is free from defects (fracture) since there is little possibility that the cracks can be arrested during the repressing stage of the deformation. Accordingly, the major conclusions have been drawn that are as follows.

The TiC-containing compacts showed better densification, thereby the stress formability index of the preform followed by Fe₃C, then Mo₂C and lowest for WC-containing compacts. However, it limited height strain to fracture.

Decreasing the aspect ratio facilitates uniform deformation resulting in improved densification and formability behaviour of the preform; however, it limits the height strain to fracture.

The variation in aspect ratio and hard carbide particles in aluminium composite made nil impact in the stress ratio behaviour against densification, however, against true axial strain induced is prominent.

The amount of pores and grain structure varies significantly in the composites tested at 50% deformation. The final grain distribution reveals strong orientation along the compression direction, resulting in a fibre structure.

The relationship between stress ratio and formability against axial strain and relative density is established using polynomial curve fitting results.

Footnotes

Appendix 1 Acknowledgements

The authors would like to acknowledge the help rendered by Mr Sanjay Singh, Mr Shiu Prasad and Mr Navneel Prasad, Metal Workshop Technicians at the University of the South Pacific, during the experimental phase of this project.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

References

Sahin

. Preparation and some properties of SiC particle reinforced aluminium alloy composites. Mater Design2003; 24: 671–679.

Sahin

. The effect of sliding speed and microstructure on the dry wear properties of metal matrix composite. Wear1998; 214: 98–106.

Derakhshandeh

Jahromi

. An investigation on the capability of equal channel angular pressing for consolidation of aluminum and aluminum composite powder. Mater Design2011; 32: 3377–3388.

Zebarjad

Sajadi

. Dependency of physical and mechanical properties of mechanical alloyed Al–Al₂O₃ composite on milling time. Mater Design2007; 28: 2113–2120.

Fogagnolo

Velasco

Robert

. Effect of mechanical alloying on the morphology, microstructure and properties of aluminium matrix composite powders. Mat Sci Eng A: Struct2003; 342: 131–143.

Prabhu

Suryanarayana

. Synthesis and characterization of high volume fraction Al–Al₂O₃ nanocomposite powders by high energy milling. Mat Sci Eng A: Struct2006; 425: 192–200.

Sheibani

Fazel

. In situ fabrication of Al-TiC metal matrix composites by reactive slag process. Mater Design2007; 28: 2373–2378.

Zhong

Hojamberdiev

. In situ fabrication of titanium carbide particulates-reinforced iron matrix composites. Mater Design2011; 32: 3790–3795.

Lui

Wang

Jia

. Evaluation of mechanical properties of 1060-Al reinforced with WC particles via warm accumulative roll bonding process. Mater Design2013; 43: 367–372.

10.

Selvakumar

Ganesan

Radha

. Modelling the effect of particle size and iron content on forming of Al-Fe composite preforms using neural network. Mater Design2007; 28: 119–130.

11.

Taha

El-Mahallawy

El-Sabbagh

. Some experimental data on workability of aluminium-particulate-reinforced metal matrix composites. J Mater Process Tech2008; 202: 380–385.

12.

Rajeshkannan

. Workability studies on cold upsetting of sintered copper alloy preforms. Mater Res2010; 13: 457–464.

13.

Narayanasamy

Ramesh

Pandey

. Some aspects on workability of aluminium-iron powder metallurgy composite during cold upsetting. Mat Sci Eng A: Struct2005; 391: 418–426.

14.

Abdel-Rahman

El-Sheikh

. Workability in forging of powder metallurgy compacts. J Mater Process Tech1995; 54: 97–102.

15.

El-Domiatry

Shaker

. A note on the workability of porous-steel preforms. J Mater Process Tech1991; 25: 229–233.

16.

Spigarelli

Cerri

Cavaliere

. An analysis of hot formability of the 6061 + 20% Al₂O₃ composite by means of different stability criteria. Mat Sci Eng A: Struct2002; 327: 144–154.

17.

Rahimian

Ehsani

Parvinb

. The effect of particle size, sintering temperature and sintering time on the properties of Al–Al₂O₃ composites, made by powder metallurgy. J Mater Process Tech2009; 209: 5387–5393.

18.

Dannininger

Jangg

Weiss

. Microstructure and mechanical properties of sintered iron. Part I: basic consideration and review of literature. Powder Metall Int1993; 25: 170–173.

19.

Lee

Park

. A new method of preform design in hot forging by using electric field theory. Int J Mech Sci2002; 44: 773–792.

20.

Solhjoo

. Analysis of flow stress up to the peak at hot deformation. Mater Design2009; 30: 3036–3040.

21.

Kuhn

Downey

. How flow and fracture affect design of preforms for powder forging. Int J Powder Metall Powder Tech1974; 10: 59–66.

22.

Shima

Oyane

. Plasticity theory for porous metals. Int J Mech Sci1976; 18: 285–291.

23.

Green

. A plasticity theory for porous solids. Int J Mech Sci1972; 14: 215–224.

24.

Narayanasamy

Senthilkumar

Pandey

. Effect of titanium carbide particle addition on the densification behaviour of sintered P/M high strength steel preforms during cold upset forming. Mat Sci Eng A: Struct2007; 456: 180–188.

25.

Narayanasamy

Ponalagusamy

Subramanian

. Generalized yield criteria of porous sintered powder metallurgy metals. J Mater Process Tech2001; 110: 182–185.

26.

Narayanasamy

Anandakrishnan

Pandey

. Effect of geometric work-hardening and matrix work-hardening on workability and densification of aluminium–3.5% alumina composite during cold upsetting. Mater Design2008; 29: 1582–1599.

27.

Vujovic

Shabaik

. A new workability criterion for ductile materials. J Eng Mater: T ASME1986; 108: 245–249.

28.

Doraivelu

Gegel

Gunasekara

. A new yield function for compressible P/M materials. Int J Mech Sci1984; 26: 527–535.

29.

Park

Yoo

. The effects of SiC particle volume fraction on the microstructure and hot workability of SiC_p/AA 2024 composite. J Mater Process Tech1999; 95: 210–215.

30.

Narayanasamy

Senthilkumar

Pandey

. Some features on hot forging of powder metallurgy sintered high strength 4% titanium carbide composite steel preforms under different stress state conditions. Mater Design2008; 29: 1380–1400.

31.

Narayanasamy

Senthilkumar

Pandey

. Some aspects of workability studies on hot forging of sintered high strength 4% titanium carbide composite steel preforms. Mat Sci Eng A: Struct2006; 425: 121–130.

32.

Narayanasamy

Ramesh

Pandey

. Workability studies on cold upsetting of Al–Al₂O₃ composite material. Mater Design2006; 27: 566–575.

33.

Qin

Hua

. Deformation and strengthening of sintered ferrous material. J Mater Process Tech2007; 188: 694–697.

34.

Han

Lee

. Analysis of forging limit for sintered porous metals. Scripta Metall Mater1995; 32: 1937–1944.

35.

Lewis

Khoei

. A plasticity model for metal powder forming processes. Int J Plasticity2001; 17: 1659–1692.

Workability studies of sintered aluminium composites during hot deformation

Abstract

Keywords

Introduction

Experimental details

Materials and characteristics

Powder blending and compaction

Ceramic coating, drying and sintering

Hot deformation and measurements

Theoretical analysis

Results and discussion

Conclusion

Footnotes

Appendix 1

Acknowledgements

Declaration of Conflicting Interests

Funding

References