Original Article

, Volume: 16( 2)

Influence of Aluminum Addition on the Structural and Physical Properties of Tin- Antimony Alloy

*Correspondence:

Abbas Al-Bawee Materials Engineering Department, University of Diyala, Diyala, 32001, Iraq
Tel: +0201009061678; E-Mail: abbash77@engineering.uodiyala.edu.iq

Abstract

The aim of this study is to investigate the effects of adding different ratios of aluminum on the structural, mechanical, electrical and thermal properties of tin-antimony bearing alloy Sn87Sb13. Many experimental techniques were used. The results showed that crystallinity (peak intensity), crystal size (peak broadness) and orientations (peak position, 2θ) of Sn87Sb13alloy changed after adding different proportions of aluminum. Many mechanical and physical properties changed; for example, the hardness and minimum shear stress of the alloy increased while its internal friction dramatically decreased. Furthermore, the addition decreased the melting temperature of the alloy and increased its electrical resistivity. Thus, the properties of the tin-antimony alloy improved after adding the aluminum portions.

Keywords

Tin-antimony alloy; White metals; Internal friction; Hardness; Physical properties.

Introduction

White metals can be classified into two types: tin and lead as its main component. Tin-based white metals are common for their high thermal conductivity, compression strength, fatigue, and impact strength and corrosion resistance in addition to their low tendencies towards segregation. Tin- aluminum and lead- aluminum alloys are slightly different in their mechanical properties. Indium and lead have the lowest elastic modulus of all soft alloying phases with aluminum, Xie et al. [1]. Sn-Sb alloy is one of the most important types of Sn-based alloys. Tin based white metals are called bearing alloys due to the intermetallic compound (SbSn), which is harder than the matrix. Resistance to wear is the most significant property of bearing alloys as they are typically used for sliding bearings to transmit loads between moving surfaces because of their lubrication property such as Babbitt alloys (white metals), which have a good combination of strength and surface properties referring to Wang et al. [2].

Many prior studies have investigated the properties of tin based bearing alloys. El-Bediwi, Al-Bawee and Kamal [3] have inspected the effect of adding alloying elements, such as Cu, Bi, Al, Pb, Zn and Ag on the structural, physical and mechanical properties of Sn₈₇Sb₁₃ alloy. The study indicated that the microstructure of the alloy and present phases were changed after adding the alloying elements. The results also showed that Sn₈₇Sb₁₀Pb3 alloy was the most interesting alloy due to its lowest corrosion rate and internal friction, acceptable hardness, self-lubricate property and good thermal properties, which are significant for automotive applications. In addition, El-Bediwi, Grayb and Kamal [4] studied the Influences of adding titanium oxide (TiO₂) nanoparticles on the mechanical and thermal properties of tin-aluminum-antimony comparing to tin-antimony-lead bearing alloy. The results revealed that the tin antimony lead alloy has better bearing properties such as low internal friction, high elastic modulus and thermal diffusivity for industrial applications. Furthermore, El-Bediwi, El-Shishtawi and Abdullah [5] have studied the different compositions of tin based alloys (Sn₈₀Al₁₀Sb₁₀, Sn₈₀Al₁₀Bi₁₀, Sn₈₀Al₁₀Bi₅Sb₅, Sn₇₆Al₁₀Sb₁₀Cu₂Zn₂ and Sn₇₆Al₁₀Bi₁₀Cu₂Zn₂. Sn₇₆Al₁₀Sb₁₀Cu₂Zn₂ alloy was the best alloy for bearing applications because of the high values of corrosion resistance, hardness, thermal diffusivity and low internal friction. They have also investigated at the same year the microstructure and mechanical properties of Sn₈₀Al₂₀, Sn₇₀Al₂₀Bi₅Zn₃Cu₂, Sn₆₅Al₂₀Sb₅Pb₅Cd₅, Sn₇₀Al₂₀Sb₅Ag₃Zn₂ and Sn₆₃Al₂₀Sb₁₀Pb₅Zn₂ and found that Sn₇₀Al₂₀Sb₅Ag₃Zn₂ alloy has the best properties for automotive industries. Moreover, Dawood [6] studied the properties of Sn₈₀Sb₁₅X (X=Cu_2.5Al_2.5 or Cu_2.5Pb_2.5 or Pb_2.5Zn_2.5 or Pb_2.5Bi_2.5) and stated that Sn₈₀Sb₁₅Sn₈₀Cu_2.5Al_2.5 alloy was the best one due to its low internal friction, high thermal diffusivity and temperature coefficient of resistivity values. Finally, El-Bediwi, Samir and Kamal [7] detected the structural, mechanical and thermal properties of several bearing alloys such as Sn₈₂Bi₁₅Zn₃, Sn₇₇Bi₁₅Zn₃Sb₅, Sn₇₉Bi₁₅Zn₃Ag₃ and Sn_81.3Bi₁₅Zn₃Cu_0.7 and proved that Sn₇₇Bi₁₅Zn₃Sb₅ has the lowest corrosion rate value. Also, the melting temperature and contact angle of the alloy decreased after adding an amount of bismuth.

The aim of this research is to investigate the effect of adding different portions of aluminum on the structural, mechanical and physical properties of tin-antimony Sn₈₇Sb₁₃ bearing alloy. Investigations the properties of Sn-based alloys are crucial for many engineering applications such as in die casting alloys, cable casing, battery grids, automobile and shipbuilding industry, Jiang et al. [8].

Materials and Methods

Sn_87−xSb₁₃Al_x (x= 5, 10, 15 and 20) alloys were prepared using tin, antimony and aluminium with a purity of 99.5%. The alloys were first melted in a muffle furnace. Next, the formed ingots were re-melted again to increase the homogeneity. From these alloys, long ribbons of approximately 4 mm width and 80 μm thicknesses were prepared by single roller melt spinning technique. Finally, the samples were cut by a double knife cuter to the required dimensions. The representative stream sediment sample was collected from different locations of wadi El Reddah of ore grade 380 ppm U₃O₈ for carrying out the study. The stream sediment sample was ground to desired particle size. After sample preparation, the sample was subjected to analysis and leaching study.

The microstructure of the samples was investigated by Shimadzu X-ray Diffractometer, (Dx–30, Japan), Cu–Ka radiation with l=1.54056 Å at 45 kV and 35 mA and Ni–filter, in an angular range 2q ranging from 0℃ to 100℃ in a continuous mode with a scan speed 5 deg/min. X-ray diffraction is an important tool used to identify phases, quantify changes in the cell parameters, orientation, crystallite size and other structural parameters. It is also used to determine the (crystallographic) structure (i.e. cell parameters, space group and atomic coordinates) of novel or unknown crystalline materials.

Differential scanning calorimetry (DSC) thermographs were obtained by SDT [Q600 (V20.9 Build 20), U. S. A] instrument with a heating rate of 10 k/min at a temperature range 0-400℃. Furthermore, a digital Vickers micro-hardness tester, (Model-FM-7- Japan), was used to measure the hardness of the samples. Moreover, the internal friction Q-1 was determined using the dynamic resonance method according to Cullity [9] and J.N [10] by plotting the amplitude of vibration against the frequency of vibration. Finally, thermal diffusivity Dth was derived and calculated from Berry and Pritchett equation according to Eichler et al. [11].

Finally, double-bridge method was used for measuring the electrical resistance of melt spun alloys. This method provides high accuracy for measuring small resistance owing to the fact that the series resistances of the contacts and current conductors have no effect on the potentials at the points (f) and (c) to which a nanometer is connected. Since the arm resistances R₁, R₂, R₃ and R₄ are substantially greater than these series resistances. Measurements are made as follow: by varying the resistances R₂, R₄ while resistances R₁, R₃ are left unchanged with a constant reference resistance RN until the potentials at points (f) and (c) are equalized which correspond to the zero reading of the nano-ammeter. At this moment the voltage drops across sections (af) and (fe) are equal to the voltage drops across sections (ac) and (ce) respectively. Then the resistance of the specimen (x) was calculated from the equation:

If R₁=R₃ and R₂=R₄ then:

To check that the bridge circuit readings are corrected a known reference resistance R_F is connected into the bridge instead of the measured unknown resistance X. The actual value Xact of the unknown resistance can then be found from the proportion,

Where X' = circuit reading with resistance X connected into it. R_F’ = reading with X replaced by R_F.

This checking can reduce substantially the measuring error of the double-bridge circuit (approximately from 0.1- 0.2 to 0.001 - 0.002 per cent).

Results and Discussion

According to the phase diagram of SnSb alloy (Figure 1), Sn₈₇Sb₁₃alloy forms two phases Sn and Sb-Sn.

The results of X-ray diffraction patterns showed Sn_87−xSb₁₃Al_x (x= 5, 10, 15 and 20) alloys having lines corresponding to β Sn, Al, Sb and Al-Sn phases, (Figure 2-6).

Table 1-5 identifies changes in the cell parameters, orientation, crystallite size (2q, intensity, d Å, full width at half maximum of peak, phases and Miller indices) of Sn_87−xSb₁₃Al_x alloys. The results showed that the started baseline, peak intensity (crystallinity), peak broadness (crystal size) and position (orientation) changed by adding different ratios of Al to the base alloy Sn₈₇Sb₁₃ since Al atoms dissolved in the Sn-Sb matrix alloy and formed a solid solution. Other accumulated Al atoms formed (Al or AlSn intermetallic phases).

Table 1. X-ray analysis of Sn₈₇Sb₁₃ alloy.

Table 2. X-ray analysis of Sn₈₂Sb₁₃Al₅ alloy.

Table 3. X-ray analysis of Sn₇₇Sb₁₃Al₁₀ alloy.

Table 4. X-ray analysis of Sn₇₂Sb₁₃Al₁₅ alloy.

Table 5. X-ray analysis of Sn₆₇Sb₁₃Al₂₀ alloy.

The crystal size was given by measuring the diffraction pattern broadening using Scherer formula mentioned in Cullity (1959). Table 6 shows the calculated lattice parameters, (a and c), unit volume cell (V) and crystal size (t) of the tetragonal tin phase. It can be clearly seen that adding different ratios of Al to Sn₈₇Sb₁₃ alloy led to a little variation in the lattice parameters and unit cell volume of the tetragonal tin phase in addition to a significant change in the crystal size.

Table 6. Lattice parameter, unit cell volume and crystal size of β- Sn in Sn_87-xSb₁₃Al_x alloys.

Resonance technique was used by taking the first amplitude of the ribbon at which the frequency appears, which is used to calculate the elastic modulus, thermal diffusivity, internal friction of the alloys, Figure 7.

Figure 8 states the calculated internal friction and thermal diffusivity values of Sn_87−xSb₁₃Al_x alloys as follows:

Internal friction = 0.57735026 * full width at half maximum of peak (FWHM)/Frequency (f)

Thermal diffusivity =2*thickness ^2 * frequency/ 3.14159

The results reveal that the internal friction of the base alloy decreases a lot after adding the different ratios of Al nevertheless, thermal diffusivity values are approximately stable at the same rate.

Figure 9 illustrates the calculated elastic modulus, shear modulus and balk modulus of Sn_87−xSb₁₃Al_x (x= 5, 10, 15 and 20 wt. %) alloys using Minitab17. The results show that Sn₆₇Sb₁₃Al₂₀ has the highest values concerning the three modules.

The modules were calculated as follows:

E= 38.32* (density (ρ)*(Poisson’s ratio (ν) ^4) *Frequency (f) ^2)/ (thickness of the sample ^2)

The density (ρ) of the sample= (mass)/ (0.04*0.004* thickness)

Poisson’s ratio (ν) of an alloy = summation of the Poisson’s ratio (ν) of elements * % per cent of the element in the compound.

Shear modulus = young modulus / (2*(1+Poisson ratio))

Bulk modulus= young modulus / (3*(1-2* Poisson ratio))

Furthermore, the results show that the hardness at a load (10 g) and indentation time (5 sec) and minimum shear stress of the Sn₈₇Sb₁₃ alloy increased after adding the Al proportions, Table 7. It seems that the intermetallic phase (Al Sb) formed after adding aluminum portions increased the hardness of the alloy as it’s harder than the matrix. Increasing the hardness improves wear resistance which is important in dry sliding condition.

Table 7. Vickers hardness and minimum shear stress of Sn₈₇-xSb₁₃Al_x alloys.

Moreover, Figure 10 exhibits the values of electrical resistivity of Sn_87−xSb₁₃Al_x (x= 5, 10, 15 and 20 wt. %) alloys. Generally, the electrical resistivity of the base alloy increased after adding Al content as dissolved Al atoms work as a scattering center for conduction electrons, which changed the measured electrical resistivity. In many applications, the electrical behavior of a material is more critical than its mechanical behavior: El-Bediwi et al. [13]. In order to select and use the alloys for industrial applications, we must understand how properties such as electrical resistivity is produced and controlled.

In general, plastic deformation raises the electrical resistivity as a result of increasing the number of electron scattering centers. Crystalline defects serve as scattering center for conduction electrons in metals. This depends on temperature composition and the degree of cold working. Electrons scattering may be caused by several types of lattice defects: by phonon scattering, impurities and by mechanical deformation. An experimental fact, known for a long time, is that the contributions to the resistivity ρ from these several sources are additive. This rule, known as Mathiessens rule that can be written as:

ρ total = ρ temperature + ρ impurities + ρ deformation

For this reason, increasing the rate of electrical resistivity with temperature is dependent of the small amount of present alloying elements and the state of deformation. The actual cause of resistivity must therefore be sought in deviations from the periodicity of the potential in which the electron moves that cause resistivity due to: (1) boundaries (2) lattice defect (3) lattice vibration (4) foreign impurity atoms.

Finally, DSC thermographs were obtained with a heating rate of 10℃ /min at a temperature range of (0 – 400℃). DSC thermographs of Sn_87−xSb₁₃Al_x (x= 5, 10, 15 and 20 wt. %) alloys are shown in Figure 11-15. Thermal analysis is used to study solid-state transformations as well as solid-liquid reactions. They depend on the nature of solid phase and its temperature. As it shown, a little variation in the shape of the thermal peak of SnSb₁₃ alloy caused by adding the different ratios of aluminum.

The melting temperatures of Sn_87−xSb₁₃Al_x alloys are presented in Figure 16. It can be clearly seen that the melting temperature of Sn₈₇Sb₁₃ alloy decreased after adding the different ratios of aluminum.

Conclusion

In conclusion, it can be clearly seen that adding different ratios of Al to Sn-Sb based alloy indicated forming new phases and differences in the lattice parameters and the unit cell volume of the tetragonal tin phase in addition to a substantial change in the crystal size. In terms of the mechanical properties, Sn₆₇Sb₁₃Al₂₀ alloy has the highest values of elastic, shear and balk modules. The hardness and minimum shear stress of the base alloy increased after adding the Al proportions. Moreover, the results showed that the internal friction of the origin alloy significantly decreased after adding the different fractions of Al, however, thermal diffusivity values were nearly settled at a fixed rate. The addition also increased the electrical resistivity and clearly decreased the melting temperature of the base alloy. Sn₇₇Sb₁₃Al₁₀ alloy is the most interesting alloy due to its low melting point and acceptable hardness, lowest internal friction, highest thermal diffusivity and electrical resistivity for bearing applications.

Competing Interests

No conflict of interest

Acknowledgements

The authors would like to acknowledge the high ministry of education and scientific research for support.

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