Mechanical Behavior of AA7475-B4C Composites

Mechanical Behavior of AA7475-B4C Composites

Shivraj Koti, S B Halesh Department of Mechanical Engineering,

Sir MVIT, Bangalore-562157, Karnataka, India

Madeva Nagaral

Aircraft Research and Design Centre,

HAL, Bangalore-560037, Karnataka, India

V Auradi

R&D Centre, Department of Mechanical

Engineering, SIT, Tumkur-572103,

Karnataka, India

AbstractThe work is carried out to investigate and study the mechanical properties of B4C reinforced AA7475 alloy metal matrix composites. In the present work AA7475 alloy was taken as the base matrix and B4C particulates as reinforcement material to prepare metal matrix composites by stir casting method. For composites the reinforcement material was varied in 6 & 8 wt. %. For each composite, the reinforcement particulates were preheated to a temperature of 300ºC and dispersed into a vortex of molten AA7475 alloy. The microstructural characterization was done using scanning electron microscope. Mechanical properties like hardness, ultimate tensile strength and yield strength were evaluated as per ASTM standards. Further, scanning electron microphotographs revealed that there was uniform distribution of B4C particulates in AA7475 alloy matrix. Hardness, ultimate tensile strength and yield strength increased as wt. % of B4C increased in the base matrix

Keywords: AA7475 Alloy, B4C, Stir Casting, Hardness, Tensile Strength

to improve their mechanical and physical properties. Fabrication of composites having metals with limited mutual solubility can be produced, utilizing the properties of the alloy and the resulting composite [5, 6]. With their high formability and low work hardening rates, these composites can be produced at low cost. In terms of applications, AlCu- Mg alloys shares a larger fraction of aluminium alloys due to their high specific mechanical properties and wide range of alloys and properties. The basic idea of developing metal matrix composites is to derive high strength materials.

Large number of products have been designed and manufactured for various applications. Many of the investigations have shown improved mechanical properties, but limited with low & poor ductility. In the present investigation, an attempt has been made to achieve a good combination of strength & hardness properties with composites.

1. INTRODUCTION

   Metal matrix composites (MMCs), a metal/alloy with ceramic reinforcements, are known for their high specific strength, modulus and hardness properties [1]. These share a good fraction in aerospace applications. Though there are many applications with MMCs, secondary processing, compatibility between matrix and reinforcement and characterization are still the major problems in the manufacturing of these composites. To limit the limitations associated with metal-ceramic composites, metal-metal composite systems were chosen. Thought has been given by Madhu et al. [2], to have the advantage of MMCs and metal metal combination system by choosing the conventional alloy systems for the manufacture of composites with restricted solubility, termed as metal-metal composites. To have good compatibility between the matrix and the reinforcement, an established alloy system with proven application needs to be chosen, where the solvent acts as the matrix and the solute as the reinforcement. Solute dissolution needs to be restricted / controlled to have the reinforcement effect. Strengthening of the resultant composite can be achieved by a combination of reinforcement, alloying, and heat treatment. The other advantages include use of conventional foundry facilities for manufacturing of these composites [3, 4], secondary processing of these composites at much lower energy level than the alloy and the MMCs etc.

   Particle reinforced aluminium metal matrix composites (AMCs) have been developed in the last few years, to reduce the wt. of components used in structural applications and also

   There are several aluminium alloys are available to fabricate the metal matrix composites. Among all the alloys 2XXX, 6XXX and 7XXX series are the most commonly using materials for aerospace, structural and aerospace applications. AA2024 and 2014 alloys are wrought alloys used in aerospace structural applications. AA6061 alloy is the light weight and medium strength Al alloy commonly considering for the automotive and home appliances [7].

   AA7475 alloy is the class of 7XXX series alloy having high strength and good fracture toughness. In the present investigation an attempt has been made to develop AA7475- B4C reinforced composites by melt stirring concept. Further, prepared AA7475-B4C composites were evaluated for mechanical properties to know the influence of reinforcement particulates on the AA7475 alloy.

2. MATERIALS AND EXPERIMENTAL DETAILS AA7475 is one of the 7xxx series alloys, in which zinc is

   the major alloying element; usually it is in combination with the magnesium and copper. Alloys of this series have the highest strength among all series. Alloys of this series possess the highest mechanical strength when heat treated.

   The matrix material used in the experimental investigation is Aluminium 7475 alloy whose chemical composition is listed in Table 1. AA7475 alloy is one type wrought Aluminium alloy, which is suitable to carry out the secondary operations. The density of AA7475 is taken as

   2.81 g/cm3 theoretically.

   TABLE I. CHEMICAL COMPOSITION OF AA7475 ALLOY

   Elements	Symbol	Wt. %
   Zinc	Zn	5.2-6.2
   Copper	Cu	1.2-1.9
   Magnesium	Mg	1.9-2.6
   Iron	Fe	0.12
   Silicon	Si	0.10
   Titanium	Ti	0.06
   Manganeese	Mn	0.06
   Chromium	Cr	0.18-0.25
   Aluminium	Al	Balance

   The main advantage of introducing reinforcement material to base metal or alloy is to increase the properties there by enhancing the mechanical and tribological properties of composites. In the current research Boron Carbide particulates of size 70-80 microns (m) were used as a reinforcement material, which was procured from Speedfam (India) Pvt. Ltd., Chennai. Boron carbide is non metal material that poses very useful physical and chemical properties. This material among excellent potential material because it known as third hardest material after diamond and boron nitride and density of Boron carbide is 2.51g/cm3 which is lower than the base Al matrix, contributes in weight saving. Boron carbide retains high melting point which is 2450 ºC as well as high resistance to chemical agents.

   A. Preparation of AA7475-B4C Composites

   The fabrication of AA7475-B4C composite used in this study was carried out by using a stir casting method. In this, firstly AA7475 alloy in the form of 25 mm diameter rods cut into 75mm length was placed in a clay graphite crucible. It was then melted in a resistance heated muffle furnace to the desired temperature of 750C. In the mean time B4C particulates of size 70-80 m were heated in another crucible to a temperature of 300C to remove moisture, and the die was preheated to a temperature of 200C. Then the boron carbide particulates were mixed into the molten meal. The crucible was covered with a flux and degassing agents to improve the quality of Aluminium composite casting. The mixture was stirred continuously by using mechanical stirrer for about 10-15 minutes at an impeller speed of 300 rpm. The melt temperature was maintained at 730C during addition of

   (a)

   (b)

   (c)

   Fig.1. Showing SEM micrographs of (a) as cast AA7475 Alloy (b) AA7475- 6% B C (c) AA7475-8% B C

   4 4

   the particles. The molten metal was then poured into the

   preheated die to cast rods. Same procedure is followed to fabricate AMCs with 6 and 8 wt. % B4C.

3. RESULTS AND DISCUSSION

   1. Microstructural Analysis

      Figure 1 (a) – (c) shows the scanning electron microscope micrographs of as cast AA7475 alloy and its composites. Figure 1(b) (c) shows the SEM micrographs of 6 & 8 wt. % of B4C particulate composites. This reveals the uniform distribution of B4C particles and very low agglomeration and segregation of particles. The vortex generated in the stirring process breaks solid dendrites due to higher friction

      between particles and Al matrix alloy, which further induces a uniform distribution of particles.

   2. Hardness

      The figure 2, it is observed that there is an increase in the hardness of AA7475 with addition of 6 and 8wt % of B4C particulate. The graph shows the variation of hardness of AA7475 alloy with B4C reinforcement particulate. It can be concluded that the addition of wt. % of B4C particulate results in increasing the hardness. The hardness of a soft material such as aluminum matrix is increased when it is reinforced with a hard particulate i.e., B4C [8].

      140

      130

      120

      210

      Ultimate Tensile Strength (MPa)

      Ultimate Tensile Strength (MPa)

      200

      190

      180

      170

      160

      0 2 4 6 8 10

      Wt. % of B4C Particulates

      Hardness (BHN)

      Hardness (BHN)

      110

      100

      90

      80

      70

      Fig. 3. Showing the ultimate tensile strength of AA7475-B4C composites

      D. Yield Strength

      180

      170

      60

      0 2 4 6 8 10

      Wt. % of B4C Particulates

      Fig. 2. Showing the hardness of AA7475-B4C composites

      160

      Yield Strength (MPa)

      Yield Strength (MPa)

      150

   3. Ultimate Tensile Strength

      Figure 3 shows the variation of ultimate tensile strength (UTS) of base alloy, when reinforced with 6 and 8 wt. % of B4C particulates. The ultimate tensile strength of AA7475- B4C composite material increases as compared to the cast base AA7475 alloy. The microstructure and properties of hard ceramic B4C particulates control the deformation of the

      140

      130

      120

      0 2 4 6 8 10

      Wt. % of B4C Particulates

      composites. Due to the strong interface bonding, load from the matrix transfers to the reinforcement resulting in increased ultimate tensile strength.

      This increase in ultimate tensile strength mainly is due to presence of B4C particles which are acting as barrier to dislocations in the microstructure [9]. The improvement in ultimate tensile strength may also be due to alloy strengthening of the matrix, followed with a reduction in grain size of the composites, and the formation of a high dislocation density in the AA7475 alloy matrix due to the difference in the thermal expansion between the metal matrix and the B4C reinforcement [10].

      Fig. 4. Showing the yield strength of AA7475-B4C composites

      Figure 4 shows variation of yield strength (YS) of AA7475 alloy matrix with 6 and 8 wt. % of B4C particulate reinforced composite. It can be seen that by adding 6 wt. % of B4C particulates yield strength of the AA7475 alloy increased from 130 MPa to 164 MPa. This increase in yield strength is in agreement with the results obtained by several researchers, who have reported that the strength of the particle reinforced composites is highly dependent on the volume fraction of the reinforcement. The increase in YS of the composite is obviously due to presence of hard B4C particles which impart strength to the soft zinc-aluminum matrix resulting in greater resistance of the composite against the applied tensile load [11]. In the case of particle reinforced composites, the dispersed hard particles in the matrix create restriction to the plastic flow, thereby providing enhanced strength to the composite [12].

4. CONCLUSIONS

The present work entitled, Microstructure and tensile behavior of AA7475-B4C composites, has led to following conclusions:

• AA7475 alloy- B4C particulate composites were successfully produced by liquid stir casting route with different weight percentage (6 and 8) of reinforcement.

• Aluminum based metal matrix composites have been successfully fabricated by liquid stir casting method by two step addition of reinforcement combined with preheating of particulates.

• The hardness of AA7475-6 and 8 wt. % B4C composites increased with the addition of B4C particulates in AA7475 base alloy.

• Improvements in ultimate tensile strength of the AA7475 alloy matrix were obtained with the addition of B4C particulates. The extent of improvement obtained in B4C alloy after addition of 6 and 8 wt. % B4C particulates were

1. and 23.49 percentages respectively.

   • Improvements in yield strength of the AA7475 alloy matrix were obtained with the addition of B4C particulates. The extent of improvement obtained in alluminium alloy after addition of 6 and 8 wt. % B4C particulates were 10 and 18 percentages respectively

REFERENCES

1. H. R. Ezatpour, S. A. Sajjadi, M. H. Sabzevar, and Y. Z. Huang, An investigation of the tensile and compressive properties of Al6061 and its nano composites in as-cast state and in extruded condition, Material Science & Engineering A, 607, pp. 589-595, 2014.

2. Madhu Raja Ezatpour, Seyed Abolkarim Sajjadi, Mohsen Haddad Sabzevar, and Yizhong Haung, Investigation of microstructure and mechanical properties of Al6061 nano-composite fabricated by stir casting, Materials and Design, 55, pp. 921-928, 2014.

3. A. Baradeswaran, and A. Elaya Perumal, A., Study on mechanical and wear properties of Al 7075/Al2O3/graphite hybrid composites, Composites: Part B, 56, pp. 464-471, 2014.

4. S. Suresh, Shenbaga N. Vinayaga Moorthi, S. C. Vettivel, and N. Selvakumar, Mechanical behavior and wear prediction of stir cast Al- TiB2 composites using response surface methodology, Material and Design, 59, pp. 383-396, 2014.

5. S. Baskaran, V. Anandkrishnan, and Muthukannan Duraiselvam. Investigations on dry sliding wear behavior of in situ casted AA7075- TiC metal matrix composites by using Tagushi technique, Materials and Design, 60, pp. 184-192, 2014.

6. S. Suresh, N. Shenbaga, and Vinayaga Moorthi, Process development in stir casting and investigation on microstructures and wear behavior of TiB2 on Al6061 MMC, Procedia Engineering, 64, pp. 1183-1190, 2013.

7. S. Sankaranarayanan,R. K. Sabat, S. Jayalakshmi and M. Gupta, Microstructural evolution and mechanical properties of Mg composites containing nano B4C hybridized micro Ti particulates, Materials Chemistry and Physics, Vol. 143, pp. 1178-1190, 2014.

8. Madeva Nagaral, V. Auradi, and S. A. Kori, Microstructure and Mechanical Properties of Al6061-graphite Composites Fabricated by Stir-Casting Process, Applied Mechanics and Materials, 766-767, pp. 308-314, 2015.

9. Yang Zhou, Naiqin Zhao, Chunsheng Shi, Enzuo liu, and Xiwen Du, In situ processing and aging behaviors of MgAl2O4 spinel whisker reinforced 6061Al composite, Materials Science and Engineering A, 598, pp. 114-121, 2014.

10. Pattanashetty Basavaraja, Madeva Nagaral, S. Vidyashankar, and V. Auradi, Evaluation of Mechanical properties of Al2219-SiC-Graphite reinforced hybrid Composites, International Journal of Applied Enginering Research, 10, 9, 2015.

11. Mohsen Hossein Zadeh, Omid Mirzaee, and Peyman Saidi, Structural and mechanical characterization of Al based composite reinforced with heat treated Al2O3 particles, Materials and Design, 54, pp. 245-250, 2014.

12. J. H. Shin, H. J. Choi, and D. H. Bae, The structure and properties of 2024 aluminium composites reinforced with TiO2 nanoparticles, Materials Science and Engineering A, 607, pp. 605-610, 2014.