FABRICATION AND WEAR PROPERTIES OF ZA-27 ALLOY MATRIX HYBRID COMPOSITE REINFORCED WITH NANOPARTICLES

In this research stir casting technique is used to produce (ZA-27)alloy hybrid composites reinforced by nano particles (BN and Si3N4) with various weight percentage.The wear test were used pin on disk for both (ZA-27) alloy and all composites.The results indicate that the value of hardness increased with increasing the additives of nano (BN and Si3N4) percentage for ZA-27 hybrid metal matrix composites. It was found that the nano particles play an important role in improving the wear properties of alloys. Since nano particles impede dislocations movement, causing enhancement in the mechanical properties.


INTRODUCTION
Zinc -aluminum alloy (ZA alloys) are widely used in for both scientific researches and industry, as a tribomaterials , due to its good castibility and other properties, which become an alternative alloy for bearing Copyright  2018 Al-Qadisiyah Journal For Engineering Sciences.All rights reserved.bronze [S. S. Owoeye,, et al., 2018].They are known to be cheap material processed energetically, efficiently and without endangering the environment ] W. K. Meyers , et al. 2003 The aims of this study is to evalute the mechanical behavior and study the microstructure of ZA-27 alloy by using a recycled alloys produced by stir casting, and to study the effect of hybrid composite with different percentages of nanoparticles on mechanical properties.

A. Materials Selection a. Matrix Materials
Matrix material used in this research was ZA-27 which has excellent properties with a wide range of applications .Its chemical analyzes composition is shown in table (I).

B. Production of Matrix Material and its Composite
Producing of matrix material (ZA-27alloy) it used The weight of melting alloys was approximately equal to 264 gm , which included 185 gm electrolytic zinc (99.99%) ,78 gm pure aluminum , 20 gm master alloy 50 Cu-50Al and 10 gm AA2024.The melting for all alloys was done in a graphite crucible by using gas furnace to about 700 °C (above its melting point) to ensure full melting [13].The molten material was mixed using a mechanical stirrer to get a homogenous mixture.Flux cleaning (KCl-NaCl-NaF) with weight percentage 0.25% were used which usually rich in chlorides to facilitate wetting of the oxide inclusions for easier separation from the melt.For reducing the impurities and gases by using hexachlorethane as degassing [14].
The reinforcement nanoparticles (BN, Si3N4) with different weight percentages were added to the melted matrix as packaged in aluminum foil and continuously stirred using a mechanical stirrer at speed 1000-1200 rpm. to obtain a uniform mixing for all melted material.The slag was removed and then pouring the molten material into a cylindrical graphite mold (permanent mold casting) for casting and the temperature was gradually lowered.Fig. 2 and Fig. 3 show the mold preparation and sketch of mold with dimensions, respectively.(All dimensions are in mm)

C. Hardness:
Hardness test was carried on the base metal alloy and its composites using standard Vickers hardness test machine.The applied load was 0.2kg.Square based diamond pyramid indenter was used during the test.

D. Wear test:
Resistance to surface wear of cast ZA-27 alloy and reinforced with nano boron nitride (BN) and nano silicon nitride (Si3N4) were evaluated using pin on disc type wear tester as per ASTM G99-95 standards.The sample dimensions were Φ10mm with 30 mm for length.The tests were made using applied load of 5, 10,15,20 N , the disk rotential speed was 950 rpm and a sliding velocity 6 cm/min and for 15 min.The disk was made of stainless steel with 63 HRC hardness.

E. Microstructure:
Scanning Electron Microscope was used to study the microstructure of ZA-27 alloy and its composites.
were investigated by using (Inspect S 50 device) and the magnification employed on the samples ranges from 100-20000X.

RESULTS AND DISCUSSION
A. Micro hardness measurement: The value of vickers micro hardness are given in figure (4).The results indicate that the hardness of ZA-27 alloy and its composite increased when increasing the weight percentage of nano particles.The presence of hard reinforcement particles (BN and Si3N4) increase load bearing capacity of the composite material and also limit the matrix deformation by impeding the movement of dislocation.The maximum value of hardness were observed at (3%BN and 3%Si3N4) weight fraction.

B. Wear rate measurement:
The wear rates of composites were increased with increaseing ia addition of nano particles additives as shown in fig (5).Moreover, Increasing the load caused an increase in wear, showing the transition of mild wear to severe wear.
However the transition was more pronounced in alloy compared to composites.Perhaps the increase in reinforcement has resulted in the increased hardness and causes the surface barrier for the penetration of hard particles into the surface.The minimum wear loss was observed for ZA-27/3%BN/3%Si3N4 reinforcement compared to other materials studied at all different loads considered in the study.
The wear rate decreasing for hybrid composites with increasing nanoparticles.This due to the presence of both of the reinforcing materials, which assume to form a stable film on surface composites [15].The nano Copyright  2018 Al-Qadisiyah Journal For Engineering Sciences.All rights reserved.
ceramic particles improve the hardness and wear properties of the composite, and help in the formation of a stable tribolayer.
The hybrid composites improved the hardness and lower wear rate than the unreinforced matrix.
The wear properties of ZA-27 hybrid composites with nano particles additives as a solids lubricant.The hybrid composites had 3 wt% BN with 3wt% Si3N4 It was greater mechanical properties than the single reinforced ZA-27 2wt% BN composite and ZA-27 2wt% Si3N4. The ZA-27 composite reinforced with (3% BN+3%Si3N4) hybrid nanocomposites exhibited lower wear rate due to presence of two ceramic materials.
 The microstructure of ZA-27 alloy is dendritic structure, and the hybrid composites reinforced with nano particles observed to be uniform distribution and noting to presence the agglomeration particles.Thus, increased nanoparticles with weight percentage increases the mechanical properties of the composite.
 The hybrid composites were cast successfully with liquid metallurgy technique.
[ .ZA-27 have higher aluminium and copper contents which give high strength, excellent wear and creep resistance, and lower density [ S. C. Krishna, et al. 2006], However, when wear resistance properties are needed, ZA-27 has demonstrated extraordinary performance ] Y. H. ZHU , 1999[.Nowadays, the most researchers studied the properties of the ZA-27 alloy reinforced with ceramic dispersions such as silicon carbide and graphite for improving mechanical and tribological properties[W.K. Krajewski, 2006].The purpose of applied metal matrix composite based on ZA-27 matrix because its light weight and good mechanical and wear properties [M.Kaplan,, et al..2008 [ .The hybrid metal matrix composites contains two or more types of reinforcements.Reinforcements in terms of mixtures of particles, whiskers and fibers with different weight percentage and varying sizes are utilized in metal matrix for multi functional properties [H., J. Looney, et al. 2002].Seah et al. 1995, investigated that the hardness of artificial aged specimens of ZA-27/Gr increased as compared with as cast alloy.Babic et al. 2009, tested ultimate tensile stress for ZA-27 alloy and heat treated it, The results showed reduce UTS and hardness with increase in elongation for as cast alloy while for heat treated alloy showed reduction in hardness.Ranganath et al. 2002 evaluated the effect of weight percentages of TiO2 additives as a reinforced particles on the mechanical properties of ZA-27.The results indicated that the reinforce particles reduced alloy ductility, and improved UTS, yield strength and hardness.Prasad, et al. ,1998, studied the effect of micro structure composition on the tensile property of ZA-27 alloy.Experiments were performed at different strain rates and temperature.The results showed that the tensile strength of the alloy was improved with increase in the strain rate.
26.23 Actual chemical Composition of ZA-27 alloy b.Nano Boron Nitride and Nano Silicon Nitride as a Reinforcing Materials Ceramic powders (nano BN and nano Si3N4) were used as reinforcement materials for ZA-27alloy.The morphology of raw powders were studied with Scanning Electron Microscopy (SEM) as shown in Fig. 1. nano BN nano Si3N4 Fig. 1 SEM of nano (BN and Si3N4 ) powders.

Fig. 3
Fig.3 Sketch of mold with Dimensions.(All dimensions are in mm)

Fig. 4
Fig.4 micro hardness values of ZA-27 alloy and its composites.

Fig. 5 
Fig.5 Wear rate values for ZA-27 alloy and its composites of different loads