The Effect of Porosity and Cu Rate on Microstructure and Mechanical Properties of Co Alternative Powder Metallurgy Compound

Cu-Ni-MX1480 powder compound were prepared at six different alloy ratios and cold pressed with a pressure of 300 MPa. Then 12 samples were obtained by hot pressing at 700 C with 20 MPa pressure and at 800 C with 30 MPa pressure for 4 minutes in a vacuum atmosphere hot pressing machine. Porosity ratios of samples were calculated by the different between theoretical density and measured density of samples. Effect of porosity on microstructure and mechanical properties were investigated by performing SEM, XRD, hardness measurements and TRS tests. It was observed that as a result of increased hot pressing parameters and increased Cu ratio, porosity ratio was decreased and better mechanical properties achieved.


Introduction
Diamond cutting tools have a large usage area in quarrying, cutting, processing of natural stones [1].In these tools, diamond which performs cutting is bonded with a metal matrix.This metal matrix must cushion the impact forces which come on diamond during cutting process, by supporting the diamond.Also the other function of this matrix is that to wear at a rate compatible with diamond wear [1][2][3][4].Physical and chemical properties even more important thermal resistance and wear resistance of metal matrix that is used diamond cutting tool applications, are determinant factors [5].Matrix is produced via powder metallurgy (PM) route.
Density is an important factor on microstructure and mechanical properties of the products which are produced via powder metallurgy.Densification of the product is determined by controlling the porosity in the microstructure [6].Shape, size and locate of porosities have a great importance on densification.Reducing the surface area energy that is connected with pores, is the determinant factor of densification and elimination of pores.Surface area energy of pores has bigger values than energy of grain boundaries [7].
Hot pressing technique is a method which is used production of PM products.It has advantages compared to conventional sintering process.By this technique, it is achieved a structure that is nearly free of pores by heating and pressure are applied together.Resistance to plastic deformation of metals was decreased by increasing temperature, so it was possible work at much lower temperatures.Also precision of density is relatively less related with properties of alloy powders while they are more important at cold pressing and sintering process [8].
Cu is widely used as matrix component at diamond cutting tool production.Copper acts as binder in matrix through its low melting point and also decreasing its density.Cu is used as matrix component due to provide required softness and ductility to diamond cutting tool.And also Cu is added to matrix via further process for increased density [1,9,13].
Aim of this study is investigation of effect of porosity which has important effects on mechanical properties of matrix, at various rate of Cu and various hot pressing parameters.Cu-Ni and a melt alloy powder MX1480 (50% Cu-25% Fe-25% Co+20% Fe) which is used as alternative of Co are used as matrix component in this study.Samples were produced at different hot pressing parameters and different alloy rates.Porosities of samples were calculated by comparing density of sample to theoretical density value.Effect of porosity on density, hardness and tensile strength of samples were studied.Also microstructure and phase analysis of samples were performed.

Experimental Works 2.1. Materials and processes
Ni, Cu and MX1480 commercial powders were used in this study as alloy powders.Characteristics of powders are summarized at Tab. I. Also SEM images of powders are shown in Fig. 1.Six different powder mixtures were prepared for experiments and they are summarized at Tab. II.Initial ratio of alloy was used as by weight 3%Ni+10%Cu+87%MX 1480 powder.Then while % rate of Cu was increased ten by ten up to 60 %, rate of MX1480 was decreased ten by ten down to 37 %.Powders were weighed by using a precious weight according to rates that are shown at Tab. II.1,5 % by weight PEG (polyethylene glycol) was added in the mixture to avoid outpouring the powders from graphite dies during hot pressing and to obtain a homogenous structure.Powder mixture was mixed by a turbula mixer for 30 minutes and at rate of by volume 1/3 steel balls used for avoid agglomeration.Powder mixtures were weighed 20 grams for produce of segments.Weighed powders were loaded graphite dies at and cold pressed with a pressure of 300 MPa by using a double-effected hydraulic press.40x10x6 mm size green segments were produced, loaded graphite dies and then hot pressed under vacuum atmosphere by a direct hot pressing machine.Hot pressing process was applied at two stages.Initially, samples were held at 480 °C under a pressure of 10 MPa 2 minutes for deburring and then heated up to 700 °C and 20 MPa pressure for samples A and 800 °C and 30 MPa pressure for samples B. Samples were held at this hot pressing parameters for 4 minutes.Then samples left cooling down to 300 °C with a constant pressure force.And then dies were taken out from machine and were left cooling to room temperature.

Characterization
Densities of specimens were determined according to the standard of ASTM B311-92 Archimedes principle.Hardness of the samples were measured by using measurement tip which has 2.5 mm diameter with a pressure of 62,5 kgf by using "Emco Test Durajet" branded hardness measurement device.All specimens surfaces were polished by using Metkon Forcimat branded grinding device for microstructure observation and then samples were etched by dipping in solution which compounds 30 % HNO 3 + 30 % Acetic Acid+ 6.5 % H 3 PO 4 + 33.5 % H 2 O. TRS tests were performed in accordance with standard of ASTM B528-83a by using "Shimadzu AG-IS 100 kN" test machine with Trapezium X software.Investigation of microstructure and fracture surfaces of the specimens and EDS analysis were performed by using JEOL JSM-7001F branded scanning electron microscope.And also analyses of phases were determined by using Rigaku RadB-DMAX II branded device.

Results and discussions 3.1. Microstructure
While Fig. 2 shows the SEM images of A samples, Fig. 3 shows the MAP analysis of A samples which were hot pressed at 700 °C with 20 MPa pressure.And while Fig. 4 shows the SEM images of B samples, Fig. 5 shows the MAP analysis of A samples which were hot pressed at 800 °C with 30 MPa pressure.MAP analyses were made for understanding how distribution was occurred in the microstructure of samples.As seen in the figures porosity rate of B samples of B series were decreased compared to porosity rate of A samples thanks to effect of high temperature and pressure parameters.Macro porosities were mostly observed in Ni-Fe and MX1480 alloy and partially observed at grain boundaries of Cu.Micro porosities were observed mostly compound of Fe-Ni-MX1480.Decrease in size and amount of porosities has occurred by increasing rate of Cu amount in the compound.It was observed that bonding between particles hasn't fully formed because of hot pressing parameters.When the samples of B series were analyzed, porosity amount in the microstructure were decreased by increased hot pressing parameters.Also as a result of increased temperature and pressure, new bonds have formed between Cu particles and grain growth has occurred for Cu particles.

Density and hardness
Fig. 6 shows density chart according to the changes in rate of Cu in the samples A and B. As shown in the figure, theoretical density and density of the specimens increased by increasing rate of Cu.Somunkiran and Balin achieved same resuls [14].Density of A1 specimen was measured 7,89 g/cm 3 while theoretical density was calculated 8,36 g/cm 3 and the difference is calculated 0,47 g/cm 3 .With increasing rate of Cu up to 60 %, this difference value was calculated 0,26 g/cm 3 .Tab. III shows the relative density and porosity rates of specimens.As can be seen from table, for specimens group of A by increasing Cu rate porosity rate was decreased and for A6 it was achieved maximum relative density.But for group of B specimen's maximum relative density was obtained for B5 specimen.It was observed that for both samples A and B, increasing Cu rate was decreased porosity rate as can be seen in the Fig. 2. Cu has filled the porous in the microstructure and it has covered the other components in the alloy thanks to its dendritic shape and bigger grain size from other components.Hot pressing parameters lead to important changes in amount of porosity and microstructure of specimens.It was observed that porosity rate of A1 specimen was calculated 5.7 % while porosity rate of B1 specimen which has same alloy rate with A1 specimen but has higher pressure and temperature hot pressing parameters, was calculated 1,41 %.Fig. 7 shows hardness chart according to changes in rate of Cu in the samples A and B. As seen in the figure, conversely to density, hardness was decreased by increasing ratio of Cu.As known, Cu is a soft metal and gains to softness metal matrix regarding hot pressing and sintering characteristics.However hot pressing parameters are effected on the hardness of products.It can be seen clearly that hardness of B specimens are higher from A specimens which were produced lower temperature and pressure parameters of hot pressing compared to B specimens.For same alloy ratios, increasing hot pressing temperature and pressure leads to increasing hardness of specimens by decreasing ratio of porosity.TRS graphs of specimens A and B are shown in the Fig. 8 and Fig. 9.As can be seen from both graphs, ductility of matrix is increased by increased ratio of Cu.Also toughness of matrix was increased upward to 30 % ratio of Cu in the matrix.Fracture strength of B samples which were hot pressed at 800 °C with 30 MPa pressure, are higher than A samples which were hot pressed at 700 °C with 20 MPa pressure.In series of B samples, bonding between particles in B samples have enhanced by increasing of pressure and temperature.It was observed that strength of samples were decreased but elongation % was increased by increasing rate of Cu.Elongation has raised up to 5.5 % for A series of samples thanks to increasing ratio of Cu.For B series of samples elongation has raised up to 10 % by increasing temperature and pressure of hot pressing parameters.While decreasing ratio of porosity, TRS of specimens have increased.Elongation of samples has increased by increasing density.Kır et al. has achieved similar results, as a result of their study which was named "effect of the cBN content and sintering temperature on the transverse rupture strength and hardness of cBN/diamond".They have obtained high TRS results thanks to increasing sintering temperature [10].Fig. 10 shows the SEM images of fracture surfaces of A2 and B2 samples which have 3%Ni+20%Cu+77%MX1480 alloy.Fracture images were taken at low and high magnification.It can be clearly seen from images porosity amount is much more at A2 than B2.When the images studied, it is seen that plastic deformation occurred on Cu particles in both samples.Cu has formed strong bonds with Ni-MX1480 and these bonds haven't deformed result of fracture.It is understood from both images that at areas which is poor of Cu, necking and bonding is relatively poor and as a result of this fracture mode is intergranular at these areas.Pores are largely seen in the particles and partially seen at grain boundaries in specimen A2.It is seen that enhancement necking and bonding has increased in sample B compared to sample A. In specimen B2; pores which were on the grain boundaries are significantly eliminated and pores in the granules are reduced.It was observed that porosity was decreased as a result of improved bonding between particles due to increasing of hot pressing temperature and pressure, therefore much better mechanical properties achieved.

XRAY results
Fig. 11 shows the XRD graphs of A series sample and also Fig. 12 shows the XRD graphs of sample B. While blue lines in the both graphs indicate the XRD pattern of 2 series, red lines in the both graphs indicate XRD pattern of 4 series of samples.Green lines in the both graphs indicate the XRD pattern of 6 series of samples.Fe 3 Ni 2 , FeCu 4 and Co 3 Fe 7 intermetalic phases were observed all samples which were produced via hot pressing.Changing rate of Cu didn't cause a significant change in terms of phases which occurred in the microstructure.Only, changes were observed at peak intensity of the samples.It was observed that increased of FeCu 4 intermetalic phase for B samples which were hot pressed at 800 °C with 30 MPa pressure.It's thought that, this phase improved the mechanical properties of the samples.

Conclusions
In this study, manufacture of a matrix which is used for diamond cutting tools that contains 10-60 % Cu, MX1480 (50% Cu-25% Fe-25% Co +25% Fe alloy powder) and 3 % Ni was performed successfully.Microstructure, hardness, density, porosity rate, three point bending tests and XRAY investigations were applied for matrix by changing the porosity rate, and obtained results that listed below.

Fig. 6 .
Fig. 6.Changing in density of hot pressed samples based on Cu rate.

Fig. 10 .
Fig. 10.Fractured surface SEM images of A2 and B2 segments which were taken at low and high magnification.

Tab. I Characteristics of powders.
Tab. III Relative density and porosity rates of specimens.A B