Sintering studies on iron-carbon-copper compacts

Sintered Iron-Carbon-Copper parts are among the most widely used powder metallurgy product in automobile. In this paper, studies have been carried out to find out the sintering characteristics of iron-carbon-copper compacts when sintered in nitrogen atmosphere. The effects of various processing parameters on the sintering characteristics were studied. The various processing parameters considered were compaction pressure, green density and sintering temperature. The sintering characteristics determined were sintered density, porosity, dimensional change, micro hardness and radial crush strength. The results obtained have been discussed on the basis of micro structural observations. The characteristics of SEM fractography were also used to determine the mechanism of fracture. The fracture energy is strongly dependent on density of the compact.


Introduction
Powder Metallurgy (P/M) has become an attractive processing technique because of its flexibility and versatility.Pistons made of powder metal are used on a large scale in assembling Shock absorber.Among other compaction techniques, the most widely used technology for manufacturing sintered compacts is the die compaction method due to its flexibility.The powder metallurgy (P/M) is rapidly growing in the market.Since most of the parts made by P/M are used in the automotive industry, cost is a major driving force.In developing better parts by P/M, an important consideration is not only the cost but also the performance.An important step in the process of producing powder metal (P/M) components is compaction.The green compact should have sufficient strength so that it does not crumble during handling until it is charged into sintering furnace.The green strength depends upon the green density, which in turn depends upon compaction pressure, type of powder, size, shape, type and amount of lubricants etc.
First to increase the density of the material, compaction is done on the powder materials.This process is responsible for its densification, and this in turn results in attaining minimum porosity, which is our ultimate goal, since loose powder materials have about 65 to 75% of porosity.To effectively reduce porosity to about 20%, the process of axial loading in a die is carried out.Hence in this present study, nitrogen atmosphere has been selected as the sintering protective atmosphere.[1] D. Roshan Kumar et al. carried out experiments on dynamic compacting and quasistatic compacting of Fe, Fe-C-Cu-Mn powder and Cu-Zn powder and they have reported that densification and dimensional accuracy is better in dynamic compaction than quasistatic compaction.[2] D. Korachkin et al. investigated the tensile failure of Distaloy AE ferrous powder and Alumina powder using three point bending test and diametral compression test and they have reported that ferrous powder showed a nonlinear increase in powder strength at higher density level whereas Alumina powder showed a linear increase in tensile strength.[3] C.C. Degnan et al. have evaluated the green strength of Distaloy AE densmix powder.Data's from three point bend test and Brazilian disc test have been analysed using weibull analysis and optimal green strength obtained successfully.[4] Z. Zhang et al. studied Fe-Mo prealloyed powders and obtained an increase in tensile strength, yield strength and hardness with increasing Fe-Mn-Si alloy and carbon.Higher sintering temperature showed higher mechanical properties and the structure was found to be bainite and martensite at the end of sintering process.[5] A.  was obtained.[7] Wen-Fung-wang investigated the effect of powder type and compaction pressure on the density, hardness.Compacting pressure of around 500 MPa shows maximum hardness on sintered specimen.Density of 6.4 g/cc shows better results in mechanical properties.[8] Necati Ozkan et al. studied the characterization of die pressed Alumina green compacts.In their investigation they have concluded that increase in pressure increases the density and green strength of the compact.[9] H. Rutz et al. carried out experiments on Ancorsteel powder using double press/double sinter and warm compaction and they have reported that density of 7.35 g/cc was obtained in both the process and a radial crush load of 38 KN in double press/double sinter and 32 KN in warm compaction process noticed.
The aim of the present study is to investigate the effect of compaction pressure, green density and sintering temperature on the microstructure and mechanical properties of P/M Fe-C-Cu compact samples.In addition, the pore formation and fractured surfaces of the radial crush strength samples were investigated via SEM.

Materials and methods
The premixed iron powder with carbon and copper were analyzed to find out their apparent density, flow rate and composition.The premix iron powders were used for floating die compaction experiment.The powder used for floating die compaction was a mixture of 100 mesh high purity with 97 % of atomized iron powder with 0.2 % of carbon, 2% of copper and 0.8 % of zinc stearate (ASM100.25).Hoganas India Ltd has supplied the premixed powder.The powder mixtures were cold compacted in a rigid carbide die to produce a cylindrical green compact with dimension of 25.305 -25.31 mm in diameter and 7 mm high were compacted at a pressure of 200 to 400 MPa using a Dorst 50 ton mechanical press as per the ASTM standard [10].For compaction pressure studies, the powder samples was pressed at 6.0 (+/-0.05)g/cm 3 , 6.3 (+/-0.05)g/cm 3 , and 6.6 (+/-0.05)g/cm 3 .Green density and sintered density was determined by water immersion as per ASTM standard [11].
The samples were sintered for 1 hour in a nitrogen gas atmosphere at different sintering temperature 1130 °C, 1115 °C and 1090 °C for 20 min in the high heating zone.The sample dimensional change after sintering was measured by micrometer.After sintering, the sintered specimens were analyzed to find their density, dimensional change, micro hardness [12] , radial crush strength [13] and microstructure were also studied using optical microscope ( Leica DM 2500 M ).The fractured surfaces obtained from radial crush strength samples were analyzed by SEM.

Result and discussions 3.1. Material characterization
Tab.I gives the results of characteristics of Fe -C -Cu premix iron powder used for this work.The premix iron powder particles are of irregular and have a homogeneous structure in Fig. 1.The flow rate of the premix iron powder was approximately 30.5 seconds for 50 grams as per the ASTM standard [14].It had an apparent density of approximately 2.87 g/cc as per the ASTM standard [15].Fig. 2 shows the density-pressure curve for atomized iron powder (ASM 100.25).

Effects of Sintering Temperature on Sintered density
Fig. 3 shows the effect of sintering temperature on sintered density of atomized iron powder compacts.Sintered density increases between the temperature ranges of 1090 -1130 °C.The increase in sintered density can be justified by the following reasons: During heating the elastic energy stored in the compact during compaction tend to relax their stresses resulting in slight growth.Sintering starts to take place around 1130 °C by diffusion and fusion mechanism.Alpha iron sinters more rapidly than gamma iron.The lubricant zinc stearate to vaporise at a temperature of 470 °C and its primary function is to heat the components.Carbon diffuses readily into iron within five minutes at a temperature of 1040 °C.Copper becomes a liquid at 1086 °C and settles at the grain boundaries, but does not penetrate the iron in the same manner as the carbon.Copper migrates to the grain boundaries and fusion of iron grains during sintering.During slow cool sections the temperature of the part from 600 °C to room temperature, precipitation of copper takes place and make the specimen softer.
Thus it can be concluded from above that the increase in density with increasing temperature in the range of 1090 -1115 °C is due to release of elastic energy and recrystallisation.Relax of elastic energy leads to a slight growth thereby increasing the density.The further increase in density in the range of 1115 -1130 °C may be associated with alpha to gamma transformation.As the density increases, the porosity decreases which is evident from the microstructures as shown in Fig. 4.This trend is in agreement with the work of others [16, 17, 18, 19, 20, 21, 22, 23, and 24].The microstructure investigation of green compacts was performed after sintering of the specimens.A few samples made of atomized iron powder with 0.2 % carbon specimens obtained by cold compaction, were selected for investigation, and sintered at 1130 ºC, 1115 °C and 1090 °C in 100 % Nitrogen atmosphere for about 75 minutes are shown in Fig. 4. Sintered pore distribution, porosity , grain boundaries and internal particle structure can be seen from the optical micrograph.The porosity is controlled by compaction pressure and re-arrangement of powder grains filling large pores.Increasing pressure leads to decreasing porosity with formation of new contacts of powder grain surfaces, therefore, porosity decreases as the number of contacts and contact area increases.Particle size, particle shape, particle distribution, particle density, particle hardness and particle chemical composition have influence on mechanical properties like compressibility and hardness.

Effects of Sintering Temperature on Microstructure
Fig. 4 shows the microstructure of atomized iron powder compacted to a density of 6.0 g/cc to 6.6 g/cc and sintered for 1 hr at 1090 °C in 100 % nitrogen atmosphere.The matrix shows sintered pore formed in inter-granular space.The matrix also shows fine golden color of free copper not dissolved in iron solid solution and also the mechanical properties are very low due to low and early stage of sintering temperature.Pores with irregular shape and un-dissolved copper as free copper in the metal matrix of ferrite is present in the optical micrograph of the samples sintered at 1115 °C.Irregular pore formation, no proper copper migration to the grain boundaries and fusion of iron grains caused micro hardness, radial crush strength to decrease in 1090 and 1115 °C groups of samples.In microstructure of the samples sintered at 1130 °C, presence of free copper is low with ferrite at the grain boundaries due to two different stages of sintering process i.e local bonding between adjacent particles and pore rounding and pore shrinkage.At high sintering temperature which produces a significant improvement in the diffusion which increase dramatically, accelerate the atomic motion between particles (better sintering necks), improve the surface reduction of the particles (activate sintering), increase the sintered density, improve the homogenization, better mechanical properties and improve the porosity (rounded and closed).From the microstructure of the samples it can be concluded that the sintering temperature of 1130 °C is characterized by producing fine ferrite and pearlitic structure, low porosity which is important to maintain compressive strength and size control during the sintering process.The size and shape of porosity decrease with increasing sintering temperature.These findings are well agreed with earlier studies by others [17, 18, 19, 21, 22, and 25].

Effects of Sintering Temperature on Dimensional Change
The linear measurements of test specimens of 6.6 g/cc sintered at 1090 °C in Fig. 5 showed a growth of about 0.310 % from the green to sinter size.This growth is primarily due to the diffusion of carbon in the iron matrix.It is confirmed by noting the presence of ferrite and pearlite structure in the photomicrograph of a test specimen at 1090 °C in Fig. 4. Some un melted copper was present in the structure near the pores.A further increase in sintering temperature to 1115 °C caused an increase in linear growth to about 0.319% .This increase in growth is due to melting and diffusion of molten copper in the iron-carbon-copper alloy.The photomicrograph of the sintered specimen at 1115 °C in Fig. 4 confirmed the absence of un melted copper in the structure.It showed a complete diffusion of carbon in the iron matrix, as evidenced by the absence of ferrite in the structure.Furthermore, it showed an onset of particle to particle bonding or sintering.A further increase in sintering temperature to 1130 °C caused the growth to increase to 0.339%.These results are consistent with the trends reported by some investigators [17, 18, 19, 20, 22, and 26].This is to near complete particle to particle bonding or sintering, as evidenced in Fig. 5.A comparison of Fig4i revealed that a sintering temperature close to 1130 °C is required to obtain near complete sintered structure.

Effects of Sintering Temperature on Micro hardness
The micro hardness of test specimens sintered at 1090 °C in Fig. 6 showed a hardness value of about 55 HRC.This hardness is due to the extent of sintering and diffusion of carbon in the iron matrix, as discussed earlier.The hardness value increased from 55 to 63 HRC by increasing the sintering temperature from 1090 to 1115°C.This increase is related to both complete diffusion of copper and carbon in the iron matrix.A further increase in the sintering temperature to 1130°C increased the hardness value to 75 HRC, as shown in Fig. 6.This increase is related to near complete particle to particle bonding or sintering.The hardness data in Fig. 6 also confirmed that a sintering temperature close to 1130 °C is required to obtain near complete sintered structure.These findings are well agreed with earlier studies by other researchers [17, 23, 27, 28 and 29].

Effects of Sintering Temperature on Radial crush strength
The radial crush strength of test specimens of 6.6 g/cc sintered at 1090°C in Fig. 7 showed a strength value of about 250 MPa.Once again, this radial crush strength is due to the extent of sintering and diffusion of carbon in the iron matrix.The strength of sintered radial crush specimens increased dramatically from 250 to 268 MPa by increasing the sintering temperature from 1090 to 1115°C.This dramatic increase in strength is related to both complete diffusion of copper and carbon in the iron matrix.A further increase in the sintering temperature to 1130°C increased the strength to 286 MPa, as shown in Fig. 7.This increase is related to near complete particle to particle bonding or sintering.This is well agreed with earlier investigation [18, 21, and 24].The radial crush strength values in Fig. 7 once again, revealed that a sintering temperature close to 1130°C is required to obtain near complete sintered structure.
In the metallographic analyses, it was observed that the strength of the porous Fe-C-Cu alloy increased with decreasing porosity (%) and pore size.In the fracture surface analysis of a specimen with 20 % porosity, it was observed that there was the smallest amount of sinter neck formation and the largest pore size for the different specimens.The largest pore size was observed to be 60 µm, and an irregular pore formation occurred in the sample with 20% porosity is shown in Fig. 8a.In addition, it was also observed that a 40 µm pore size and 20 to 30 µm pore sizes were observed in the 15% and 10% porosity specimens respectively which are displayed in Fig. 8b and Fig. 8c.Among the different specimens the most ductile structure was found in the structure with 10% porosity Fig. 8(c).As a result of the low porosity the high amount of sinter neck formation, the regular spherical pores and the small pore size in a specimen of 10% porosity, the ductility of the specimen increased.This trend is in agreement with the work of others [16, 17, and 19].To obtain a small pore size and regular pore formation there should be an increase in density and a decrease in porosity, which can be seen in Fig. 4i.

Conclusion
In the present investigation studies were carried out to find the sintering characteristics of iron-carbon-copper compacts when sintered in nitrogen atmosphere.The purpose of this study was to find the effect of compaction pressure, green density and sintering temperature on sintered density, microstructure, dimensional change, micro hardness, and radial crush strength.From the results obtained, it can be concluded that all these parameters had an effect on the sintered properties.
1. Sintering temperature close to 1130 °C and sintered density 6.6 g/cc are required to obtain near complete sintered structure and produce sintered components with good surface finish and properties.2. Sintered density, dimensional change, micro hardness and strength of components sintered from iron-carbon-copper powders change significantly by increasing the green density and temperature in nitrogen atmosphere.3. The optimum compaction pressure for atomized ferrous powder has been identified as 400 MPa.At the end of the compaction stroke the desired powder grain deformation and density equal to 96 % of the material capability has been achieved.4. Investigations into the microstructure contained pearlite phase in grey color with ferrite in white color.Some un-dissolved copper as free copper in the metal matrix of ferrite is present.5. Mechanical properties were affected by porosity and the sintered Fe-C-Cu alloy with 10% porosity had the best mechanical properties such as micro hardness, radial crush strength and dimensional change as compared to the other specimens with 15% and 20% porosity.6.An average pore sizes of 60 µm, 40 µm, and 20-30 µm were obtained for the specimens with 20%, 15% and 10% porosity respectively, which proved that pore size increased with increase in porosity and the pore size did not affect the chemical purity of the sintered parts.

Fig. 3 .
Fig. 3. Effect of sintering temperature on sintered density of atomized iron powder.

Fig. 5 .
Fig. 5. Effect of sintering temperature on dimensional change of atomized iron powder.

Fig. 6 .
Fig. 6.Effect of sintering temperature on Micro hardness of atomized iron powder.

Fig. 7 .Fig. 8 .
Fig. 7. Effect of sintering temperature on radial crush strength of atomized iron powder Cias et al. made another study on sponge NC 100.24 and atomized ABC 100.30iron powders and obtained tensile strength of 300-600 MPa and transverse rupture strength of 640 -1260 MPa.[6] P. Jonsen et al. studied tensile strength, crack initiation, crack propagation and fracture in Distaloy AE iron powder during diametral compression testing.Tensile strength of around 17.46 MPa and fracture energy of 1594 J/m 2