CHAPTER 3 PREPARATION OF THE POWDER METALLURGICAL POROUS BRONZE COMPACTS

Transcription

1 CHAPTER 3 PREPARATION OF THE POWDER METALLURGICAL POROUS BRONZE COMPACTS 3.1 INTRODUCTION: In this chapter, the procedure for preparation of the Powder Metallurgical (P/M) porous bronze compacts has been briefly explained. The selection and properties of metal powders are also discussed because the ability of a metal powder to be effectively compacted and the resulting properties of the compact before and after sintering are influenced to a substantial degree by the characteristics of the starting powder. Further the composition of the various bronze samples used in the present work is briefly summarized. The details regarding the rate of raising and cooling of furnace temperature and sintering temperature are given. The procedure for polishing and grinding to get a plane and acoustically smooth surface is also discussed Metal powder characteristics: The basic powder characteristics are given below: i) Particle size and size distribution ii) Particle shape Hi) iv) Apparent density Flowability v) Compressibility. In the powder metallurgial process, most of the properties of the green powder compact and the final sintered compact are directly related to the extent to which powder particles establish contact

2 54 with their neighbours. This, in turn depends on both the size and shape of the particles. The author has used the electrolytic copper powders from Loba Chemie, Bombay for making bronze compacts. The maximum grain size of the powder is 53 V m (325 mesh). Electrolytic copper consists 99.5% of copper and the remaining percentage consists of oxygen, acid insolubles and metallic impurities like Antimony, Lead, Tin, Silver, Iron and Manganese. The particle shape is dendritic or fernlike and it has medium to high surface area. Due to fine grains, the interparticle contacts are enormous. This promotes subsequent sintering. But the dendritic shape of the grains increases the surface area of the powder and hence the interparticle friction is also increased. In turn, this affects the ease with which particles can move during powder flow, when settling in a die and when under pressure during compaction. Further the chemical reactivity of the powder is also increased due to increased particle surface area. This leads to greater adsorption of atmospheric gases and water vapour, resulting in the formation of oxides on the particle surfaces. After making the compacts, they are prefired in hydrogen atmosphere at 300 C. Metallic oxides are reduced into their metals and occluded oxygen is evolved as water vapour by this process. Since the fine and irregular grains are used, the apparent density of the powder is about 15% of the maximum density of copper and hence during compaction the reduction in volume of the powder is 80%. The flow rate of a powder is the ability of a metal powder to flow. The high flow rate of the powder is characterised by rapid and smooth movement of the powder particles leading to higher densification. The flow rate is reduced by interparticle friction. The flow rate is increased by adding lubricants which reduce the friction between the powder and the tooling surfaces. But the addition of lubricants like graphite, zinc stearate and lithium stearate is generally detrimental to compact quality and inconvenient in subsequent sintering

3 55 operations. The author has prepared the samples with and without addition of lubricants so as to evaluate the effects of lubricants in powder metallurgical compacts. Compressibility of the metal powder depends upon its initial density, particle size and shape. For the electrolytic powders the compressibility is high due to their irregular shape and low density. 3.2 COMPOSITION OF BRONZE POWDER COMPACTS: Powder metallurgical bronze typically originates as premixes consisting of elemental copper and tin powders plus 0.5 to 1.75% dry organic lubricants such as stearic acid or zinc stearate. The nominal composition of 90 Cu - 10 Sn is favoured because the homogenization of the alloy is formed at lower sintering temperatures 800 to 850 C. Depending on the specified grade in the manufacture of porous, self - lubricating bushings and bearings and for more complex structures requiring superior bearing and mechanical strength the other constituents such as graphite, iron, zinc and nickel are also used in small percentages. The author has initially used lithium stearate and zinc stearate as lubricants in the powders to be compacted. Eventhough the dry organic lubricants are used to reduce interparticle friction and to enhance the sinterability of the metal powders, they produced some stains on the sintered surfaces of the samples. Further soot is formed in the inner surfaces of the preheating furnace and the mechanical properties are weaker in those sintered samples. Therefore the addition of dry organic lubricants is avoided in the later work and graphite is taken as lurbicant. Table 3.1 shows the details of the percentage of composition of the metal and non-metallic powders used in the present work. The compositions are chosen based on the Metal Powder Industries Federation Standard. The addition of other constituents like Iron,

4 56 Nickel, Zinc and Silica is slightly increased in their percentage to study their effects in the properties of sintered compacts. Table 3.1: Composition of the bronze compacts Weight percentage Grain size Copper 88 to mesh Tin (grey) 9 to mesh Graphite 0.5 to mesh Iron 1 to mesh Nickel 1 to mesh Silica 1 to mesh Zinc 1 to mesh These powders are grinded and mixed simultaneously by pestle and mortar for 30 minutes. This may further reduce the grain size by fragmentation. After sieving the grinded powders by 53 y m sieve plate it is concluded that the maximum grain size of the powder is 53 y m. In the above compositions, silica is not normally used in powder metallurgical bronze compacts and it is used only in ferrous powder metallurgy. The author has included it to study the changes in elastic properties of the bronze compacts. The importance of other constituents is given in Chapter 1. The addition of these constituents is very small so as to retain the alpha bronze microstructure in the sintered products. 3.3 PREPARATION OF POWDER COMPACTS: There are several ways to compact the powders. Powder metallurigal processing may involve compaction in rigid dies, ambient or high temperature isostatic compaction in flexible tooling and high

5 57 energy rate forming or consolidation by forging or vibratory packing. Generally the largest number of powder metallurgical components are compacted in rigid dies using mechanical or hydraulic presses. For thin samples the single action compaction is used. In the single action compaction only the top punch is undergoing downward motion and the die barrel and bottom punch remain stationary. The compacted part is ejected from the die by an upward movement of the bottom punch with the top punch removed. The author has followed this process for making green compacts. There are three general events taking place in the powder during compaction : a) Bulk movement of the particles. b) Elastic and plastic deformations, and c) Fragmentation of the particles. Bulk movement of the particles results in the rearrangement of the particles which is limited by interparticle friction as well as by the friction between the powder and the compaction tooling. This bulk movement of the particles occurs generally at relatively low compacting pressures and accounts for most of the early densification in the compact [ 29 ]. The extent of plastic deformation depends on the ductility, yield strength and strain hardening characteristics of the constituent particles. The particle fragmentation leads to still further densification by virtue of the fact that some of the newly formed small pieces can move into the remaining pore spaces between the larger particles. Green density is used as a measure of the effectiveness of the compaction operation. In the powder compact, there are density variations which result from stress distributions in the compact promoted by frictional forces at the tooling/powder interfaces. In the case of single action compaction, the green density is greatest near the moving top punch and decreases in the regions remote from the punch, particularly near the die walk

6 58 Kuczynski and Zaplatinsky t40 ] determined the density distribution of a nickel powder compact, pressed only from the top, through hardness measurements and found that the density of the compact varied from 6400 to 7300 kg/m**. The strength of the green compacts is increased by using fine powders (325 mesh) and using powders with irregular particle shapes and rough surfaces (Electrolytic powders). Further the higher green density and decreased amounts of admixed lubricant or other elemental powder additives result higher green strength. The higher green density is achieved by applying high pressure. Heckel [21] deduced an equation (1.1) between the applied pressure *P' and relative density D and that equation holds good for iron, nickel and copper powders when the applied pressure is in between 150 MPa and 700 MPa. James [41] related the constant C' in that equation (1.1) to the yield strength of the metal with an approximately linear relationship between C and reciprocal of yield strength. For relatively soft powders of aluminium, copper, zinc and even iron, a terminal density appears to be reached at very high pressures and after that the density of the compact does not change eventhough the pressure is increased further. Higher compacted density significantly shortens the sintering time needed to produce a given density. Hence the author has used high pressure (550 MPa) to attain higher green density. 3.4 SINTERING OF POWDER COMPACTS Sintering is a process in which an assembly of compacted particles metallurgically bond into a coherent body at elevated temperatures. During sintering, the fragile green compacts are heated in a protective atmosphere to the sintering temperature which is normally below the melting point of the major constituent of the compact. Sintering time and sintering temperature are selected to establish the desired mechanical properties by causing the powder particles to form coherent bonds and to alloy any admixed elements. Sintering process has been

7 59 explained already in Chapter 1. This section deals with the sintering time and sintering temperature for bronze compacts^ rate of heating and rate of cooling during sintering and protective atmospheres in the sintering furnace Sintering Temperature and Sintering Time: Typical sintering furnace temperatures for bronze range from 815 to 860 C and total sintering time within the hot zone is from 15 to 30 minutes, depending on the furnace temperature selected, required dimensional change and most Importantly the presence of an optimum alpha grain structure [22]. In the case of self - lubricating bearings, the porosity in the bearings is an important factor. To get the desired amount of porosity the sintering time is made so small. Increasing the sintering temperature decreases the amount of time required to achieve the desired porosity. The sintering temperature is selected as 850 C and the corresponding sintering time is selected as 30 minutes. Previously the desired porosity was obtained by the vacant sites originated from the burning off of the organic lubricants during sintering. But the organic lubricants produced so many unwanted effects in the samples and sintering furnace. Now-a-days without addition of organic lubricants the desired porosity and microstructure are obtained by controlling the sintering time. The author has also studied the effect of sintering time and sintering temperature on homogenization of sintered compact. The duration of sintering required to achieve a desired degree of homogeneity is critically dependent on sintering temperature because interdiffusion coefficient (D) is exponentially dependent on absolute temperature [42 ]. D ** D e o -Q/RT...(3.1) where Dq is the pre-exponential factor and Q is the activation energy. R and T have the usual meanings. The total sintering time *t*

8 60 required to achieve complete homogenization is inversely proportional to D much that t = kc Jl2/5...(3.2) where i is the average diameter of the dispersed particles and is a constant whose value depends upon the alloy system parameters and powder particle compositions [ 13). The evaluation of the state of homogeneity is done through the X-ray compositional line broadening technique using X-ray powder diffractometer [ 14 ] Sintering atmosphere: Sintering atmospheres primarily control the chemical reactions between the materials being processed and the furnace surroundings. Normally the sintering atmospheres are protective and reducing to facilitate sintering. The type of sintering atmosphere directly affects tensile properties and dimensional change of the sintered compact. In bronze bearings, the important function of the sintering atmosphere is to aid in the reduction of oxides on the surfaces of the metal particles in the compact. For copper, the oxidation rate is very higher at elevated temperatures. The presence of oxides in the sample reduces its mechanical thermal and electrical properties. The author has chosen hydrogen and vacuum as reducing and protective atmospheres respectively. The hydrogen atmosphere is used during presintering at 300 C and vacuum is used during sintering at 850 C. The presintering in hydrogen atmosphere produces effective reduction and hence the compacts are free from oxygen. The final vacuum sintering at 850 C reduces the formation of oxides at higher temperatures. The vacuum is used at 850 C because handling of hydrogen at this temperature is very difficult with the available furnace system. Commercially bronze bearings are sintered in only one atmosphere rather than two atmospheres. But author has taken two atmospheres to achieve effective sintering. Other protective atmospheres are nitrogen and dissociated ammonia. Eventhough the dissociated ammonia 4

9 61 seems to be an effective atmosphere for sintering, the water formed during chemical reaction between evolved oxygen and hydrogen from dissociated ammonia is to be removed from the furnace. So the furnace design is little complex in the case of atmosphere with dissociated ammonia. Further the dry dissociated ammonia is necessary to avoid the change of colour of the samples during cooling [ Pre-heating of the green compacts: The preheating furnace is cleaned and hydrogen is passed through it for 30 minutes. Then the samples to be reduced are kept in quartz boats and placed at the hot zone of the pre heating furnace. The temperature of that furnace is raised to 300 C at the rate of 10 C/min. The samples are pre-heated at 300 C in hydrogen atmosphere for 3 hours and cooled to room temperature by placing them in the furnace itself. After that each sample is transferred to separate quartz tube of length 15 cm and a vacuum of 10-2 mm of mercury is created in each quartz tube. Using a flame of oxygen and indane gas the tubes are sealed carefully without heating the samples. Before sealing, the quartz tubes are heated for few minutes using a bunsen burner to liberate any occluded gases which may be formed during transferring the samples from preheating chamber to quartz tubes. The sealed quartz tubes with the samples are kept in the muffle furnace Sintering of the samples with different sintering times: To study the effect of sintering time on microstructure and properties of bronze compacts the sintering furnace temperature is initially maintained at the sintering temperature (750 C). Five minutes were given to the sample to reach the sintering temperature. The compacts, made from a mixture of 92% of copper and 8% of tin are used. The sintering time is varied from 15 minutes to 48 hours. After the completion of the sintering time, the samples are quenched in coconut oil at room temperature.

10 Sintering of the samples with different sintering temperatures: The compacts made from a mixture of 90% copper and 10% tin are used to study the effect of sintering temperature on microstructure and properties of bronze compacts. A constant sintering time of 15 minutes is given to each sample. The sintering temperatures are set at 850 C and 900 C. After the completion of the sintering time, the samples are quenched in coconut oil at room temperature Sintering of the samples having different compositions: To study the effect of composition on the properties of the samples, the sample are sintered by maintaining the sintering temperature at 850 C and allowing 30 minutes as the sintering time. The rate of heating and cooling is shown schematically in figure 3.1. After loading the furnace with samples which are kept in evacuated quartz tubes, the furnace temperature is raised at the rate of 10 C/min. At 300 C, the temperature is maintained constant for 30 minutes. Then the temperature is raised to the sintering temperature (850 C) at the rate of 3 C/min. The sample is kept at that temperature for 30 minutes. After that the furnace is cooled at the rate of 2 C/min. by switching off the electrical power supply given to the heating elements of the furnace. So the samples are cooled in the furnace itself. When 400 C is reached, the samples are taken out from the furnace and are cooled faster to room temperature by forced convection using a fan. The adopted heating and cooling rates of the samples are almost same with the commercial production of the bronze bearings [ 8 ]. But here the samples are kept stationary instead of moving continuously as in the commercial sintering furnaces.

11 63 10 V w TIME ( hours ) Fig 31: Rate of heating and cooling of bronze compacts (D.l 3«mva3dW3X

12 POLISHING AND GRINDING OF THE SINTERED COMPACTS: For the microstructure studies and ultrasonic velocity measurements, the sample surfaces are to be polished. Particularly for velocity measurements using pulse echo methods, the surfaces of the sintered compacts are to be perfectly parallel, acoustically smooth and optically plane. The deviation from the plane condition is maintained within 5 microns. Using lapping machine the surfaces are polished. Then using hand lapping the surface are made plane with higher accuracy. The fine grade (3/0 and 4/0} emery polishing papers (John Oakey and Mohan Ltd, Uttar Pradesh) are used for hand lapping. Optical testing is a better method to determine the accuracy of the planeness of a surface. An optical flat is placed over the polished surface of the sample. In the presence of sodium light, yellow coloured interference fringes are appeared. If the fringes are straight, then the surface is optically plane. Instead if the fringes are curved in the form of convex or concave, then the surface is curved. The curved part of the surface is made plane by successive careful lapping of that part. It is found that when the surfaces of the sample are optically plane and parallel, the echo pattern is appeared with more contrast on the oscilloscope screen. When the surfaces are not plane and parallel, the echoes are not appeared on the screen.