A Description of the Copper Infiltration Process Used in Iron-Based P/M Parts
Infiltration is basically defined as "a process of filling the pores of a sintered or unsintered compact with a metal or alloy of a lower melting point." In the particular case of copper infiltrated iron and steel compacts, the base iron matrix, or skeleton, is heated in contact with the copper alloy to a temperature above the melting point of the copper, normally within the range of 2000¯ to 2100¯F (1095¯ to 1150¯C). Through capillary action, the molten copper alloy is drawn into the interconnected pores of the skeleton and ideally fills the entire pore volume.
Filling of the pores with higher density copper can result in final densities in excess of 95% of the composite theoretical value. Completely filled skeletons also allow for secondary operation such as pickling and plating without damaging the structure through internal corrosion. Pressure tight infiltrated components are also possible for specific applications that demand the absence of interconnected porosity.
Although non-infiltrated high densities are obtainable through multiple pressing and sintering operations, the extraordinary cost of machine/labor time may exceed the cost of the infiltrating materials required. The choice of high density, multiple compacting versus infiltration would essentially depend upon the required properties of the finished components.
Basic Infiltration Methods
The infiltration process is generally subdivided into two fundamental methods: single step or double step. The single step or single pass is presently the preferred infiltration method that consists of one run or pass through the furnace. In this process, the unsintered (green) iron and copper alloy compacts are placed in contact prior to furnace entry. The typical arrangement is to place the copper alloy infiltrant compact on the top surface of the iron compact. In some cases, it is preferred to place the iron compact on top of the infiltrant compact, or, infiltrate from top and bottom simultaneously.
During the full furnace cycle, the iron base compact is ideally partially sintered prior to attaining the melting points of the infiltrant composition. Preferably, multi-independent zone furnaces are employed allowing for preheat, or lubricant burn-off, followed by pre-sintering (graphite solution) and finally infiltration.
The double step or double pass infiltration method consists of pre-sintering or full sintering of only the iron compact in one pass through the furnace. After the first sintering pass, the unsintered (green) infiltrant compact is placed in contact with the sintered iron part, and the full furnace cycle is repeated.
The infiltrating powders available may be used for both the single and double step processes. Most, if not all, infiltrating powders are prepared as a pre-blended and/or a pre-lubricated lot or batch and are designed for typical compacting operations. Shapes of infiltrant compact forms vary substantially depending upon the amount required and the configuration of the iron skeleton. Usually, simple infiltrant shapes, such as bars, cylindrical slugs, or annuli are compacted to a specific weight and are placed on the iron components in single or multiple contact arrangements.
Infiltrants consist of a variety of compositions. Typically, they contain copper or copper alloy base plus a substantial amount of iron to minimize erosion or localized dissolving of iron beneath the contact area as infiltration proceeds. Other ingredients may include lubricant, graphite, nickel, manganese, and miscellaneous materials that tend to form non-adhering residuals.
Within this variety of available powders, there are two basic types: those that leave essentially no residue and those purposely designed to leave a substantial residue. Primary consideration must be given to the type of infiltrant used in order to properly calculate the efficiency or yield of the infiltrant with regard to attaining specific final densities and related copper content requirements.
The maximum amount of infiltrant that may be used depends solely upon the available void or pore volume of the iron compact, since only the porosity may be filled. Braze infiltration of assemblies may require a greater amount to accommodate the initial void volume between mating components.
The primary factors to be considered with regard to the total amount of infiltrant required are:
- Void or interconnected pore volume
- Specified final infiltrated copper content
- Specified final infiltrated density
- Efficiency of the infiltrant to be used
Other secondary, yet important, factors are related to the basic iron blend used for the components and the sintered characteristics of the iron. These factors include total weight change of the base compact resulting from loss of lubricant, carbon and oxides, and total volumetric changes of the compact directly related to the sintered dimensional characteristics of the base iron and/or base iron blend composition.
Infiltrant compacts are normally prepared employing common press equipment and simple or convenient tooling. The compacting characteristics of infiltrants may differ as a result of their various compositions and methods of manufacture. The best approach is to obtain preliminary information from the powder supplier who can provide information regarding the characteristics specifically related to their product. For example, the infiltrating efficiency of a particular product may be related to green density of the compact, or minimum green densities may be suggested to attain green strengths adequate for normal production handling.
The shapes of the infiltrating compacts to be employed are generally the choice of the process operator; however, their configuration must be compatible with the part to be infiltrated and also depend upon the characteristics of the infiltrant. Typically, an infiltrating compact should be of such a size that it does not extend over openings or beyond the outer boundaries of the base part. Such over extensions may results in run-off and possible loss of infiltrant. Should over extension of the infiltrant compact be unavoidable, then an infiltrant having minimum run-off characteristics should be selected.
Short cylindrical slugs of infiltrant are an acceptable and multi-purpose shape for most parts having flat surfaces. The cylindrical compacts also allow for multiple placement on a single part. The advantages of multiple placement are a more even distribution of infiltrant and a minimum of erosion. If possible and/or practical, the contact area per unit weight of infiltrant should always be maximized to avoid erosion.
Annular infiltrating compacts are best suited for infiltration around inside diameters of iron parts having a minimum flat supporting surface such as bevel gears, pinion gears, and those having hubs. Annular compacts are usually not used on large flat areas due to the low contact area per unit weight that may result in erosion of the contact area.
The infiltrating processing conditions are quite similar to those commonly employed for the typical sintering of iron-based components with regard to temperature and atmosphere protection. One departure is that the components to be infiltrated are usually not placed directly on the furnace belt. Graphite or ceramic trays or plates are placed beneath the parts to be infiltrated to prevent furnace belt damage by excessive molten infiltrant and prevent infiltrant loss through the mesh belt. In some cases, infiltrating materials are placed between the plates and the components for bottom infiltration.
Common protective sintering atmospheres are suitable for infiltration; however, certain types of infiltrants require atmosphere dew points of 32¯F (0¯C) or higher to prevent objectionable and tenacious adherence of constituent residuals. Tough, adhering residuals may only be removed through secondary operations such as scraping, grinding, or abrasive sanding.
The sintering temperatures and times employed are essentially those used for standard parts sintering. The exception being the necessity for an intermediate pre-conditioning temperature for the solution of carbon (graphite) prior to reaching infiltration temperature when infiltrating by the single step method.
Assuming effective delubrication is complete, other gaseous reactions occurring at temperatures prior to reaching the infiltrant melting point can result in internal pressures and outward flow of gas through porosity. Excessive outgassing persisting after melting of the infiltrant could inhibit the inward flow of the infiltrant. This could cause the molten infiltrant to flow over and down the outside of the part, and as long as outgassing persists, effective infiltration cannot occur. Matrix outgassing is the result of expansion of trapped air/lubricant residuals within the green part; water forming reactions between the reducing atmosphere constituents and residual oxides; and formations of CO and/or CO2 from added graphite and residual oxides.
The potential difficulties caused by inherent outgassing may be eliminated or at least minimized by double step infiltration or by extended sintering time prior to reaching infiltration temperature during the single step process.
Components treated through the double step process are given a compete sinter prior to the second, or infiltration step; therefore, complete degassing has occurred and infiltration is accomplished during the second step without interference. The time required for pre-infiltration conditioning would be affected by the completeness of initial lubricant removal, the oxide content of the base iron powder, and the amount of graphite contained.
Suggested infiltration cycles for single and double step infiltration are as follows:
- Single Step - Single pass through the furnace
- 15 minutes at 1250¯ to 1400¯F (677¯ to 760¯C) for lubricant burn-off;
- 10 minutes (minimum) at 1850¯ to 1900¯F (1010¯ to 1038¯C) for graphite solution;
- 30 minutes at 2050¯F (1121¯C) for sintering-infiltration.
- Double Step - Two complete passes through the furnace
- Pass One - 15 minutes at 1250¯ to 1400¯F (677¯ to 760¯C) for lubricant burn-off plus 30 minutes at 2050¯F (1121¯C) for sintering;
- Pass Two - Repeat the above cycle with infiltrant.
INFILTRANT QUANTITY CALCULATIONS (Suggested Methods)To accurately predict the amount of infiltrant to be used, there are three assumptions that must be made in relation to the following calculations:
1. Only 97% of the void volume consists of interconnected porosity; therefore, only 97% of the void volume may be filled with infiltrant. (The average value of 97% is determined empirically.)
2. The change in part volume due to dimensional changes after infiltration is not considered. Such changes would normally be related to the particular characteristics of the base iron composition being used.
3. The average value of 7.7 g/cm^3 has been used for iron/steel theoretical density.
Infiltration to Maximum Density The percentage of infiltrant required in a green iron part to achieve maximum density is given by the equation
[(0.97Dt - Ds)/Dg] x (Di/Dt) x (100/E)
[(7.47 - Ds)/Dg] x If
Dt = Theoretical density (average) of iron/steel
Ds = Density of sintered, non-infiltrated part
Dg = Density of green part
Di = Theoretical density of the metallic portion of the infiltrant
E = Infiltrant efficiency
If = Infiltrating factor inclusive of the terms Di/Dt x100/E (see Table I for examples of If values for infiltrants produced by U.S. Bronze)
Infiltration to a Specific Density The formula for "Maximum Density" (Equation 2) must be applied first to verify that the specific density required can be attained based upon available porosity and the infiltrant being used. If the percentage infiltrant required for maximum infiltration is less than the amount calculated by the formula for specific density, the required infiltrated density may not be attained with the initial green density of the part. The percentage infiltrant for a specific density may be calculated according to the equation
[(0.97Dr - Ds)/Dg] x (Di/Dt) x (100/E)
[(0.97Dr - Ds)/Dg] x If
Dr = required final infiltrated density of the part.
Infiltration to a Specified Final Copper Content
The formula for "Maximum Density' (Equation 2) must be applied first to verify that the specified copper content can be attained based upon available porosity and the infiltrant being used. If the percentage infiltrant required for maximum infiltration is less than the amount of copper calculated by the formula for copper content, the required infiltrated copper content may not be attained with the initial sintered density of the part.
In this relationship, the sintered uninfiltrated weight is considered since the final copper content is based upon the chemical analysis of the infiltrated part. It has also been assumed in the following formulations that 100 percent of the copper in the infiltrant is recovered during infiltration. The infiltrant percentage of sintered uninfiltrated part weight is equal to
[%Cur/(100 - %Cur)] x (100/Cui)
then, the weight of infiltrant required for specified final copper content is given by
infiltrant % x Ps
Cur = Final copper percentage requirement by analysis
Cui = Percentage of copper (Cu) contained in the infiltrant expressed as a decimal (i.e., % Cu in infiltrant/100)
Ps = Sintered uninfiltrated part weight
Infiltrant Efficiency Calculation
The following calculation is as shown in MPIF Standard 49:
Gross Efficiency % = [(F - B)/S] x 100
F = Weight of infiltrated specimen, in grams, after residue has been removed<
B = Weight of sintered uninfiltrated specimen, in grams
S = Weight of infiltrant slug, in grams