Methods

Frequently, LPM methods for shape forming with metal powders are extensions of processes developed for ceramics. The majority of LPM processes can be classified based on the binder system under two categories: dilute polymer solutions and polymerizable systems. Variations under each of these categories are discussed.

Processes Using Dilute Polymer Solutions. A straightforward approach to reducing binder viscosity is to use polymer solutions. The viscosity of a polymer solution is lowered with decreasing concentration and molecular weight of the polymer (Ref 10). Both aqueous and non-aqueous systems have been practiced, although the former is preferred for environmental and safety reasons, provided the metal system is not reactive toward water. Two examples are presented using this method.

Slip casting is a classic example of a forming method used primarily for ceramics (Ref 17). Although the method had been explored several decades ago for metal powders (Ref 21), it was rarely practiced until more recently (Ref 22, 23). Slurries consisting of aqueous polymer solutions as binders and metal powders have been used for slip casting. Polysaccharides, vinyl polymers, waxes, and various organometallic compounds have been used in solutions (concentration varying between 0.5 and 5 wt%) for slip casting (Ref 24). A homogeneous feedstock is obtained by the low shear mixing of the ingredients at room temperature. The viscosity of the feedstock can be reduced by more than an order of magnitude under conditions of low shear (e.g., 450 Pa • s at a shear rate of 40 s-1 for a 316L stainless steel feedstock, solids loading of 60 vol%). The feedstock is then poured into molds at ambient conditions of temperature and pressure and dried before ejection of parts. The use of porous tooling helps achieve green densities in the range of 70 to 74% of the theoretical density (Ref 23). The parts obtained are nearly 99% debound and have a green strength of 9.5 MPa (1380 psi). The remaining binder is rapidly removed in a thermal debinding step (500 °C). Because the amount of carbon in the starting feedstock is low, carbon control, especially for the fabrication of thick sections, is relatively easy. Carbon analysis performed on 17-4 PH stainless steel parts formed by slip casting resulted in 0.045 wt% residue (Ref 23). Final sintering is conducted under standard conditions. Starting with identical powders and solids loading, comparable properties can be obtained for 316L stainless steel parts fabricated by slip casting and powder injection molding (Ref 25), as shown in Table 4. The latter process is an example of a high-pressure molding process. Depending on the mold geometry, pressures in the range of 3.5 to 70 MPa (500 to 10,000 psi) are typical for powder injection molding (Ref 18). In a separate study, a sintered density of 7.74 g/cm3 and hardness of 40 HRC have been reported for slip cast 17-4 PH stainless steel (Ref 23). This powder is unsuitable for use in the fabrication of parts by die compaction due to its hard and spherical nature. Powder injection molded parts for the same material are 7.5 g/cm3 and 35 HRC, respectively (Ref 26).

Table 4 Sintered properties of 316L stainless parts obtained by slip casting and powder injection molding

Property

Slip casting(a)

Powder injection molding(b)

Shrinkage after sintering, %

14.8

13.3

Sintered density, % theoretical

95.6

91.0

Ultimate density strength, MPa

492

448

Elongation to fracture, %

38

34

Hardness, HRB

59

57

Feedstock: water-atomized 316L stainless steel (d50 = 14.3 /,!m) at a solids loading of 62 vol% was used in both processes. The binder system used in slip casting was an aqueous alginate solution. The binder system used in powder injection molding consisted of paraffin wax, polypropylene, and stearic acid. Sintering conditions for both processes: 1350 °C, 1 h, H2.

The Quickset process is another variation of binder systems using aqueous polymer solutions (Ref 8). Here, the parts are frozen in the mold, and the water molecules are removed by sublimation. Additives are introduced in the feedstock to minimize volume changes during freezing. The residual binder is removed in a thermal debinding step. For Sialon, flexural strength of 975 MPa and Weibull modulus of 19.4 have been reported using the process. Comparative runs with isopressed parts resulted in values of 785 MPa and 8.7, respectively, for the flexural strength and Weibull modulus.

Polymerizable Systems. The viscosity and melting point of a polymer increase with its molecular weight (Ref 10). Therefore, a logical alternative to using polymer solutions is to use low-molecular-weight polymer precursors (referred to as monomers, oligomers, or prepolymers) in order to obtain low binder viscosities at room temperature (typically <1 Pa • s). A catalyst is used to initiate the polymerization reaction, often in combination with heat. The polymers are of the thermosetting type because a cross-linking agent is usually added to the mixture to solidify the part in the mold. This approach owes its origins to fabrication techniques used in polymer-matrix composites. Two examples of this approach are presented.

The debinding reaction injection molding (D-RIM) process uses a low viscosity (0.2 to 1.0 Pa • s), thermosetting liquid polymer to fluidize the powder (Ref 27). Solids loading of 60 to 65% for metal powders can be achieved. The furfuryl alcohol-based polymer undergoes solidification as a result of a cross-linking reaction initiated by the addition of heat or a catalyst. A green strength of 6.5 MPa (950 psi) is typical. Water molecules are evolved in the gaseous state as a by-product of the reaction and leave behind an open pore network. Nearly 50% of the initial binder is removed in this manner at the end of the molding stage. The remaining binder is thermally decomposed in air, hydrogen, or vacuum at 400 to 500 °C. Mold residence times depend on part dimension and typically vary from 1 to 5 min. An important aspect of processes using thermosetting resins is that thermal decomposition occurs without any liquid phase being formed. The porous structure formed in the present process as a result of the condensation reaction provides a convenient pathway for the escape of the gaseous decomposition products. This minimizes the formation of internal stresses that could lead to defects such as cracks and blisters during the thermal debinding stage. Tensile strength for carbonyl iron D-RIM parts was reported to be 215 MPa. A value of 220 MPa reported for powder injection molded processed iron (Ref 26).

Gelcasting is another variation based on the use of cross-linkable monomers (Ref 28). A monomer solution provides a low-viscosity vehicle for suspending the powder. For example, the viscosity of a 62 vol% slurry of alumina has been reported to be 1.8 Pa • s at a shear rate of 10 s-1. The monomers used are a mixture of acrylamides (10 to 20%), and the solvent is usually water (80 to 90%). This is introduced into a mold made of aluminum or polyethylene where it is cross linked to form a solid polymer-solvent gel. The cross-linking reaction can be performed by the addition of heat (50 to 60 °C) and a suitable catalyst. The gelled part can be removed from the mold without distortion and dried to remove the solvent. The mold residence time depends on part geometry and varies from 10 min to an hour. Drying is dependent on the part and occurs outside the mold cavity in a matter of hours. The residual binder was removed at 350 to 500 °C. The process was originally developed for processing ceramics (e.g., alumina and silicon nitride) and has been extended to metal powder systems (e.g., 17-4 PH stainless steel and H13 tool steel). A sintered density of 7.58 g/cm3 and hardness of 37 HRC have been reported for gelcast 17-4 PH stainless steel. Values for powder injection molded parts for the same material are 7.5 g/cm3 and 35 HRC, respectively.

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