Difficulties in the metallographic preparation of powder metallurgy parts

The main challenge during the preparation is to show the true porosity after grinding and polishing. Depending on the hardness of the material this can be more or less successful.

During the grinding of soft metals, abraded metal is pushed into the pores, which then has to be removed by polishing. Samples from parts in which hard and soft materials are mixed are prone to show pronounced relief. The preparation of green parts needs particular care and patience as they are very fragile. There are various recommendations for the preparation of powder metallurgy parts.

For sectioning a powder metallurgy component of a specific metal or alloy one can select an appropriate cut-off wheel using the recommended charts and guidelines. For mixed materials it is recommended to use a cut-off wheel suitable for cutting the material which constitutes the major part. For sintered carbides a resin bonded diamond cut-off wheel is recommended. Green parts need to be mounted in cold mounting resin before cutting, so that they are not crushed by clamping.

In order to assure good adhesion of the mounting resin to the sample material it is essential to degrease the sample thoroughly with acetone, toluene or Isopar C before mounting. Use proper safety precautions when handling solvents.

Experimental powder metallurgy stainless steel colour etching

Sintered parts can be mounted with a resin suitable to the mechanical properties of the sample material. For hot compression mounted either phenolic resin (MultiFast) or resins containing harder filler material (DuroFast, LevoFast). For a cold mounting acrylic resins with filler (DuroCit-3 or LevoCit) can be used.

Green parts need to be re-impregnated after sectioning under vacuum with a cold mounting epoxy resin (CaldoFix-2, EpoFix, SpeciFix-40). Powders can be mounted by mixing a small amount (about 1/2 teaspoon) of powder with a slow curing epoxy resin and pouring it into a mounting cup. During the approximate12 hours curing process the particles will form a layer by settling at the bottom of the cup.

Experimental powder metallurgy stainless steel colour etching

Hard metal powders can be hot compression mounted by mixing with one measuring spoon of the fine-grained mounting resin DuroFast. The mixture is then poured into the cylinder of the mounting press and is topped with phenolic resin.

Grinding and polishing
The grinding and polishing procedure for powder metals follows the same rules which are applied to prepare ingot-based samples of the same material.

Plane grinding high volumes of samples of materials >150 HV can be carried out on an aluminium oxide grinding stone or a diamond grinding disc (MD-Piano). Materials <150 HV can be plane ground on silicon carbide foil or paper. For fine grinding with diamond the fine grinding disc MD-Allegro is suitable for materials >150 HV and MD-Largo for materials <150 HV. This is followed by a thorough diamond polish with 3μm and a brief final polish with 1μm or with oxide polishing suspensions. As one of the major goals of preparing a powder metallurgical sample is to show the true porosity, it is important that the diamond polishing step is carried out long enough to achieve this goal. For large or soft samples, it can take up to 10 to 15 minutes of diamond polishing to remove the metal pushed into the pores during grinding and reveal the correct porosity. For soft metals the final polishing time should not be prolonged unnecessarily, as this leads to rounding of the pore edges. Starting with 500# or 800#, green parts are ground manually on silicon carbide foil or paper to 4000#. The surface is re-impregnated as needed. Polishing can be done on a semi-automatic polishing machine for individual samples.

Experimental powder metallurgy stainless steel colour etching

In order to establish the exact polishing time for specific alloys and parts, it is recommended to check the structure during polishing every two minutes with the microscope, and only proceed to the next polishing step when all residual metal has been removed from the pores.

For further details contact Advanced Laboratory Solutions (ALS) on TEL: 011 462 1363 or email info@advancedlab.co.za or visit www.advancedlab.co.za