The Metal Powder Industries Federation (MPIF) has published a collection of nine existing MPIF Standard Test Methods that can be applied for the characterisation of powders used in metal Additive Manufacturing processes. The standards, intended to present and clarify the technology as an aid in conducting business, relate to those activities that concern designers, manufacturers and users of metal AM parts.
Titled ‘Collection of Powder Characterization Standards for Metal Additive Manufacturing’, the publication includes the following MPIF Standards:
MPIF Standard 01- Method for Sampling Metal Powders
Scope of the Standard: Describes methods for sampling powders received in containers, and for sampling powders in the process of being packaged from blenders or other containers.
Purpose: Establish the method for obtaining samples needed for powder testing which can be agreed-to by all participants in the supply-chain, including powder producers, powder blenders and end users.
Relevancy to Metal AM: The first step for process control using Metal AM powder is the ability for all parties to check powder suitability. To accomplish this, all parties in the supply-chain need agreement on how to obtain a representative sample for verification testing. This standard describes how this is accomplished for both in-process and packaged material.
MPIF Standard 02 – Method for Determination of Loss of Mass in a Reducing Atmosphere for Metal Powders (Hydrogen Loss)
Scope of the Standard: Describes the method to determine loss of mass for metal powder. This is done with the powder heated in a high temperature furnace under hydrogen atmosphere.
Purpose: This test indicates the purity of powder with regards to interstitial elements like oxygen, carbon, sulfur etc. Typically, the primary information learned in this test is the amount of easily-reduced oxides contained in a powder. It can also be show if there is appreciable moisture content in the powder.
Relevancy to Metal AM: There are many alloys used in Metal AM where interstitial elements can affect the process and subsequent material properties negatively. One direct measurement of the prevalence of these elements is to see what % of the powder mass is lost when heated in the presence of a powerful reducing agent like hydrogen. The higher the % loss, the higher the interstitial content of the powder. Those interstitials can affect alloy quality and process control.
MPIF Standard 03 – Method for Determination of Flow Rate of Free-Flowing Metal Powders Using the Hall Apparatus
Scope of the Standard: Describes a method to determine how well powder will flow by using a standardized funnel.
Purpose: This test measures how easily a powder will flow, using a specified weight (50 grams) of powder. The powder is put into a standardized funnel and the time is measured for how long it takes to completely pass, under the force of gravity only. If the powder flows through the funnel it indicates it will flow in other conditions.
Relevancy to Metal AM: In most Metal AM processes there is a process step where the powder is spread in a thin layer with minimal force. During this step a powder that flows easily is typically a positive attribute, so a shorter time reported for this test means that the powder flows more easily, and that should be a beneficial for Metal AM processing. Factors that will affect this test include powder shape (the more spherical the powder, typically the better it will flow) and particle size distribution (a broad particle size distribution powder typically flows better than a monosize particle size distribution powder).
MPIF Standard 04 – Method for Determination of Apparent Density of Free-Flowing Metal Powders Using the Hall Apparatus
Scope of the Standard: Describes method to determine the apparent density of a powder when the powder is flowing freely under the force of gravity only (i.e. no outside energy or force is added to change the void space between particles).
Purpose: This test measures how well a powder packs after flowing through a funnel (the same funnel used in MPIF Standard 03), using the force of gravity only, and into a “density cup” that has a specified geometry and volume.
Relevancy to Metal AM: In most Metal AM processes there is a process step where the powder is spread in a thin layer, using minimal force. A powder that packs easily (under gravitational force only) to a high density implies that it can be moved/spread with the result that there is minimal void space between the powder particles. Reducing the void space between powder particles during powder spreading will make it easier to subsequently densify the powder. In general, the higher the apparent density of a powder, the better it should process for Metal AM provided other attributes like particle size remain the same. Powder shape will influence this characteristic, because the more spherical the powder, typically the better it will pack together. Particle size distribution will also play a role; in the case of a broad particle size distribution powder, the smaller particles can fit in the space between larger particles, while a monosize particle size distribution powder does not have this opportunity.
MPIF Standard 05 – Method for Determination of Sieve Analysis of Metal Powders
Scope of the Standard: Describes method to assess the particle size distribution of a powder lot by having the powder pass through a series of screens with progressively smaller openings, and weighing the material left on each screen.
Purpose: This test measures powder sizes directly.
Relevancy to Metal AM: Particle size distribution is critical to all Metal AM processes. It will influence the minimum thickness that can be spread while building a powder bed. It will also directly affect the densification in sintering for the Metal AM processes that require consolidation in a furnace.
MPIF Standard 28 – Method for Determination of Apparent Density of Non-Free-Flowing Metal Powders Using the Carney Apparatus
Scope of the Standard: Describes the method to determine the apparent density of a powder when the powder does not flow freely under the force of gravity alone. This test is used for powder that cannot be tested by MPIF Standard 04.
Purpose: This test measures how well a non-free flowing powder packs after passing through a funnel which has a larger orifice size than the funnel used in MPIF Standard 04. The technician can assist the flow by using a wire to “poke” the powder in order to assist the powder to pass through the orifice.
Relevancy to Metal AM: This test has the same relevancy to Metal AM as MPIF Standard 04. It is used primarily for smaller particle sizes. When the powder is smaller (i.e. < 20 um), it is more likely that interparticle friction will prevent the powder from easily flowing without energy or force being applied. The technique in this standard allows the apparent density to be determined when the powder does not flow freely.
MPIF Standard 46 – Method for Determination of Tap Density of Metal Powders
Scope of the Standard: Describes the method to determine the maximum density of a powder when vibrated under certain conditions.
Purpose: This test measures how well a powder packs when subjected to a vibrational (“tapping”) force.
Relevancy to Metal AM: This test shows how well the powder arranges (packs) when subjected to a vibrational force, as opposed to apparent density tests where the powder packing is accomplished with minimal input energy. By exposing the powder to tapping it maximizes the packing density of the powder without applying direct force. A high relative tap density value should be helpful for powder spreadability and uniformity during the Metal AM process.
MPIF Standard 48 – Method for Determination of Apparent Density of Metal Powders Using the Arnold Apparatus
Scope of the Standard: Describes the method to determine the apparent density of a powder using an alternative method from MPIF Standard 04 or 28.
Purpose: This test measures how well a powder packs when it drops into a relatively large (31.66 mm) cylinder. This will typically be used for powders that do not flow freely.
Relevancy to Metal AM: This test has the same relevancy to Metal AM as MPIF Standard 04 and MPIF Standard 28. It is used primarily for powders that do not easily flow.
MPIF Standard 53 – Method for Measuring the Volume of the Apparent Density Cup Used with the Hall and Carney Apparatus (Standards 04 and 28)
Scope of the Standard: Describes a method to determine the volume of the cup used to determine apparent density in Standards 04 and 28.
Purpose: Good process control for Standards 04 and 28 requires that the volume of the cup used in these tests has suitable accuracy. This procedure is a check to make sure that the volume of a cup in use is acceptably accurate as over time it may change (i.e. worn edge of the cup).
Relevancy to Metal AM: If Standards 04 or 28 are in use, then this Standard is necessary for ensuring ongoing accuracy of their results.