GKN Sinter Metals has produced a new case-hardened low carbon alloy steel – 20MnCr5 – for use in additively manufactured gear prototypes (Courtesy GKN Sinter Metals Blog).
“Metal Powders for Additive Manufacturing Market to reach US$1,783.9 Million by 2025; Burgeoning Automobile and Space Industries Spur Market’s Growth – Transparency Market Research”
Metal powder plays a very important role in the additive manufacturing processes. Indeed the quality of metal powder used will have a major inﬂuence on mechanical properties but it can also inﬂuence:
- The build-to-build consistency
- The reproducibility between AM machines
- The production of defect-free components
- The manufacturing defects on surfaces
The variety of materials available for Metal Additive Manufacturing systems is continuously expanding. A very wide range of alloys are used on additive manufacturing machines thanks to the availability of metal powders:
- Steels such: 316L, 17-4PH etc.
- Nickel and cobalt base superalloys: 625, 718, 939, CoCr F75 etc.
- Titanium alloys: Ti6Al4V, TiCP, Ti6Al4V ELI etc.
- Aluminium alloys: AlSi10Mg etc.
But many other metals are also evaluated and developing:
- Copper alloys
- Magnesium alloys
- Precious metals such as gold, silver, platinum
- Refractory metals such as Mo alloys, W and WC
- Metal Matrix Composites (MMC’s), etc.
The most common materials for Selective Laser Melting (SLM) processes are shown in the table below. The material trade names vary depending on the manufacturer, therefore the name used here corresponds to the specifications on the material data sheets and in some cases the European nomination is given.
2. Powder Manufacturing processes
Metal powders for additive manufacturing are usually produced using the gas atomization process, where a molten metal stream is atomized thanks to a high pressure neutral gas jet into small metal droplets thus forming metal powder particles after rapid solidification.
Gas atomization is a physical method (as opposed to chemical or mechanical methods) to obtain metal powders, like water atomization. But powders produced by gas atomization have a spherical shape, which is very beneficial for powder ﬂowability while powders produced by water atomization will have an irregular shape.
Gas atomization is the most common process for additive manufacturing because it ensures:
- A spherical powder shape
- A good powder density, thanks to the spherical shape and particle size distribution.
- A good reproducibility of particle size distribution
Besides a very wide range of alloys can be produced using the gas atomization process.
2.1 The Gas Atomization (GA) process
Gas atomization process has been widely used in industry due to its advantages such as high capacity and high flexibility for the production of a wide range of ultrafine spherical metal powders. The rapidly solidified metal powder formed as a result of high cooling rate and deep under-cooling exhibits fine microstructure, chemical homogeneity, extended solid solution and metastable phase formation.
The gas atomization process starts with molten metal pouring from a tundish through a nozzle. The stream of molten metal is then hit by jets of neutral gas such as nitrogen or argon and atomized into very small droplets which cool down and solidify when falling inside the atomization tower.
Powders are then collected in a can. The gas atomization process is the most common process to produce spherical metal powders for additive manufacturing. It is used in particular for steels, aluminium alloys, precious metals, etc.
Gas Atomization (GA) Process
2.2 The VIM (Vacuum Induction Melting) Gas Atomization (GA) process
Commercial vacuum induction melting (VIM) was developed in the early 1950s, having been stimulated by the need to produce superalloys containing reactive elements within an evacuated atmosphere. The process is relatively flexible, featuring the independent control of time, temperature, pressure, and mass transport through melt stirring. As such, VIM offers more control over alloy composition and homogeneity than other vacuum melting processes.
In the VIM gas atomization process, the melting takes place in a vacuum chamber. This process is recommended for superalloys so as to avoid in particular oxygen pick-up when working with alloys with reactive elements such as Ti and Al.
VIM Gas Atomization (GA) Process
2.3 Plasma Atomization (PA) process
A new atomization process, Plasma Atomization (PA), has been developed to produce fine, spherical powders. Unlike conventional high-pressure atomizers, PA utilizes multiple non-transferred direct-current arc plasmas to accelerate the atomization gas. In the PA process, metal wires are fed into the apex of the multiple plasmas, where they melt and are atomized in a single step. This process offers the unique ability to produce spherical powders of reactive metals with a typical average particle size of 40 μm.
Plasma atomization and spheroidization consists of in-ﬂight heating and melting thanks to a plasma torch of feed material followed by cooling and solidification under controlled conditions.
Depending on processes, the raw material can be particles as well as bar or wire feedstock. Plasma atomization can be used in particular to spheroidise refractory metals such as Mo alloys, W and WC.
Plasma Atomization (PA) Process
2.4 Centrifugal Atomization (CA) process
Centrifugal atomization is a well-established method for the production of fine metal powders. This method has been successfully used to produce various metal powders such as Sn, Pb, Al, Mg, Zn, Ti, Ni, and their alloys. Compared with liquid or gas atomization techniques, centrifugal atomization can produce highly spherical metal powders with low impurity content, narrow particle size distributions, and high production yields.
Centrifugal atomization, also known as Plasma Rotating Electrode Process (PREP), consists in melting with a plasma torch the end of a bar feedstock rotating at high speed and thus ejecting centrifugally the molten droplets of metal.
2.5 Other Powder Manufacturing processes
Powder blending and Mechanical alloying (Ball Milling), to produce Metal Matrix Composites (MMC’s).
3. Metal powder characteristics for Additive Manufacturing (AM)
Key metal powder characteristics for additive manufacturing can be sorted in four main categories:
- Chemical composition
- Powder size distribution (PSD)
- Physical properties
In all cases, there are several useful existing standards to determine methods for characterizing metal powders. Additional points are important to consider when selecting metal powders for additive manufacturing processes:
- Storage and aging of powders
- Reusability of powder after additive manufacturing cycles
- Health, safety and environmental issues
Regarding chemical composition, alloy elements and chosen measurement techniques (ICP, Spectrometry, etc.) are very important but it is also important to take into account:
- Interstitials, such as Oxygen, Nitrogen, Carbon and Sulfur, to measure by combustion and fusion techniques
- As well as trace elements and impurities
- As they may affect significantly material properties depending on alloys
With the gas atomization process, all powder particles have the same chemical composition but finer particles tend to have a higher oxygen content due to the higher specific surface.
The chemical composition will inﬂuence in particular:
- Melting temperature
- Mechanical properties
- Thermal properties (thermal conductivity, Heat capacity etc.), etc.
Last, the chemical composition can also evolve slightly after multiple uses in additive manufacturing machines.
Particle Size Distribution (PSD)
Depending on additive manufacturing technology and equipment, two main types of particle size distributions are considered:
- Powders usually below 50 microns for most powder bed systems. In this case, finer powder particles below 10 or 20 microns shall be avoided, as they are detrimental to the powder ﬂowability,
- Powder between 50 and 100 to 150 µm for EBM and LMD technologies
The Particle Size Distribution (PSD) is an index indicating what sizes of particles are present in what proportions i.e. the relative particle amount as a percentage of volume where the total amount of particles is 100 %) in the sample particle group to be measured.
The frequency distribution indicates in percentage the amounts of particles existing in respective particle size intervals whereas cumulative distribution expresses the percentage of the amounts of particles of a specific particle size or below.
Alternatively, cumulative distribution expresses the percentage of the amounts of particles below a certain size. A common approach to define the distribution width is to refer to three values on the x-axis (volume %):
- The D10, i.e. the size where 10% of the population lies below D10
- The D50, or median, i.e. the size where 50% of the population lies below D50
- The D90, i.e. the size where 90% of the population lies below D90
Powder sampling is also an important point due to the powder segregation (applicable standard ASTM B215).
Usual methods and standards for particle size distribution measurement are:
- ISO 4497 Metallic Powders, Determination of Particle Size by Dry Sieving (or ASTM B214 Test Method for Sieve Analysis of Metal Powders)
- ISO 13320 Particle Size Analysis – Laser Diffraction Methods (or ASTM B822 Test Method for Particle Size Distribution of
- Metal Powders and Related Compounds by Light Scattering)
It is important to note that the PSD results will be dependent of the chosen test methods, which can provide different results in particular depending on powder morphologies.
The particle size distribution is a major point in additive manufacturing as it can inﬂuence many aspects such as:
- Powder ﬂowability and ability to spread evenly,
- Powder bed density,
- Energy input needed to melt the powder grains,
- Surface roughness, etc.
The recommended particle morphology for additive manufacturing is spherical shape because it is beneficial for powder ﬂowability and also to help forming uniform powder layers in powder bed systems.
The powder morphology can be observed by SEM (Scanning Electron Microscope). Typical defects to be controlled and minimized are:
- Irregular powder shapes such as elongated particles
- Satellites which are small powder grains stuck on the surface of bigger grains
- Hollow powder particles, with open or closed porosity
Porosity content can be evaluated either by SEM observation or by Helium Pycnometry. The presence of excessive amounts of large pores or pores with entrapped gas can affect material properties.
Applicable Standards: ASTM B923 Test Method for Metal Powder Skeletal Density by Helium or Nitrogen Pycnometry.
Other powder physical properties
Rheological properties are very important for metal powders used in additive manufacturing equipment, both for powder handling from powder container to working area and in the case of powder bed systems to form uniform layers of powders.
Rheology is a complex matter but some standard test methods are available, though not always fully appropriate for the particle sizes typical of additive manufacturing systems:
- Density (apparent or tap):
The bulk/ apparent density of a material is the ratio of the mass to the volume (including the interparticulate void volume) of an untapped powder sample.
The tapped density is obtained by mechanically tapping a graduated cylinder containing the sample until little further volume change is observed.
The Compressibility Index (CI (%)) and Hausner ratio are measures of the products ability to settle, and permit an assessment of the relative importance of interparticulate interactions.
In a free-flowing powder these interactions are less significant and the bulk and tapped densities will be closer in value. For poorly flowing materials, there are greater interparticulate interactions and a greater difference between the bulk and tapped densities will be observed. The differences are reflected in the compressibility index and Hausner ratio.
- Flow rate:
The Hall and Carney flowmeters are widely used in the P/M industry to characterize powder flowability.
- Angle of repose:
Determining the angle of repose is relatively easy: simply form a pile of material and measure its slope. Knowing what to do with the data is the difficult part. For most materials, the angle of repose varies significantly, depending on how the pile was formed.
Furthermore, the mechanics of pile formation bear little resemblance to the formation of an arch or rat-hole in a bin or hopper, uniformity of die fill, powder homogeneity, or to the other key parameters needed when designing a material handling system. In general, the angle of repose of a material is not an accurate measure of its flowability.
- ISO 3923, Metallic powders – Determination of apparent density or ASTM B212 Test Method for Apparent Density of Free-Flowing Metal Powders Using the Hall Flowmeter Funnel
- ISO 3953, Metallic powders – Determination of tap density or ASTM B527 Test Method for Determination of Tap Density of Metallic Powders and Compounds
- ASTM B213 Test Methods for Flow Rate of Metal Powders Using the Hall Flowmeter Funnel
- ISO 4324, Powders and granules – Measurement of the angle of repose
Other powder characteristics
- Powder storage, handling and aging. For almost all alloys, shielding gas, the control of hygrometry and temperature is important and strongly recommended,
- Powder reusability, i.e. the definition of conditions of re-use of unused powders after additive manufacturing cycles (sieving of agglomerates, control, number of re-use etc),
- Health, safety and environmental issues.