Using Automated Image Analysis to Compare Metal Powders from Different Atomization Processes

Metal Injection Molding (MIM) is a growing industry. The size and shape of metal powders play a vital role in determining the process efficiency and properties of the final MIM components. Here, we look at image analysis as a tool for characterizing the size and shape distributions of atomized metal powders.

Introduction

The manufacture of complex shaped metal parts by Metal Injection Molding (MIM) is a growing industry. The shape and size of metal powders play an important role in determining process efficiency and properties of the final MIM components. Spherical powder particles are typically favored by MIM manufacturers looking to achieve best tolerances and properties in final components. Therefore, it is important to monitor atomized powder to ensure that particles of the desired shape and size are produced.

The metal injection molding process (MIM) is used in the manufacture of complexshaped, high volume, low weight parts where intricate detail may be required along with accurate tolerance control. It involves 4 crucial steps which are outlined below:

1. Atomization of molten metal to form metal powders which are further processed by sieving and/or gas classification to obtain the appropriate particle size distribution. The powder is then mixed with thermoplastic binders to form pellets of feedstock ready for the next step. 
2. Feedstock is injected into a mold or die to form ‘green’ metal injection molded parts. 
3. The binder is removed from the ‘green’ part by solvent and/or thermal processes to leave a ‘brown’ metal part. 
4. The ‘brown’ part undergoes a sintering process in a high temperature furnace where the metal particles fuse together. Particle size is important during this stage, but so is particle shape since spherical powders will have a higher packing density. This means more touching surfaces, faster sintering times and reduced shrinkage resulting in better dimensional control. Therefore, the size and the shape of the original metal particles produced in step 1 will affect the final product and must be carefully controlled.

This application note describes how atomized powders with similar particle size distributions produced by two different atomizing processes, can have very different shape properties and how such parameters can be assessed using automated image analysis.

Materials and Methods

Two powder batches of stainless steel alloy 17-4PH (Sandvik Osprey Ltd, -32mm grade) produced by different atomization techniques were analysed using a Morphologi G3 (Malvern Instruments) to determine both size and shape. The Morphologi G3 is an automated particle characterization instrument which disperses the powder particles on a glass substrate using an integrated dry powder disperser. The instrument then captures images of individual particles by scanning the sample underneath the microscope optics and then performs image analysis to determine size and shape parameters. Sample A was prepared by a process of gas atomization whereas sample B was prepared by water atomization. An aliquot of each sample was automatically dispersed and analysed according to a standard operating procedure (SOP) which contained all the hardware and software variables for the measurement. Dispersion using the Morphologi G3’s integrated sample dispersion unit (SDU) involved the application of a pulse of positive pressure on the sample at 4 bar for 8 ms then allowing the sample to settle for 240 s. A 20x objective was used for analysis combined with a 3 plane zstack. Images of touching particles were excluded from the results. More than 55000 particles were characterized for each sample. The samples dispersed well as can be seen from the field of view images shown in Figure 1.

 >> Download the full Application Note as PDF

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