FIRST Newsletter - Bruker

FIRST NewsletterJul 2017, Issue 42Analytical Solutions for PowderMetallurgy and Additive ManufacturingProcesses: Determination of Ar, O, N, Hand C, S by Gas Fusion Analysis andCombustion MethodsBy Dr. Peter Paplewski, Product Manager OES,Combustion/Gas Analysis, Bruker AXS GmbH,Karlsruhe, GermanyIntroductionAccording to ASTM F2792-10, additivemanufacturing (AM) is defined as “the process ofjoining materials to make objects from 3D modeldata, usually layer upon layer, as opposed tosubtractive manufacturing methodologies” [1] suchas machining. In the recent years AM technologiesdeveloped rapidly and techniques like ElectronBeam Melting (EBM), Selective Laser Melting (SLM),Selective Laser Sintering (SLS), Laser MetalDeposition, Powder Bed Fusion, just to mentionsome variants, prepared the ground for AMtechnologies being increasingly established in themetal part production industry, not only forprototyping. Generally AM is complementing otherpowder metallurgy (PM) technologies like HotIsostatic Pressing (HIP) or Metal Injection Molding(MIM). HIP is used to produce massive, near netshape parts of several 100 kilograms (with fine andfull isotropic microstructure) but also as adensification step for parts produced by AMtechnology. In contrast MIM, like other press &sintering technologies, is widely used to producelarge series of small near net shape parts. Althoughthe choice of a suitable PM process depends on thetype and size of part to be produced and on therequirements and possibilities of the user, all1techniques share one common element: the metalpowder.Metal Powders for Additive ManufacturingAmong the metal powders regularly used for AMare different steels, nickel and cobalt basesuperalloys, titanium- and aluminum alloys. Variousother metals like copper alloys, precious metals andrefractory metal alloys are under evaluation. Thepowders are produced by a process called“atomization”.In the gas atomization (GA) process, a stream ofmolten metal, pouring from a tundish through anozzle is hit by a jet of inert gas such as nitrogen orargon. This inert gas jet nebulizes the molten metalinto very small droplets which cool down andsolidify while falling down inside the atomizationtower. The particles are then collected as powder.This process is the most common applied process toproduce spherical metal powders [1]. Since theliquid metal is not protected from ambient air, theGA process is suitable for less reactive metals likesteels, certain aluminum alloys and precious metals.For more reactive elements like titanium, highpurity aluminum or in order to avoid particularlyoxygen build up in superalloys a modification of theprocess is recommended [1], where the meltingtakes place under vacuum. The Vacuum InductionMelting (VIM) gas atomization process [2] is such amodification where the melting is done by inductionheating under vacuum. The droplet formation is stilldriven by an inert gas jet. In case of titanium oraluminum, high purity argon shall be used asnebulizer gas since nitrogen would react with themolten metal and form stable nitrides of theseelements.

Interim ConclusionIt got clear the liquid metal is exposed to processgases like argon or nitrogen during the atomizationprocess. These process gases can be entrapped intothe powder particles in form of micro pores whichare sealed during the solidification process. Actuallythe initial argon content of available Inconel andTi6Al4V powders exanimated varies vastly from 100 ng/g (ppb) to over 1500 ng/g. Despite usingoptimized process parameters for the AM process,defects like pores and shrink holes cannot becompletely avoided (e.g. the SLM process itselftakes place under argon atmosphere). One key aimof the AM process is to build parts without porosityand thus the remaining porosity is one main qualityfeature of the parts. Especially argon filled poreswould affect the quality and mechanical propertiesof the final part [3]. The HIP process is attractingmore and more attention as post-processingtechnique for AM parts, since HIP can stronglyreduce the internal porosity of AM produced parts,thus enhancing product quality [3]. Porosity filledwith a process gas like argon, prevents a perfectpost-densification by means of HIP. Anotherproblem arises when the part is exposed to highertemperatures because entrapped gases wouldcreate internal stresses and may support cracking.Thus, an argon content over 400 ng/g is notpermitted for HIPed parts [4] with safety limitsdown to 50 ng/g [5].The remaining porosity can be measured by microCT, but this technique cannot give informationabout whether or not porosity is filled by a processgas. Thus an argon analysis shall be conducted.Quality ControlThe quality of the initial metal powder used in AMplays an important role on the quality andproperties of the final product but also fordeveloping process parameters to ensure robustbuild-to-build consistency or reproducibilitybetween different AM machines. An establishedparameter is the particle size, its distribution andthe particle morphology (spherical vs. irregularshaped or porous particles). However, as explainedabove in case of entrapped argon, these more orless mechanical parameters are not sufficient for a2full quality control process. Although AM methodsbecome increasingly introduced in productionprocesses, they are still new in terms of requiredprocess and quality control steps. For examplethere are currently no industry-wide standardssettled describing or defining the necessary criteria.The metal powders are usually analyzed for theirinitial elemental composition in regard to all majoralloying elements (e.g. Ni, Cr, Mn, Al, Cu, etc.) bywavelength dispersive X-ray fluorescence likeBruker’s S8 TIGER, which is perfectly equipped forthis task. But while these alloying elements usuallydo not change during processing the powder and aone or two point control step along the productionchain might be sufficient, the content and effect ofvarious “foreign” elements does. These “foreign”elements (that usually do not belong into a metal)are also referred to as “gases in metals”. Thenegative effect, especially that of hydrogen, isknown since a long time and became so prominentthat its effect was entitled “the hydrogen disease ofcertain metals” [6]. Elements like (O, N, H but alsoAr) can easily vary during the production chain ofAM manufactured parts. Likewise the carbon undsulfur concentration can be affected during thevarious melting (heat treatment) steps or due tocontamination of the base powder material. Twoother important points that underline the need fora close monitoring of mentioned elements,especially oxygen and hydrogen are: Storage conditions (like humidity, temperature,atmosphere, etc.) and aging of the metalpowders. Packing shall be in air tight containersor better under argon atmosphere, especiallyfor alloys containing reactive elements liketitanium or aluminum.The reusability of the powder after subsequentadditive manufacturing cycles. This is alsoreferred to as “recycling” powder that was notused to build the part in the powder bed of theAM machine. Since a high purity powder withthe right quality can be quite expensive, it isobvious unused powder is not thrown away butreused for the next production cycle. But sincepowder with a particle size between 5 µm and150 µm has a high specific surface, the surface

oxygen content will increase with subsequentrecycling steps.Effect of Oxygen, Hydrogen and Nitrogen onMaterial PropertiesWhereas the influence of carbon and sulfur on theproperties of metals, especially steel is well knownand summarized in Table 1. The influence ofoxygen, nitrogen and hydrogen is less commonlyknown and worth to be briefly discussed.Table 1. General influencing trends of carbon and sulfuron the physical properties of steels.Physical PropertyCarbon InfluenceSulfur InfluenceTensile StrengthStrongly increasingSlightly reducingHardnessStrongly increasingSlightly increasingStrainSlightly reducingStrongly reducingStretching LimitStrongly increasingNo effectNotch Impact StrengthSlightly reducingStrongly reducingLong-term StrengthStrongly increasingStrongly reducingThermal ConductivitySlightly reducingStrongly reducingElectrical ConductivitySlightly reducingStrongly reducingWear ResistanceStrongly increasingSlightly increasingCold WorkabilityStrongly increasingStrongly reducingHot FormabilitySlightly reducingStrongly reducingCutting QualityStrongly increasingSlightly increasingCorrosion ResistanceSlightly reducingSlightly reducingOxygenOxygen is generally an unwanted, parasite elementin all metals. In steel it causes ageing brittleness. Intitanium even small amounts of oxygen do affectthe mechanical properties like hardnessconsiderably. Even small differences in the oxygencontent may determine the difference between atop-quality (grade 1: 0.18% O) or low-quality(grade 3: 0.35% O) titanium alloy. One differentiatesbetween bulk oxygen which can be present forexample in form of oxidic inclusions and the oxygenlayers on the surface (corrosion). Especially reactiveelements like titanium and aluminum have a highoxygen affinity.3NitrogenIn steel the nitrogen content may be differentiatedinto a desired (e.g. nitriding as surface hardeningtreatment) and undesired fraction. There arespecial steel applications which allow high nitrogencontent but in these cases its chemical formmatters. In its elemental form nitrogen appearsalong the grain boundaries and influences theductility significantly. Nitrogen chemically boundedto other elements is usually not considered to beimportant. This changes when the steel is alloyedwith significant amounts of titanium or otherelements that form stable, sometimes refractorynitrides. Titanium nitride with a melting point of 2930 C is an excellent example for refractoryinclusions that finally represent failures in theunderlying metal structure and thus affectbrittleness.HydrogenHydrogen-induced material damage is a widespreadand dreaded phenomenon. It is manifested in a waythat a component, even without visible indication ofcorrosion, fails or breaks unforeseeable under theinfluence of mechanical stress, which may lead tolife- and environment-threatening damages.Particularly high-tensile and high-strength lowalloyed steels tend to this kind of hydrogen inducedstress corrosion cracking which commonly is calledhydrogen embrittlement. Hydrogen can be suppliedduring various manufacturing and processing steps,e.g. during melting (incl. AM), etching and annealingas well as through corrosion and during weldingprocesses. Apart from the interstitial diffusion ofatomic hydrogen (more precisely a proton) in themetal lattice, hydrogen can accumulate in areas ofthe metal which are exposed to tensile stress. Thus,hydrogen in metals can be present as: Diffusible hydrogen (DH): hydrogen atoms,located on interstitials, high mobility, solubilityincreases with temperature, released from aferritic steel even at room temperature over aperiod of time.Residual hydrogen (RH): weakly to stronglytrapped hydrogen atoms and molecules, with adifferentiation between reversible and