LPW views AM from the perspecve of the metal powder, understanding and preparing for the impact of serial producon on powder characteriscs and the subsequent built parts. Serial producon highlights two key benefits of addive manufacturing, the low material waste and, for powder bed systems specifically, the ability to reuse powder which has not been melted. It also presents challenges for AM machine systems and process chains. Several studies have demonstrated that powder changes, both chemically and physically, with successive build cycles. Renishaw has demonstrated that tanium alloy Ti6Al4V picks up oxygen and nitrogen with successive builds, resulng in increasing UTS (1) . Other invesgaons have demonstrated degradaon in stainless steel 17-4PH (2) and Inconel 625 and stainless steel 316L (3) (4) . However, the relaonships between processing and powder degradaon have not been fully quanfied. If the metal powder is to be reused, parcularly in safety crical industries, the degradaon of a process’s feedstock must be understood and limits determined and implemented if the process and parts are to be qualified for operaon. Part I of the case study describes the mechanisms which drive powder degradaon, and introduces the two most widespread powder reuse regimes. In part II results are presented from an extensive degradaon study which invesgated the impact of serial laser powder bed fusion (LPBF) processing on powder and part properes. In part III this is quanfied by relang degradaon to build parameters, creang a new metric by which to measure powder degradaon. Prototyping vs Serial Producon A key difference between prototyping and serial producon is what happens to the powder aſter a build. In prototyping facilies, a single machine may be required to produce jobs of different size, complexity and material. Powder is typically being used, sieved and stored unl it is required again. In serial producon the metal powder will no longer be stored. Powder which has been through the machine will be sieve screened to remove debris and sintered agglomerates and returned to the AM machine for the next build. The process will repeat unl the powder properes hit a pre- determined limit. ‘Virgin powder only’ serial producon is costly; powder reuse enables economic industrialisaon of the AM process. However, the limits and key performance indicators for metal powder reuse are not yet fully understood. Mechanisms for powder degradaon Degradaon can occur during the melng process itself. The process for LPBF is detailed in Figure 1. Powder is melted by a moving laser which generates a connuous melt track. The melt track has a melt pool at the point of fusion. Once the laser moves on, the liquid rapidly solidifies, remaining at high temperature for only milliseconds. However, this is sll enough me for any available oxygen LPW TECHNOLOGY CASE STUDY: SERIAL AM PRODUCTION: METAL POWDER REUSE REGIMES AND IMPACT ON PART REPRODUCIBILITY – PART I Metal powder reuse regimes and impact on part reproducibility – Part I For worldwide contact details please visit www.lpwtechnology.com Figure 1 - Oxygen and nitrogen pick up during fusion process in LPBF Spaer reacts with O and N, forming O+N rich parcles Laser Spaer O and N in atmosphere react with melt pool surface Ar atmosphere O < 1000 ppm N < 4000 ppm O+N rich / TiO x + TiN x Ti powder O and N rich parcle falls into powder bed Melng Process
Transcript
LPW views AM from the perspecti ve of the metal powder, understanding and preparing for the impact of serial producti on on powder characteristi cs and the subsequent built parts.
Serial producti on highlights two key benefi ts of additi ve manufacturing, the low material waste and, for powder bed systems specifi cally, the ability to reuse powder which has not been melted. It also presents challenges for AM machine systems and process chains.
Several studies have demonstrated that powder changes, both chemically and physically, with successive build cycles. Renishaw has demonstrated that ti tanium alloy Ti6Al4V picks up oxygen and nitrogen with successive builds, resulti ng in increasing UTS (1). Other investi gati ons have demonstrated degradati on in stainless steel 17-4PH (2) and Inconel 625 and stainless steel 316L (3) (4).
However, the relati onships between processing and powder degradati on have not been fully quanti fi ed. If the metal powder is to be reused, parti cularly in safety criti cal industries, the degradati on of a process’s feedstock must be understood and limits determined and implemented if the process and parts are to be qualifi ed for operati on.
Part I of the case study describes the mechanisms which drive powder degradati on, and introduces the two most widespread powder reuse regimes.
In part II results are presented from an extensive degradati on study which investi gated the impact of serial laser powder bed fusion (LPBF) processing on powder and part properti es. In part III this is quanti fi ed by relati ng degradati on to build parameters, creati ng a new metric by which to measure powder degradati on.
Prototyping vs Serial Producti onA key diff erence between prototyping and serial producti on is what happens to the powder aft er a build.
In prototyping faciliti es, a single machine may be required to produce jobs of diff erent size, complexity and material. Powder is typically being used, sieved and stored unti l it is required again.
In serial producti on the metal powder will no longer be stored. Powder which has been through the machine will be sieve screened to remove
debris and sintered agglomerates and returned to the AM machine for the next build. The process will repeat unti l the powder properti es hit a pre-determined limit.
‘Virgin powder only’ serial producti on is costly; powder reuse enables economic industrialisati on of the AM process. However, the limits and key performance indicators for metal powder reuse are not yet fully understood.
Mechanisms for powder degradati onDegradati on can occur during the melti ng process itself. The process for LPBF is detailed in Figure 1. Powder is melted by a moving laser which generates a conti nuous melt track. The melt track has a melt pool at the point of fusion. Once the laser moves on, the liquid rapidly solidifi es, remaining at high temperature for only milliseconds. However, this is sti ll enough ti me for any available oxygen
LPW TECHNOLOGYCASE STUDY:SERIAL AM PRODUCTION: METAL POWDER REUSE REGIMES AND IMPACT ON PART REPRODUCIBILITY – PART I
Metal powder reuse regimes and impact on part reproducibility – Part I
For worldwide contact details please visit www.lpwtechnology.com
Figure 1 - Oxygen and nitrogen pick up during fusion process in LPBF
Spatt er reacts with O and N, forming O+N rich parti cles
Laser
Spatt er
O and N in atmosphere react with melt pool surface
LPW TECHNOLOGYCASE STUDY:SERIAL AM PRODUCTION: METAL POWDER REUSE REGIMES AND IMPACT ON PART REPRODUCIBILITY – PART I
(O) or nitrogen (N) in the immediate atmosphere to react with the melt pool/melt track. In this way O and N can increase in AM parts.
A second mechanism for O and N enrichment is detailed in Figure 1. At the point of fusion a melt pool is being conti nuously generated. On a very small scale this is a violent process and molten material is ejected into the surrounding atmosphere. Just as with the melt pool, the ejected material – or spatt er – is molten or at very high temperatures and reacts with any available O and N in the atmosphere. These enriched parti cles then fall into the powder bed.
While machines may employ gas shielding/gas fl ows to minimise the amount of spatt er falling into the powder bed this is not 100% eff ecti ve. Spatt er sti ll enters the surrounding powder.
Some spatt er is large enough to be removed by sieving aft er the build, but a signifi cant amount is comparable to the alloy powder in size and is incorporated into the batch. This contributes to the increase in O and N content of the bulk powder aft er processing, which intensifi es with increasing powder cycling.
The introducti on of spatt er parti cles will also have an impact on both the powder size and morphological distributi on of the powder batch.Heat Aff ected Zones (HAZ) also contribute to powder degradati on. Figure 2 depicts the top down view of a LPBF build layout. As the part builds, its ambient temperature increases. The heat in the part radiates/conducts into the surrounding powder creati ng a HAZ. Depending on the size of the parts the ambient temperature in the HAZ can reach levels suffi cient for O or N pick-up in the powder.
When removing parts from the build chamber, oxidati on is oft en evidenced by the changed colour of the powder immediately surrounding the part, and observed when the powder is examined under an opti cal microscope. Figure 3 shows heavily used ti tanium powder. The diff erent coloured parti cles represent diff erent thicknesses/compositi ons of the surface oxide layer of the parti cle, as a result of diff erent exposure conditi ons. The colours in Figure 3 are specifi c to ti tanium oxides; however other alloy systems also display this kind of variati on.
Powder degradati on for a given build will vary depending on a range of input parameters. Figure 4 demonstrates the impact of diff erent build layouts and build volumes. In additi on, diff erences in process parameters, build processing atmosphere and material will all infl uence the rate of powder degradati on.
As the powder changes, so the parts being built with it will also display diff erences in properti es. It is criti cal that powder degradati on is understood and managed.
Figure 2 - Oxygen and nitrogen pick up in heat aff ected zones
Figure 3 - Parti cles from screening of exposed powder from LPBF process
For worldwide contact details please visit www.lpwtechnology.com
Powder bordering partlies in ‘Heat Aff ected Zone’ (HAZ)
O+N rich parti cles which have fallen into powder bed
‘Heat aff ected’ powder which has picked up O and N
‘Heat aff ected’ powder can pick up O and N
Low part: powder rati o Higher part: powder rati o = larger area of HAZ
High part surface area= larger area of HAZ
Heat aff ected powder
LPW TECHNOLOGYCASE STUDY:SERIAL AM PRODUCTION: METAL POWDER REUSE REGIMES AND IMPACT ON PART REPRODUCIBILITY – PART I
Powder reuse regimesFigures 5 and 6 illustrate two popular powder reuse regimes, Top Up (someti mes referred to as refresh) and Single Batch. In these examples it is assumed that the volume of powder used for each build is precisely that required to complete the build.
Top Up – Figure 5. Powder is taken from a master batch (Batch A) and placed into the machine. The build process is completed, and the parts and remaining powder (from overfl ow and build volume) are removed. In order to complete the next build, the powder is fi rst sieve screened, and then ‘topped up’ with virgin to make the required quanti ty, and blended – creati ng a new sub-batch. This process is repeated, creati ng increasingly more complex sub-batches, and conti nues unti l the powder hits a user-defi ned property limit.
Single Batch – Figure 6. Powder is taken from the master batch, and the build process is completed. The output (or exposed) powder is retained. The next build is completed with another quanti ty of virgin powder. This is repeated unti l there is no remaining virgin powder. At this point, all of the exposed powder is sieve screened and blended to create a new master batch. This full cycle is repeated unti l there is insuffi cient powder to complete a build or the powder hits a user-defi ned property limit.
Parts II and III of this case study consider the eff ects of a range of input parameters, providing the fi rst pieces of the puzzle to begin quanti fying these complex relati onships. Crucially, the degradati on of both powder and parts is examined, with thermo-mechanical properti es in relati on to powder degradati on being investi gated for the fi rst ti me. The absolute impact of serial producti on on Inconel 718 and Ti6Al4V is revealed.
Bibliography1. Grainger, Lucy. Investi gati ng the eff ects of multi ple powder re-use in AM. [White paper] s.l. : Renishaw plc, 2016.2. Nati onal Insti tute of Standards and Technology. Eff ects of powder recycling on stainless steel powder and build material properti es in metal powder bed fusion processes. NIST Advanced Manufacturing Series. 2017, Vol. 100, 6.3. Sarti n, B. 2017. Solid Freeform Fabricati on 2017: 316L Powder Reuse for Metal Additi ve Manufacturing. Proceedings of the 28th Annual Internati onal Solid Freeform Fabricati on Symposium - An Additi ve Manufacturing Conference.4. L.C. Ardila, F. Garciandia, J.B. Gonzalez-Diaz, P. Alvarez, A. Echeverria, M.M. Peti te, R. Deffl ey, and J. Ochoa. Eff ect of IN718 Recycled Powder Reuse on Properti es of Parts Manufactured by Means of Selecti ve Laser Melti ng. (2014) Physics Procedia. 56. 99–107.
Figure 4 - Matrix demonstrati ng impact levels of diff erent build layouts
