Configuration of specific component properties by processing AM powder mixtures

 

Subject

In the project carried out, a methodology is presented for the realization of LPBF components with properties individually tailored to the specific requirement profile by processing powder mixtures and in-situ alloy formation in the LPBF melt bath. This approach combines tool-free, geometrically flexible master molding with alloying of the material in a single process step. With a limited selection of starting powders, which are commercially available in large quantities, numerous new alloys with a tailored property profile can be produced very flexibly using the powder block principle.

Problem

Traditionally manufactured metal components have become highly specialized over the years in terms of the materials used. Alloys with special properties have been developed for numerous applications. As a result, there is a wide range of materials on the market that are suitable for different production chains. In additive manufacturing, especially laser-based powder bed fusion (LPBF), which is already widely used today, many equipment manufacturers offer only a limited selection of metal powders. For corrosion-resistant high-alloy steels in particular, there are many alloy choices in conventional production, including austenitic stainless and acid-resistant steels, low-cost ferritic heat-resistant alloys, duplex and superduplex steels, and corrosion-resistant martensitic steels.

Objectives

The research project aims to demonstrate the feasibility of the proposed process using the range of materials of corrosion resistant steels as an example. The sub-objectives of the project are as follows: Selection of the alloy composition on the basis of the material-specific requirement profile, determination of the powder composition by thermodynamic calculation methods, powder preparation by appropriate mixing and homogenization processes, determination of the optimum laser process parameters, characterization of the microstructure and qualification of the materials produced by this method by testing their mechanical and corrosive properties.

Approach

The project is divided into seven work packages (see Figure ABC). Work package (WP) 2 occupies a special position, since this work must always run alongside the other APs. It is used for continuous adaptation and estimation of the alloy design. The results of the metallurgical analysis in WP6 are used to evaluate WP3-5 and determine the further steps of all other work packages.

  Copyright: © IWM

Figure 1: Work packages

 

Results

Determination of target material properties/material procurement (WP1)

In consultation with the project committee, the target material properties were already defined during the application phase. However, this material selection had to be confirmed by a comprehensive vote of all members. In the process, the choice fell on two material groups: Corrosion-resistant tool steels and super duplex stainless steels. The following reference materials were used to compare the physical properties.

- Corrosion-resistant tool steel: 1.4125, X105CrMo17

- Superduplex stainless steel: 1.4501, X2CrNiMoCuWN25-7-4

To produce the powder mixture, the following powders were supplied by the project partners:

Table 1: Powders supplied by PA

Powder	Quantity	D10 [µm]	D50 [µm]	D90 [µm]
1.2344, X40CrMoV5-1, AISI H13 Supplier 1	22 kg	8,56	17,22	28,53
1.2344, X40CrMoV5-1, AISI H13 Supplier 2	50 kg	27,39	41,06	58,78
1.4404, X2CrNiMo17-12-2, AISI 316L	60 kg	20,71	35,84	54,32
2205, X2CrNiMoN22-5-3, 1.4462	75 kg	28,32	43,45	62,08
Cr	30 kg	19,98	37,20	54,00
Mo	3 kg	5,62	15,51	38,34
CrN	0,8 kg	21,88	59,66	99,00
FeCrC	5 kg	10,35	23,52	40,85
Cr3C2	5 kg	1,24	6,63	11,52
TiC	3 kg	10,51	28,20	52,15

Thermodynamic design of material concepts (WP2)

Super duplex steels

The 60 alloy is composed of 1.4462, 1.4404, CrN, Mo and Cr. The overall theoretical composition of the material is similar to the composition of the reference material. Only the nitrogen alloy content of >0.5% is greater than in the reference material. The 75 alloy consists of 1.4462, 1.4404, CrN, and Cr. In contrast to alloy 60, the addition of elemental Mo is omitted in this case. The reason for this is that the melting point of molybdenum is 2623 °C, much higher than the liquidus temperature of alloyed steels (about 1500 °C).

  Copyright: © IWM

Figure 2: Equilibrium calculation of the developed 60 (left) and 75 (right) super duplex steel (dashed) compared to the 1.4501 reference material (solid).

 

Tool steels

During the course of the project, there were a number of iterations in the development of the optimum alloy for the tool steel. Therefore, a table of compositions is presented below:

Table 2: Compositions developed for the tool steel.

Component	C	Si	Mn	Cr	Mo	V	Ni	Ti	Fe
88er	1,23	0,8	0,4	15,4	1,1	0,8	0,0	0,0	80,2
86er	1,28	0,9	0,4	17,6	1,1	0,9	0,0	0,0	77,8
85er	1,27	0,9	0,4	18,6	1,1	0,9	0,0	0,0	76,9
80er	0,93	0,9	0,5	13,0	1,3	1,2	0,8	2,4	79,1
Reference	1,05	<1	<1	17	0,6	0,0	0,0	0,0	81,3

 

Mixing tests in the doctor blade test rig (AP3)

From the AP, it can be deduced that the optimum mixing time for the powder quantity and mixing parameters used in the mixing tests is 15 minutes. The particle size has the greatest influence on the mixing results compared to the particle shape and density. No strong influence was found for the density and particle shape of the mixed components. The mixing method (three-dimensional shaker mixer) counteracts segregation due to density by the three-dimensional movement. Smaller deviations from the mean value were found at lower proportions of the mixed components.

The squeegee tests showed that particle size has the greatest influence on element distribution. The influence of density and particle shape is estimated to be small. For very fine mixtures, segregation occurred in the direction of the coater. There, the finer particles settled further forward on the platform due to the higher adhesion force. Squeegee tests showed that the coating thickness and the speed of the coater influenced the squeegee behavior.

Production of test specimens (AP4)

Different parameter variations were investigated for all powder mixtures. The parameter combination in Table 3 was used for the duplex demonstrators and the specimens for the mechanical as well as application-oriented corrosion tests. It reliably delivered high component densities.

 

Table 3: Parameter set for the powder mixtures

Nr.	Leistung [W]	Power [mm/s]	Hatch [mm]	Density [g/cm³]	Density [%]	VED [J/mm³]
2	195	800	0,05	7,69	98,59%	121,88

 

Optical inhomogeneities occurred in the powder layer during processing of alloy 88. Alloys 86 and 85 were first processed on cubic specimens to determine the optimum parameters. Both alloys were then processed into corrosion and wear specimens; alloy 86 was able to build up without cracking on large-area wear specimens, while alloy 85 developed cold cracks at the edges of these specimens.

No cracks were found when alloy 80 was machined into cubic specimens; the PA decision meant that the project would focus on alloys 86 and 85. For this reason, corrosion and wear specimens were omitted.

Application-oriented component testing (AP5)

Super Duplex Steels

Figure 3 compares 60s and 1.4462 (very similar to 1.4501). The results show that diffusion annealing improves corrosion resistance compared to fabricated specimens. Corrosion resistance is also improved compared to alloy 1.4462. This indicates a more homogeneous distribution of corrosion resistant elements in the matrix of the heat treated specimens. Figure 4 shows the corrosion behavior of 75s compared to PBF-LB 1.4462. Here, the corrosion behavior is very similar and the added elements have only a minor positive effect on the corrosion resistance.

  Copyright: © IWM

Figure 3 (left): Corrosion behavior of pre-alloyed powder vs. 60 mix.

Figure 4 (right): Corrosion behavior of master alloyed powder vs. 75 mix.

 

Tool steels

The corrosion resistance of the developed tool steels was characterized in terms of the tendency for intergranular corrosion in 0.1M H2SO4 and in terms of pitting corrosion tendency in 3.5% NaCl. The values can be seen in Figure 5.

  Copyright: © IWM

Figure 5: Current density-potential curves of corrosion measurements in 3.5% NaCl of the 86, 85 and 1.4125 reference alloy in the hardened as well as hardened and 300°C and 500°C tempered condition.

 

Metallurgical analysis (AP6)

Tool steels

Metallographic analysis of the 80 alloys confirms that the TiC is uniformly distributed in the structure. As the energy input increases, the TiC increasingly dissolves during the process and precipitates back out as very fine TiC. An example of a light micrograph of alloy 80 is shown in Figure 6. The gray carbides become smaller and fewer as the energy input increases. Evaluation of the image analysis did not reveal a linear correlation between the changes in energy input and the TiC residues. A systematic analysis is needed to better understand the dependencies.

  Copyright: © IWM

Figure 6: Microstructure of the 80 alloy with different energy input during the PBF-LB process and resulting different amounts of undissolved TiC.

 

Super Duplex Steels

The results of EDX measurement of blend 60s are shown in Table 7. The measured and calculated powder compositions are given for reference in each case.

Table 4: Chemical compositions measured with EDX

Element	a) %	b) %	c) %	powder % (analysed)
Si	0.64	0.62	0.63	0.59
Cr	27.67	27.74	26.80	27.02
Fe	Bal	Bal	Bal	Bal
Ni	6.55	6.62	6.83	6.23
Mo	4.44	4.73	4.68	4.82

  Copyright: © IWM

Figure 7: EDX recording of the 60s mixture

 

Development of demonstrators for validation of the process (AP7)

In order to make the demonstrators as close to the application as possible, various geometries were presented to the PA in the project meetings and evaluated. Finally, a rotor structure for the duplex steel and a blade structure for the tool steel were agreed upon. The demonstrators were then finalized at IFAM. The blade structure was sent to IWM as an STL file. During the construction of the demonstrator at IWM, there were problems with distortion in the construction process at the overhanging structures. For successful manufacturing of components, the support structures and the scanning strategy must be optimized for the tool steel with regard to low-stress process control.

  Copyright: © IWM

Figure 8: Illustration of the printed duplex demonstrator (Large: 75 mixture; Small: 60 mixture).

  Copyright: © BMWi

Funding Announcement

The IGF project 20933 N of the Forschungsvereinigung Forschungsgesellschaft Stahlverformung e.V. was funded by the AiF within the framework of the program for the promotion of industrial collective research (IGF) by the Federal Ministry for Economic Affairs and Climate Protection on the basis of a resolution of the German Bundestag. The long version of the final report can be requested from the FSV, Goldene Pforte 1, 58093 Hagen.

 

Documentation

In the course of the project, the current research results were published by the research institutions in the following scientific papers:

• Norda, M., Köhler, M..L. et al.: „Influence of Powder Properties on the Mixing Behavior of Metal Powders in LPBF”, Conference Paper, EuroPM 2021
• Norda, M., Köhler, M..L. et al.: „ Processing and Corrosion Behaviour Of Metal Powder Blends In LPBF”, Conference Paper, AWT Fachkonferenz 2022
• Köhler, M..L et al.: „Vom Pulver zum additiv hergestellten Bauteil – Teil 2: Potenziale durch Pulvermischungen“ in der Zeitschrift „Werkstoffe“
• Köhler, M. L. et. al.: “Influence of Cr3C2 Additions o AISI H13 Tool Steel in the LPBF Process”, Steel Research International
• Köhler, M. L. et. al.: „Influence Of The Powder Particle Size Distribution On The Microstructure Of Laser Powder Bed Alloyed Cold Work Tool Steel”, EuroPM2021
• Köhler, M. L. et. al.: „Resistance Against Abrasive Wear and Corrosion of a Laser Powder Bed Alloyed High Chromium Tool Steel”, Tooling2022
• Köhler, M. L. et. Al: “Resistance Against Abrasive Wear and Corrosion of Laser Powder Bed Alloyed High Chromium Tool Steels”, Steel Research International