The main goal of the present thesis was to explore the suitability of new Al-alloy options for the Selective Laser Melting (SLM) process and to investigate the interaction between the three process steps; powder, build process and heat treatment.
It has been shown that both material options, Scalmalloy and SilmagAl, benefit from the SLM process opportunities such as high cooling rates. Material properties are gained which are comparable or even slightly better than conventionally used Al-alloys in aerospace.
However, all new findings reported in this thesis, shed new light on the importance of a tight power specification for SLM with regard to obtain a stable process, which consistently gen-erates the same material quality. New characterisation methods which offer, for example, the analysis of the powder’s surface roughness after recoating have to be taken into account and described precisely. For Scalmalloy, it was demonstrated, that a surface fractal value ψPwhich is dependent on cohesiveness, particle shape and distribution, indicates favourable processability. A constant smooth weld seam is aimed for, which a jagged powders surface roughness cannot realise, as too many occasions for beam traps are generated. Hence, a pre-dictability of how the weld seam will form is not given. If during SLM the welding mode changes constantly between conduction, transition and deep penetration mode, a homoge-neous and predictable weld seam, and hence uniform microstructure, cannot be realised. This means, that a powder specification for Scalmalloy has to be quite tight, as already small de-viations from a perfectly suited powder can result in an unstable process and hence turbulent microstructure, decreasing the material performance. However, SilmagAl is more tolerant and shows a very high compensation of non-ideal powder characteristics and a non-ideal heat transfer. A limitation of the powder’s characteristics in a powder specification is nevertheless proposed for SilmagAl to ensure a stable process, even though minor deviations from perfect powder constitution can likely be compensated.
Although the discussion in Section 4.4.7 highlights that powder batches of different supplier in different quality levels are indeed processable in a research laboratory environment, is the transformation into an industrial environment for aerospace applications most likely not suc-cessful without a well-defined powder material specification. A stable process over multiple builds by utilising the full build height of a machine can not be guaranteed. If the powder quality is not appropriate, a continuous adaption of the process parameters during the SLM manufacturing process itself would be required. But no qualification for aerospace parts can be launched if the processing route is not fixed. Hence, for both material options, the defini-tion of particle size (PS), particle size distribudefini-tion (PSD), bulk surface roughness (e.g. ψP), impurities, phases and inner particle porosity is as mandatory as the chemical composition.
The process analysis demonstrated that significant differences in both material options regard-ing process stability exist. SilmagAl is stable processable over broad powder configurations and process parameters which might offer industrial interesting high build rates; however,
Scalmalloy is very sensitive to already small deviations in the powder specification, and suit-able parameter combinations do not necessarily offer sufficient build rates. An operating win-dow for Scalmalloy can be described neither by a specific volume energy density (see Equation 2.2), nor by the top view optical analysis of the morphology of single weld paths. The most effective way is to determine, by help of microsections, the welding mode by measuring the weld seam depth and width and to limit the parameters in a way that an aspect ratio ofA<1 is achieved. Heat conductivity welding is for both material options, Scalmalloy and SilmagAl, the most stable welding mode which creates a microstucture that exploits its full potential.
That does not mean necessarily that transition and deep penetration welding do not create a dense microstructure; rather, that the excessive remelting and large heat input associated with both welding modes will lead to a less perfect microstructure as obtained by heat conductivity welding. And a non-uniform microstructure necessarily will lead to a significant drop in ma-terial performance.
It can thus be suggested that different material specifications for each alloy shall be defined which also considers the safety class and criticality of a part. This approach also satisfies sup-pliers need to open SLM for a broader range of applications. Such material specifications or design allowable values for both material qualities are partially proposed and given in Table 4.15 for Scalmalloy and in Table 4.16 for SilmagAl. All values are deduced from the compre-hensive material characterisation (overview given in Table 3.2) for both materials in Section 4.4. The mechanical tests that were performed cover the static tensile strength, high cycle fatigue, fracture toughness, crack growth and crack growth threshold analysis.
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Figure 5.1.: Cause-effect diagram of powder-process-heat treatment relation for different qual-ification routes for Scalmalloy and SilmagAl
The second aim of this study was to extrapolate qualification possibilities for Al-alloys, ex-emplary for Scalmalloy and SilmagAl. These possibilities are explained by help of Figure 5.1.
The process chain, as shown in Figure 1.2, is extended by a cause-effect-diagram to illustrate possible paths of qualifying Scalmalloy and SilmagAl manufactured by SLM for aerospace applications. Each investigated process step is numbered (powder = 3, build process = 4 , heat treatment = 5 and material specification = 8) and the final parts are distinguished between principle structural element (PSE) and non-principle structural element (non-PSE).
The only possible path for manufacturing Scalmalloy PSE parts requires a well-defined pow-der (3.1), SLM build process parameters which ensure heat conductivity welding mode (4.1) and a heat treatment with an additional hot isostatic pressing (HIP) step. Although HIP of Scalmalloy at 325◦C clearly does not eliminate every gas or process porosity as no diffusion processes are activated, yet does improve mechanical properties as shown in Section 4.4. A uniform microstructure (8.1) consisting of two bands of alternating ultra-fine equiaxed (UFG) and fine columnar (FG) grains is targeted with a homogeneously distribution of full coherent Al3(Sc(1−x),Zrx)precipitations. It is assumed that such a microstructure will succeed the high material specifications of 8.3 and Table 4.15.
More options are possible for the manufacturing of non-PSE parts. The second material speci-fication 8.4 (Table 4.15) mitigates the requirements on microstructure (8.2) to mainly uniform through thickness. Both, UFG and FG, bands shall still exist but it is not mandatory to keep a certain volume fraction of these bands. This requirement can be achieved by manufacturing in all three welding modes (4.1, 4.2 and 4.3) and it can be chosen between heat treatment 5.1 or 5.2.
To obtain SilmagAl PSE parts, two different paths are possible and suggested, whereas various paths can be followed for non-PSE. PSE parts of SilmagAl shall be manufactured in either heat conductivity or transition mode 4.1 and 4.2. The T6 heat treatment also includes an additional HIP step in 5.3. A uniform fine eutectic microstructure which consists of fine columnar grains without any large Mg or Si segregation or particles is aimed to achieve the material specifica-tion 8.5 or Table 4.16. However, various welding modes and heat treatment combinaspecifica-tions are possible to achieve the requirements for non-PSE part material specification 8.4.
Nevertheless, further research should be carried out to expand the above proposed qualifi-cation routes to more heat treatment options, e.g. by solely precipitation hardening at 165◦C for SilmagAl. Also the influence of a heated platform on the evolution of the microstructure of Scalmalloy and SilmagAl was not investigated so far and should be considered.
Special requirements on the machine configuration exist additionally for processing Scalmal-loy regarding gas flow. A non-negligible portion of welding smoke is created, which counter-acts uniform welding. Machine configurations presently continue to face trouble in realising a constant laminar gas flow over the complete build plate which would also ensure that only a very low oxygen content remains in the process chamber. Hence, it is necessary to put an additional focus on the gas flow and oxygen level while processing these two aluminium alloys. The entire topic of oxygen contamination of any aluminum alloy during powder atom-isation, handling and final processing was not investigated in this thesis, but this source of contamination must also be covered for the aerospace qualification of SilmagAl and Scal-malloy. All material characterising investigations within this thesis are only based on milled coupons. On part basis, the additive manufactured surface will lead to a drop in almost all
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material properties, however, that must be examined in more detail. It is also necessary to follow a conservative approach by adding surface protection to both alloys, as indicated after analysing the corrosion test results. Scalmalloy seems to be vulnerable to corrosion attack if a, so far undefined, defect or porosity level remains in the microstructure and if it is exposed to elevated temperatures. Especially the fatigue crack propagation behaviour of Scalmalloy shows that small sharp cracks, induced by small corrosion pits or any other flaws caused by in-service usage, may lead to early structural failure. The entire surface post-build treatment of both Al-alloy options is a wide-ranging topic that requires further investigations.
A. Appendix
A.1. Powder analysis
A.1.1. Particle morphology
Microsectional and SEM images for particle morphology analysis
Figure A.1.: Exemplary microsections of Scalmalloy powder Batches with highlighted exam-ples for spheroidal and/or nodular particles
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Mechanical influences / dented spots at different locations in Scalmalloy powder Batches II, IV and V.
Figure A.2.: Dented spots on the particles surface in Scalmalloy powder Batches II, IV and V
A.1.2. Particle size distribution and particle size
Figure A.3.: Scalmalloy Batch I - Correlation of PS measurements with SEM picture analysis
Figure A.4.: Scalmalloy Batch II - Correlation of PS measurements with SEM picture analysis
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Figure A.5.: Scalmalloy Batch IIIa - Correlation of PS measurements with SEM picture anal-ysis
Figure A.6.: Scalmalloy Batch IIIb - Correlation of PS measurements with SEM picture anal-ysis
Figure A.7.: Scalmalloy Batch V - Correlation of PS measurements with SEM picture analysis
Figure A.8.: SilmagAl powder Batch I - Correlation of PS measurements with SEM picture analysis
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Figure A.9.: SilmagAl powder Batch II - Correlation of PS measurements with SEM picture analysis
Table A.1.: Dendritical surface structure of Scalmalloy and SilmagAl powders
Batch I Batch II Batch III Batch IV Batch V
Scalmalloy
SilmagAl