Technological investigation

Pag. 87

Pag. 88 Figure 56: Fracture surfaces of the PH1 tensile samples produced with virgin and reused powder. a) Virgin. b) Reused 5 times. c) Reused 9 times

Figure 57: OM images GP1 samples microstructure. a) Virgin. b) Reused 7 times.

c) Reused 8 times. d) Reused 9 times

Pag. 89 Figure 58: SEM images GP1 samples microstructure. a) Virgin. b) Reused 7 times.

c) Reused 8 times. d) Reused 9 times

In order to explain the observed structure modification of GP1, the attention was focused on powder chemical composition that had exhibited, as aforementioned, some questionable anomalies. For both PH1 and GP1, further chemical analyses were performed on all the collected powder samples (Table 18 and 22), as well as on three additional ones of virgin powder, coming from other batches, to be considered a reference. A common practice in welding applications is to use the Schaeffler diagram to predict microstructure and δ- ferrite content of stainless steel weld metals [45]. Due to the similarity of welding and L-PBF techniques, which are both characterised by rapid cooling during the process, this diagram has proved very effective also in the latter [46]

and was therefore used to understand the causes of the anomalous behaviour of GP1.

Figure 59 shows the final version of the graph proposed by Schaeffler himself in 1949 [47], while in Fig. 60, the diagram obtained for PH1 and GP1 powders is reported. The position of GP1 samples corresponding to the last two runs, highlighted by the red circle,

Pag. 90 shows how the loss of retained austenite within GP1 is caused by phase transition of metal powder, due to the variation of Cr- and Ni-equivalents.

Figure 59: Final Schaeffler diagram for welding applications on stainless steels

Figure 60: Schaeffler diagram of PH1 and GP1 for virgin (0) and reused (1 to 9) powder

Pag. 91 CONCLUSIONS AND FUTURE WORK

The ANOVA results allowed to assess the statistical significance of powder reuse on the achievable mechanical properties of:

 EOS Aluminium AlSi10Mg parts

 EOS Titanium Ti64 parts

 EOS Stainless Steel GP1 parts

 EOS Stainless Steel PH1 parts produced by DMLS technology.

AlSi10Mg alloy processed according to the experimental conditions shows statistical significance of powder reuse on yield strength and ultimate strength.

Despite the slight observed decrease of the final part features, it is worth noting that all the measured characteristics, included the fatigue resistance, resulted compliant with the company requirements.

It is necessary to underline that the recycling strategy 1 was used for this alloy.

This recycling strategy was not considered industrially convenient from the company because it implied to mix used powder, from the powder overflow container and the building platform, with the virgin one, stored in the dispenser.

A more industrial recycling strategy (recycling strategy 2) was used to assess the effects of recycling process on mechanical properties of the last three alloys, that involve only the powders on the powder overflow container and building platform.

Further studies should be done to assess the effect of using recycling strategy 2 on AlSi10Mg parts.

Ti6Al4V alloy processed according to the experimental conditions shows statistical significance of powder reuse on yield strength and ultimate strength.

These results are affected by the increase of iron content in Ti6Al4V powders after the seventh reuse. In particular, the increase of iron content raises yield and ultimate strength of Ti6Al4V specimens. It is worth noting that the percentage of iron is still acceptable for all the specimens because its values fall into the standards of ASTM B265-15. Other investigations have been carried out in order to explain the reason of this iron increase.

Pag. 92 The results suggest that a contamination occurred during the printing process. In particular, the chemical composition analysis of Job 7 shows also a small increase of Chromium and Nickel percentages, proving that the contaminating material is a stainless steel.

Considering contaminated jobs as outliners, there is no statistical significance of powder reuse on Ti6Al4V mechanical properties.

All external procedures were correctly performed in accordance with protocols previously checked by MBDA during their research activities concerning the study of the properties of parts printed with a large variety of materials. The occurrence of an external contamination can be excluded. On the other hand, any internal component of the printing process can cause contaminations. In particular, the container and the sieve are made of stainless steel. Moreover, it is important to consider two features of the Ti6AL4V printing process: the first is that the hardness of Ti6Al4V is 50% higher than of stainless steel one and the second is that sieve and container, both made of stainless steel, are involved in collisions with powder. Thus, the occurrence of a stainless steel contamination could be explained taking account of the Ti6Al4V powders abrasivity. In particular, this explanation is in accordance with the causality of the increase of iron percentage in the chemical composition of Ti6Al4V powders starting from Job 7.

A possible solution can be the use of internal components, as sieve and container, with a Vickers hardness at least equal to that of Ti6Al4V alloys.

Further studies will be conducted to assess this hypothesis.

GP1 stainless steel processed according to the experimental conditions shows statistical significance of powder reuse on all mechanical properties analyzed.

The abnormal mechanical properties of GP1 samples produced with powder reused eight and nine times are ascribable to the presence of a different microstructure. This behaviour happened although experimental chemical compositional range of these two jobs are compliant to EOS datasheet for this alloy.

These results lead the company to discard the implementation of this alloy for its applications/products.

Pag. 93 PH1 stainless steel processed according to the experimental conditions shows statistical significance of powder reuse only on elongation at break. This stainless steel showed great stability in terms of powder and mechanical properties.

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