Fracture mechanical investigations on selective laser melted Ti-6Al-4V

Available online at www.sciencedirect.com Available online at www.sciencedirect.com

ScienceDirect ScienceDirect

Structural Integrity Procedia 00 (2018) 000–000 Available online www.sciencedirect.com Available online at at www.sciencedirect.com Structural Integrity Procedia 00 (2018) 000–000

ScienceDirect ScienceDirect

Procedia Structural Structural IntegrityIntegrity Procedia1300(2018) (2016)317–321 000–000

www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia

www.elsevier.com/locate/procedia

ECF22 - Loading and Environmental effects on Structural Integrity ECF22 - Loading and Environmental effects on Structural Integrity

Fracture mechanical investigations on Fracture mechanical investigations on selective laser Ti-6Al-4V XV Portuguese Conference on Fracture, PCFmelted 2016, 10-12 February 2016, Paço de Arcos, Portugal selective laser melted Ti-6Al-4V J.-P. Brüggemann*,modeling L. Risse, G. Kullmer, B. pressure Schramm, H. A. Richard Thermo-mechanical a highB. turbine blade of an J.-P. Brüggemann*, L. Risse, G. of Kullmer, Schramm, H. A. Richard Paderborn University, Direct Manufacturing Research Center (DMRC), Mersinweg 3, 33098 Paderborn, Germany airplane gasCenter turbine engine Paderborn University, Direct Manufacturing Research(Applied (DMRC), Mersinweg 3309833098 Paderborn, Germany Paderborn University, Fachgruppe Angewandte Mechanik Mechanics), Pohlweg3,47-49, Paderborn, Germany Paderborn University, Fachgruppe Angewandte Mechanik (Applied Mechanics), Pohlweg 47-49, 33098 Paderborn, Germany

P. Brandãoa, V. Infanteb, A.M. Deusc*

Abstract AbstractaDepartment of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal(SLM) enable material efficient production of individual and Additive Manufacturing (AM) techniques such as selective laser melting b IDMEC, Department (AM) of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1,of1049-001 Lisboa, Additive Manufacturing techniques such as selective laser melting (SLM) enable material production individual and complex components in a short period of time. One typical material that is processable by SLM efficient is the titanium alloy Ti-6Al-4V. This Portugal complex components in a short period of time. One typical material that is processable by SLM is the titanium alloy Ti-6Al-4V. This c is frequently alloy used in medicine technology because of low density, very high strength and biocompatibility. The AM process leads CeFEMA, Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, alloy is frequently used medicine technology because of lowparts density, strength and biocompatibility. The AM process leads to many advantages likeinthe opportunity to produce complex withvery for high instance undercuts or lattice structures. As AM parts are Portugal to many advantages like the opportunity produce complex are parts instance undercuts or lattice structures. Asimpact AM parts are used in various high-quality sectors the to material properties ofwith greatfor interest. Many influencing factors have an on the used in various high-quality sectors the material properties are of great interest. Many influencing factors have an impact on the resulting material properties of additively manufactured Ti-6Al-4V products. For a reliable application and a fracture-safe construction resulting material properties of additively manufactured Ti-6Al-4V products. Forproperties a reliablehave application and a fracture-safe construction theAbstract influence of different changes in the production parameters on the material to be known. As Ti-6Al-4V is already the influenceand of different changesand in the production parameters on thefor material properties have to be size known. As Ti-6Al-4V is already processable the mechanical fracture mechanical properties a defined powder particle distribution are known, the During oftheir operation, engine are to increasingly demanding operating conditions, processable and the mechanical and aircraft fracture mechanical forsubjected a definedaverage powder particle size are known, influence a varied powdermodern particle size, in this casecomponents of aproperties significantly smaller, particle size is distribution investigated in the scopethe of especially highthe pressure turbine (HPT) blades. conditions cause these parts undergo types influence varied powder particle size, in this caseSuch of a significantly smaller, average sizedifferent investigated intime-dependent the scope of this paper.ofInathe detail, mechanical and fracture mechanical behavior under different heattoparticle treatments isiscompared to of existing data for one particle of is creep. model the finite element method (FEM) was developed, ordertreatment toexisting be able to predict this paper. In detail, thewhich mechanical andAfracture mechanical behavior under different heat treatments is compared to for thedegradation, higher average size. Because of theusing resulting residual stresses during the building process ainheat is data always creepaverage behaviour of HPT blades. Flight data records (FDR) for aduring specific aircraft, by commercial aviation thethe higher particle size. Because of thethe resulting stresses buildingtoprovided process heata treatment is always necessary for a reliable structure. To determine materialresidual properties, tensile teststhe according DIN ENa 10002-1 were conducted. company, were used structure. to obtainTo thermal and the mechanical data for three different flight cycles. In EN order to create the 3D model necessary for a reliable determine material properties, tensile tests according to DIN 10002-1 were conducted. For the fracture mechanical examinations compact tension specimens, according to ASTM 647-08 standard, were used. Fatigue crack needed for the FEM analysis, a HPTcompact blade scrap scanned,according and its chemical andwere material were For the curves fracture mechanical examinations tensionwas specimens, to ASTM composition 647-08 standard, used.properties Fatigue crack growth with an that R-ratio 0.1 werewas investigated. obtained. The data wasofgathered fed into the FEM model and different simulations were run, first with a simplified 3D growth curves with an R-ratio of 0.1 were investigated. rectangular block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The © overall 2018 The Authors. Published by Elsevier B.V. in Elsevier terms ofB.V. displacement was observed, in particular at the trailing edge of the blade. Therefore such a © 2018 Theexpected Authors. behaviour Published by © model 2018 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the ECF22 organizers. canunder be useful in the goal predicting turbine blade life, given a set of FDR data. Peer-review responsibility of theofECF22 organizers. Peer-review under responsibility of the ECF22 organizers. Keywords: manufacturing; selective laser melting; © 2016 Additive The Authors. Published by Elsevier B.V. Ti-6Al-4V; particle size distribution; fatigue crack growth behaviour Keywords: Additive manufacturing; selective laser melting; Ti-6Al-4V;of particle distribution; fatigue crack growth behaviour Peer-review under responsibility of the Scientific Committee PCF size 2016. Keywords: High Pressure Turbine Blade; Creep; Finite Element Method; 3D Model; Simulation.

* Corresponding author. Tel.: +49-5251-60-4388; fax: +49-5251-60-5322. * E-mail Corresponding Tel.: +49-5251-60-4388; fax: +49-5251-60-5322. address:author. [email protected] E-mail address: [email protected] 2452-3216 © 2018 The Authors. Published by Elsevier B.V. 2452-3216 © 2018 Authors. Published Elsevier B.V. Peer-review underThe responsibility of theby ECF22 organizers. * Corresponding Tel.: +351of218419991. Peer-review underauthor. responsibility the ECF22 organizers. E-mail address: [email protected] 2452-3216 © 2016 The Authors. Published by Elsevier B.V.

Peer-review under responsibility of the Scientific Committee of PCF 2016. 2452-3216  2018 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the ECF22 organizers. 10.1016/j.prostr.2018.12.053

318 2

J.-P. Brüggemann et al. / Procedia Structural Integrity 13 (2018) 317–321 J.-P. Brüggemann et al. / Structural Integrity Procedia 00 (2018) 000–000

1. Introduction Selective Laser Melting (SLM) is an additive manufacturing technology that enables the production of metallic structures with a high complexity, Gebhardt (2016). The parts are made of metal powder with a density of nearly 100%, Leuders et al. (2013), Riemer et al. (2014) and Thöne et al. (2012). It is a layer-wise process in which the three steps of coating, irradiation and lowering are repeated so that the structure grows stepwise, see Fig. 1. One important element in lightweight construction is given by the high freedom of design using additive manufacturing technologies, Gibson et al. (2010). This new technology offers the opportunity to produce complex and delicate structures, e.g. undercuts, lattice structures or topology optimized parts, Gebhardt (2013) and Riemer (2015).

Fig. 1. SLM-process, Brüggemann et al. (2016).

Another element is the use of lightweight materials – materials with low density and good mechanical properties. A typical lightweight material is titanium alloy Ti-6Al-4V. Due to its relatively low density combined with good strength properties and high fatigue strength, the material meets the most important requirements for a lightweight material, Peters and Leyens (2010). Typical application areas are automotive and aircraft industries. Because of the biocompatibility the alloy is also of great importance for medical technology, Jackson and Ahmed (2007) or Schramm et al. (2016). Ti-6Al-4V is already safely processable by SLM and Leuders et al. (2013) as well as Riemer (2015) have already analyzed the material characteristics. The material’s lightweight potential has been shown in the optimization of different bicycle and medical components, Riemer et al. (2015). In addition, the positive influence of the 1073.15 K heat treatment to reduce the process-induced residual stresses is also known. Furthermore, hot-isostatic pressing leads to the reduction of pores and thus to the improvement of material performance. Riemer (2015) also found that the additively processed Ti-6Al-4V alloy has almost isotropic material properties in the heat-treated condition, so that only a slight influence of the building direction can be observed. To use this alloy for the manufacturing of reliable components the influence of varied process parameters on the material characteristics has to be known. In this context the impact of varied powder particle size distribution on the material characteristics of laser-melted Ti-6Al-4V is investigated. 2. Experimental details In order to increase the bulk density and, thus, to generate a denser powder bed for the manufacturing process, the particle size distribution (PSD) of the powder is changed to lower values. Therefore it is to determine whether a changed particle size distribution has an influence on the material characteristics of Ti-6Al-4V. As a first step in these investigations the particle size distributions (PSDs) of two powder lots are analyzed, see Fig. 2a. Material characteristics for the powder with a median particle size value d(0.5) = 38.9 µm are already determined by Riemer (2015) and Leuders et al. (2013). The new powder lot has got a significantly smaller d(0.5) value of 31.7 µm. Furthermore, the variance in the particle sizes is increased. For the manufacturing of specimens, Ti-6Al-4V powder was produced by gas atomization. The particle shape of the new powder lot is analyzed using a scanning electron microscope (SEM) image, see Fig. 2b. A homogenous spherical particle shape can be detected which influences the processability positively.



J.-P. Brüggemann et al. / Procedia Structural Integrity 13 (2018) 317–321 J.-P. Brüggemann et al. / Structural Integrity Procedia 00 (2018) 000–000

319 3

Fig. 2. (a) Particle size distributions of the two powder lots (b) spherical particle shape of Ti-6Al-4V powder.

To evaluate the influence of the changed particle size distribution on the material properties tensile specimens based on the standard DIN 50125 (2008) and CT specimens with a thickness of 3.0 mm, a width of 40 mm and a V-shaped notch with a length of 8 mm according to the ASTM 647-08 (2008) standard were manufactured with the standard process parameters for Ti-6Al-4V. These specimens were manufactured on a SLM 280HL (SLM Solutions Group AG) system with a build chamber of 280 mm x 280 mm x 350 mm, a 400 W Yttrium laser and in inert gas atmosphere. The platform was heated up to 473.15 K. Following SLM processing the chamber was flooded with nitrogen and the oxygen content is reduced below 0.2 %. The mechanical and fracture mechanical properties were investigated for two material conditions: Some specimens were heat-treated at 1073.15 K after the building process for two hours to reduce residual stresses, the other half was set to hot isostatic pressing which leads to the reduction of residual stresses and pores. Therefore, the specimens made of Ti-6Al-4V are pressurized in a pressure chamber under inert gas atmosphere with an isostatic pressure of 1000 bar and a temperature of 1193.15 K for two hours. For the characterization of the quasi-static properties a universal testing machine, INSTRON 5569, was utilized. The tensile tests with a minimum of three specimens for each condition were conducted displacement controlled with a crosshead speed of 5 mm/min according to the DIN EN ISO 6892-1 (2009) standard. The elongation at room temperature (293.15 K) was measured using an optical extensometer. The analysis of the fracture mechanical properties was executed using an INSTRON electro-mechanical tensile testing machine ELECTROPULS E10000. The analyzed characteristics are determined at ambient conditions and under mode-I-loading. Each test series contains a minimum of three specimens. A periodic sinusoidal loading at a stress ratio of R = 0.1 was applied to the sample. The current potential drop method was used for the continuous determination of the crack length during the experiment. This process is carried out with the help of the crack length measuring system DCM-2 of the company MATELECT LDT. In this case, the measuring device has two channels for potential drop measurement and for energizing the samples a constant current source is available. In the current potential drop method, a constant current I is given into the test sample isolated from the testing machine, and the potential drop U at a defined location is measured. If now the crack propagation and the concomitant reduction of the cross section in the test sample results, according to Ohm's law a change, in this case an increase, of the electrical resistance RΩ and thus also of the potential drop U. The determination of crack growth values was conducted by the system FAMControl, see Sander and Richard (2004). A complete crack propagation curve requires two different tests. The investigation of the near threshold behavior and the threshold value (ΔKth) of the stress intensity factor range at low stress intensity factor ranges (ΔK) is performed by an exponential decrease of ΔK at a constant R-ratio and a test frequency of 20 Hz. The crack growth behavior at elevated stress intensity factor ranges (PARIS regime and above) was investigated using a constant force ratio with a test frequency of 10 Hz. 3. Results and discussion To determine the static material properties, tensile tests are performed. The results of these investigations comparing the two PSDs for the 1073.15 K heat-treated and the HIP condition are listed in Table 1. Both the yield strength and the tensile strength have only small deviations. These minor differences in the mechanical characteristics can be affected by environmental parameters such as air temperature, tolerances of the experimental system and the measurement system.

320 4

J.-P. Brüggemann et al. / Procedia Structural Integrity 13 (2018) 317–321 J.-P. Brüggemann et al. / Structural Integrity Procedia 00 (2018) 000–000

Table 1. Room temperature tensile properties of heat-treated and HIP Ti-6Al-4V SLM-specimens with two different particle size distributions. Condition heat-treated 1073.15 K HIP

PSD [µm]

yield strength [MPa]

ultimate tensile strength [MPa]

31.7 40 31.7 40

1040 962 940 912

1080 1040 1020 1005

Since no significant influence of the changed PSD regarding the static material properties can be detected, the fracture-mechanical behavior is analyzed. Fig. 3 shows the crack propagation curves for the 1073.15 K heat-treated condition and the condition after hot isostatic pressing.

Fig. 3. Crack growth curves for SLM-processed Ti-6Al-4V (d(0.5) = 31.7 µm) in the 1073.15 K heat-treated condition and the condition after HIP.

Analogous to Riemer (2015) the same specimen geometry is used for the determination of crack propagation curves. The specimen used are oriented in a way that their building direction (BD) is perpendicular to the crack path (CP) (BD⊥CPሻ according to the investigations of Riemer (2015). Overall, the detected parameters have a high reproducibility, recognizable by the little variation of the crack propagation curves. The determined threshold value ΔKI,th for the heat-treated condition is 3.6 MPa·m1/2, the standard deviation of 0.02 MPa∙m1/2 being an indication of good reproducibility. Hot isostatic pressing can significantly improve the fracture mechanical properties of the Ti-6Al 4V alloy. The threshold for fatigue crack propagation is ΔKI,th = 4.7±0.16 MPa·m1/2. A comparison of the fracture mechanical behavior of the powder lot with d(0.5) = 31.7 µm with the experimental data from Riemer (2015) with d(0.5) = 38.9 μm is visualized in Fig. 4. The threshold areas for the powders with mean particle size of 31.7 µm and 38.9 µm by Riemer (2015) are shown for the heat-treated condition (1073.15 K) and the condition after hot isostatic pressing. Comparing both powder lots in the heat-treated condition the threshold for fatigue crack propagation ΔKI,th for the powder with d(0,5) = 31.7 µm is 0.3 MPa·m1/2 lower in contrast to the data from Riemer (2015). After hot isostatic pressing it is 0.5 MPa·m1/2 higher. Both results show that there is no significant influence of the particle size distribution on the fracture mechanical behavior, especially on the threshold for fatigue crack propagation ΔKI,th. The small deviation of the threshold values may be due to environmental parameters and accuracies of the test equipment. All these factors have a minimal influence on the experimental determination of the crack propagation curves, which in total can give a slight scattering of the measurement results. Furthermore, the relatively little variation in the determined characteristics is a sign of a good reproducibility and a process-safe processing of Ti-6Al-4V alloy in selective laser melting manufacturing process. Due to the good accordance of the mechanical and fracture mechanics parameters, fatigue tests for the heat-treated condition and the condition after hot isostatic pressing are carried out for selected stress states to determine a Wohler curve for the PSD with d(0.5) = 31.7 μm. Compared to the determined fatigue properties from Leuders et al. (2013), there are no significant differences for the tested stress levels



J.-P. Brüggemann et al. / Procedia Structural Integrity 13 (2018) 317–321 J.-P. Brüggemann et al. / Structural Integrity Procedia 00 (2018) 000–000

321 5

Fig. 4. Comparison of the threshold values ΔKI,th for the powders with median particle size of 31.7 µm and 38.9 µm by Riemer (2015) for the heat-treated condition (1073.15 K) and the condition after hot isostatic pressing.

4. Conclusions The results of this study contribute to the determination of different influencing factors of the SLM-process on the material characteristics. One of them is the particle size distribution of the powder, which slightly changes from powder lot to powder lot. In this case, existing experimental data from Riemer (2015) and Leuders et al. (2013) were taken and compared to the mechanical and fracture mechanical properties of specimens with a significantly smaller mean particle size. In summary, it should be noted that the change of PSD from 38.9 μm to 31.7 μm has no significant influence on the mechanical and fracture-mechanical properties of Ti-6Al-4V alloy. In addition, the reproducibility of the material behavior is high and emphasizes the good processability by SLM. The validation of those material characteristics increases the reliability for the development of lightweight-optimized structures. References ASTM, 2008. Annual book of ASTM standards. Section 3: Metals test methods and analytical procedures, vol 03.01. Metals - Mechanical testing, elevated and low-temperature tests. Metallography, 2008: E 647-08. Brüggemann, J.-P.; Riemer, A.; Reschetnik, W.; Aydinöz, M. E.; Kullmer, G.; Richard, H. A.; Schaper, M., 2016. Optimierung von Fahrradtretkurbeln mittels additiver Fertigung. In: DVM-Bericht 401, Arbeitskreis: Additiv gefertigte Bauteile und Strukturen, Deutscher Verband für Materialforschung und -prüfung e.V., Berlin, 101-112. DIN 50125. Testing of metallic materials – Tensile test pieces; E DIN 50125:2008-10. DIN EN ISO 6892-1. Metallic materials – tensile testing - part 1: method of test at room temperature; ISO 6892-1:2009. Gebhardt, A., 2013. Generative Fertigungsverfahren: Additive Manufacturing und 3D Drucken für Prototyping - Tooling - Produktion. 1st edn., Carl Hanser Fachbuchverlag, München. Gebhardt, A., 2016. 3D-Drucken: Grundlagen und Anwendungen des Additive Manufacturing (AM). 2nd edn., Hanser, München. Gibson, I., Rosen, D.W., Stucker, B., 2010. Additive manufacturing technologies: Rapid prototyping to direct digital manufacturing. Springer, New York. Jackson, M.J., Ahmed, W., 2007. Surface engineered surgical tools and medical devices. Springer, New York. Leuders, S., Thöne, M., Riemer, A., Niendorf, T., Tröster, T., Richard, H. A., 2013. On the mechanical behaviour of titanium alloy TiAl6V4 manufactured by selective laser melting: Fatigue resistance and crack growth performance. International Journal of Fatigue 48, 300-307. Peters, M., Leyens, C. (editor), 2010. Titan und Titanlegierungen. WILEY-VCH Verlag, Weinheim. Riemer, A., 2015. Einfluss von Werkstoff, Prozessführung und Wärmebehandlung auf das bruchmechanische Verhalten von Laserstrahlschmelzbauteilen. Shaker, Herzogenrath. Riemer, A., Leuders, S., Thöne, M., Richard, H. A., Tröster, T., Niendorf, T, 2014. On the fatigue crack growth behavior in 316L stainless steel manufactured by selective laser melting. Engineering Fracture Mechanics 120, 15-25. Riemer, A.; Richard, H. A.; Brüggemann, J.-P.; Wesendahl, J.-N., 2015. Fatigue crack growth in additive manufactured products. Proceedings of the 5th International Conference on CRACK PATHS (CP2015), Ferrara, 494-503. Sander, M., Richard, H. A., 2004. Automatisierte Ermüdungsrissausbreitungsversuche. Materials Testing 46, 22-16. Schramm, B.; Brüggemann, J.-P.; Riemer, A.; Richard, H. A., 2016. Additive Fertigung in der Medizintechnik – Überblick und Beispiele –. DVMBericht 401, Arbeitskreis: Additiv gefertigte Bauteile und Strukturen, Deutscher Verband für Materialforschung und -prüfung e.V., Berlin, 21-30. Thöne, M., Leuders, S., Riemer, A., Tröster, T., Richard, H. A., 2012. Influence of heat-treatment on selective laser melting products – e.g. Ti6Al4V. Solid freeform fabrication symposium SFF, Austin Texas.