Dynamic mechanical properties of AM-manufactured stainless steel material

A.B. Spierings, G. Levy, K. Wegener

Inspire – institute for rapid product development irpd St. Gallen, Switzerland

CIRP STC-E

Paris, 27.01.2012

1

Agenda

� Background & Motivation

� Mechanical properties of stainless steel

� Dynamic material qualification

� Case study

Spierings, Adriaan © 01/2012 inspire AG2

Background & Motivation

� AM technologies, especially Beam Melting Technologies such as SLM, offer an interesting way to highly improved functional & structural parts for

– Aerospace applications

– Automotive

– Machine industry

– Medical applications (implants, instruments)

– 2

Spierings, Adriaan © 01/2012 inspire AG3

www.botzeit.de Inspire-irpd Inspire-irpd

Background & Motivation

� However, the success of AM-technologies in industry requires a furtherdevelopment in different areas:

Exemplaryapplications

Material knowhow

Processknowhow &development

Affordablemachines

AM-productionenvironment

√√√√

≈≈≈≈ √√√√

≈≈≈≈ √√√√

----

Production environment

«Lab environment» / first applications – R&D driven

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Inspire-irpd is involved in all fields

Background & Motivation

� Focus on material properties and integrity

– Static mechanical properties � Widely investigated / known

– Dynamic mechanical properties � Only partially investigated

– Fracture behaviour (da/dN, KI,II,C,-) � Almost no results available

– General material integrity � Many investigations and analysis

– Internal stresses available / R&D topic of high interest– Microstructure

– 2

– Korrosion behaviour

– 2

Industrial applications

Mechanical properties:

static, dynamic, fracture behaviour, 2

MicrostructureInternal stresses

Porosity

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Mechanical properties

� We know that mechanical properties are different to «conventionally» processed materials, due to2

– Fine grained, columnar microstructure � anisotropy

– Internal stresses– BRINKSMEIER, E., LEVY, G., MEYER, D. & SPIERINGS, A. B. 2010. Surface Integrity of Selective-Laser-Melted Components.

CIRP Annals - Manufacturing Technology, 59, 601-606.

– Some porosity

Viewing direction

Main grain

orientation

90° F

F

Main grain

orientation /

dendrits

Builddirection

Viewing direction

0°

F Fz

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SPIERINGS, A. B. & LEVY, G., 2009. Comparison of density of stainless steel 316L parts produced with selective laser melting

using different powder grades. In: BOURELL, D. L., ed. Proceedings of the Annual International Solid Freeform Fabrication Symposium,August 3-5 2009 Austin, Texas. 342-353.

Mechanical properties

� Static mechanical properties

– For many materials known

– Typically good static properties(Hall-Petch !)

���� Many materials meet the

requirements for static applications.

� Dynamic mechanical properties– Partially analysed

– Typically lower strength compared to conventionally processed materials

� Is this dramatic and hindering a use of AM-materials in more dynamic

(industrial) applications?

Rm

0

100

200

300

400

500

600

700

800

900

Type 1 Type 2 Type 3

Powder Types

(MPa)

0

100

200

300

400

500

600

700

800

900

30um - 0° 45um - 0°

30um - 90° 45um - 90°

Literature

Spierings, A.B. et.al; Influence of the particle size distribution on surface quality and mechanical properties in additive manufactured stainless

steel parts; Rapid Prototyping Journal, 2011. 17(3): p. 195 - 202.

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Dynamic material qualification

� Analyse of dynamic properties of AM-processed stainless steel

– Machines & Materials

Material: SS-316L

ConceptLaser - M1

• ≈ 100 W• v = 425 mm/s• Layer thickness 30 µm• Hatch 0.13 mm

• Build direction: vertical

• Powder- D50 ≈ 15 µm

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SLM machine @ Inspire-irpd

Dynamic material qualification

� Analyse of dynamic properties of AM-processed stainless steel

– Test setup

S-N specimens

• Surface finishing- as-built- machined

- polished

• Testting (ASTM-E739) at- R = 0.1 («severe»)- 50 Hz

• 50 samples for SS 316L

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� Results

– As-built

– & Machined

– & polished

Dynamic material qualification

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� Conclusions

– Values are comparable toconventional materials, dependingon the specific conditions(annealed or not, 2)

polished

as-built

machined

– As-built samples: Lower endurancestrength compared to machined, polished

– Only small differences in fatigue rangebetween as-built – machined – polished

samples!

Dynamic material qualification

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Dynamic material qualification

� Open questions

– Surface quality

– Interestingly, the surface quality did not play a highly significant role.↔ conventional machining (grooves) !!� other factors play a more important role.

This behaviour was also shown on other materials!

– Differences in S-N curves: z- versus xy-direction

– The grain size plays an important role in fatigue,especially during crack initiation (∆Kth ↑ for d↓).

– Crack growth in the current (z-direction) specimens:� load acting ⊥ to the long axis of the grains� for cracks: «coarse grained structure»

– Expectation for xy-built specimens: Lower values compared to z-built specimens� «fine grained structure»

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Z-specimens (left), xy-specimens (right)

Dynamic mechanical properties

� Challenges: Are the dynamic mechanical properties sufficient forindustrial applications?

– We need to qualify the materials� Material database

– We can optimise the part-design� Topological optimisation

� Optimization of stress flow

– We need to optimise the processes� Reduction of porosity

� Reduction of internal stresses Answer

Optimised

processes

Design

Optimization

Qualification

of materials

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Design

• Geometry• Joints• Material selection• Surface condition• 2

Stress distribution

Material

• Static• Dynamic• da/dN• Fracture

toughness

Application

• Leightweight• Functionally

optimised

14

Case study

� Formula student car 2011

Production of heavily loaded brackets for the Formula Student Cars 2010 & 2011, ETH Zurich

� Bracket development (leightweight)

���� Dynamically loaded

AMZ Formula Student Car 2010 & 2011, 1st place winner in Silversone 2010.

�Exemplare case study:Motivation for an industrial use of SLM for functional, load-bearing parts.

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15

Formula student car of ETHZ

� Version 2011:

Different brackets (Version 2011)

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Brackets, produced by inspire-irpd

SPIERINGS, A. B., et.al, 2011. Production of functional parts using SLM – Opportunities and limitations. In: BÀRTOLO, P. J., ed. Virtual and Rapid Manufacturing: Advanced Research in Virtual and Rapid Prototyping, September 28-October 1 2011, Leira, Portugal. CRC Press, Taylor & Francis, London, 785-790.

Dynamic material qualification

� Outlook («Materials»)

– Analyse more materialsS-N measurements done

– 316L

– 1.4313

– Some measurements on 1.4542

– Analyse not only S-N curves, but also– Crack propagation curves (da/dN)

– KI,C

– Planned: Measurements on Aluminium (S-N, da/dN)

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Materials

SLM machine

equipment

Applications

Processdevelop-ment &

Simulation

SLM fields of activity @ inspire-irpd

Dynamic material qualification

� Participation of inspire-irpd in CWG proposal «Laser in Production»

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