CASE STUDY

Baked to Perfection:

Renishaw’s Insights Into AM Heat
Treatment Sensitivity Using PIP
In collaboration withPlaceholder

Challenge
Heat treatment is a common step in the additive
manufacturing (AM) process, used by engineers
to control the performance and properties ofadditively manufactured parts. It’s often a crucialpart of turning these intricate designs into strong,
usable components that can stand up to thedemands of its intended operating environment.
We rely on this process to make sure that partproperties are optimised for their application,
ensuring high performance and longevity inservice.

But what if this process, designed to enhanceand strengthen, could in fact leave us open

to local property variations? In this case study
with Renishaw, we uncover how the varying

temperatures throughout a furnace have asignificant impact on the mechanical propertiesof AM parts, in this case showing over 10%
variation in yield strength, and how thesefindings have led Renishaw to optimise their heat

In this case study with Renishaw,
we uncover how the varying
temperatures throughout a furnace
have a significant impact on themechanical properties of AM parts,
in this case showing over 10%
variation in yield strength.

treatment process, ensuring consistency andincreasing confidence in part performance.

2 ] Baked to Perfection: Renishaw’s Insights Into AM Heat Treatment Sensitivity Using PIP

Objectives

This case study aims to highlight the variation

in mechanical properties across a set of builds
which can arise during heat treatments ofAM parts and how these variations can becharacterised quickly and effectively.

AM materials are commonly heat treatedto optimise the properties of the parts andmaximise their performance. However, non-uniform heating and cooling conditions withina furnace, due to variable gas flows and heater

Unknown variations in heating
conditions and resulting mechanicalproperties can increase part-topart variation or result in differential
properties across a large build.

positioning, can generate unknown variations

the entire gauge length). Again, this option is notproperties. This can increase part-to-part

Hardness testing could generate mechanicalin heating conditions and resulting mechanicalvariation or result in differential properties acrossa large build. Furnace temperature variation

can be mapped using thermocouple arrays;

however, this does not capture the temperatureacross the whole build or any direct information
about the resulting effect on the materialproperties.

Another option for verifying and characterising

this variation in heat treatment conditions could
be to tensile test a large number of witness

coupons from across the part (although there

is no guarantee that a witness coupon will haveexperienced a uniform heat treatment across

To use tensile testing to characterise variation in heat
treatment conditions would require high volumes ofmaterial and time, without a guarantee of a tensile
coupon having experienced a uniform heat treatment
across the entire gauge length.

just time, but also material and cost intensive.
data on a finer scale, however these tests

generate far less information than a full stressstrain curve does.

This case study will show how valuable insight
into the inhomogeneity associated with rapid

cooling heat treatment, can be obtained using

Profilometry-based Indentation Plastometry (PIP)
due to the technique’s fine spatial resolution,
allowing for accurate mapping of variations

in mechanical properties, whilst accelerating
testing times and reducing the substantial

material requirements associated with pulling
witness coupons.

PIP testing allows for accurate mapping of variations inmechanical properties, whilst accelerating testing times
and reducing material requirements substantially.

3 ] Baked to Perfection: Renishaw’s Insights Into AM Heat Treatment Sensitivity Using PIPMaterials

Two Inconel 718 builds, each consisting of 16

tensile coupons were built by Renishaw for thiswork, using laser powder bed fusion. The tensile

coupons were all printed vertically, such that theprobed tensile direction was parallel to the builddirection. Both builds were then heat treated byRenishaw, together in the same vacuum furnace,using their standard Inconel 718 heat treatment.
A 1 bar argon gas quench was used after thesolutioning and ageing stages, with the gas inlet
positioned at the front of the furnace.

16 of the 32 coupons were positioned at theback of the furnace, ~280 mm from the argon

inlet, whilst the other 16 were positioned at thefront of the furnace, as close as 15 mm from theargon inlet (Figure 1). Having been heat treated,the coupons were tested in uniaxial tension andPIP tests were subsequently carried out on thethreaded tensile ends. This allowed any variability
in the mechanical properties to be characterised
through both the tensile and PIP results.

Figure 1: Diagram of a vacuum furnace with 16 coupons placed near the back, about 280 mmfrom the argon inlet, and 16 coupons positioned about 15 mm from the argon inlet at the frontof the furnace.

4 ] Baked to Perfection: Renishaw’s Insights Into AM Heat Treatment Sensitivity Using PIP

Measurements

The mechanical properties of the Inconel 718

samples were determined using conventional
tensile testing and PIP testing.

PIP is an indentation-based technique which canrapidly derive the stress-strain curve of metallic
materials. An indent, approximately 1 mm indiameter and 150 µm in depth, is generated using
a spherical indenter tip and the residual indent

shape is then characterised using a contacting
stylus profilometer. Using the indent shape andan integrated inverse finite element simulation
method, accurate stress-strain curves can beinferred from the indent. The test process is fullyautomated using a PLX-Benchtop device and thefull test takes only around 5 minutes to complete.

The PIP tests were carried out directly on six of thethreaded tensile ends such that the indentation

direction was parallel to the build direction. A basic
surface preparation to a mirror finish was applied
to the indented surfaces prior to PIP testing.

PLX-Benchtop by Plastometrex.

5 ] Baked to Perfection: Renishaw’s Insights Into AM Heat Treatment Sensitivity Using PIPResults

The trends identified from the PIP results provided
significant insight for Renishaw about thesensitivity of mechanical properties on the variabletemperatures generated within the same heat
treatment run.

effect on the measured UTS values, but a more

The PIP-derived stress-strain curves for thesignificant effect on the yield stress: the average

samples at the front and back of the furnace,shown in Figure 2, were compared to unlock thisPIP-derived yield stresses, shown in Table 1, wereinsight. The position within the furnace had minimal

FurnaceFront~140 MPa (11%) lower for the samples at the front ofthe furnace compared with the back.

11%Furnace
Backvariation

The average PIP-derived yield stresses, shown in
Table 1, were ~140 MPa (11%) lower for the samples at
the front of the furnace compared with the back.

Figure 2: Representative PIP-derived nominal stress-strain curves from each heat

treated Inconel 718 build, situated either at the front or back of the furnace at Renishaw.

6 ] Baked to Perfection: Renishaw’s Insights Into AM Heat Treatment Sensitivity Using PIP

Front of Furnace

Back of Furnace

Average Yield Stress / MPa

1068

1205

Average UTS / MPa

1554

1574

Table 1: Averaged PIP-derived yield stress and UTS values, for Inconel 718 heat treated at thefront and back of the furnace at Renishaw.

The tensile stress-strain curves also exhibitedpart with only a small amount of preparation.

for samples at the front of the furnace andacross a part to be mapped at a far finer

similar trends, with a reduction in yield stressinsignificant differences between measured

ultimate tensile strengths, whilst also showing
good agreement with the PIP-derived results.
However, identifying this trend using PIP isa much faster process, with no significantmachining requirements. In fact, PIP can becarried out on a printed and heat-treatedThis allows the effects of the heat treatment

resolution compared with tensile testing using

a separate coupon, providing invaluable insight
into the efficacy of heat treatments across alarge or complex part. Through PIP testing, thisunderstanding can be achieved with significantly
less demand on time and printed material.

7 ] Baked to Perfection: Renishaw’s Insights Into AM Heat Treatment Sensitivity Using PIP

Outcomes

Where gas quenching is used, the location ofa part inside the heat treatment furnace cansignificantly affect its mechanical properties,including its yield strength. Our findings showthat parts placed at the front of the furnace(closer to the argon inlet) have an 11% lower
yield stress compared with those at theback. This study has enabled Renishaw to
identify variation in gas flow conditions when
multiple builds are in the furnace, allowing forthe implementation of heat treatment bestpractices to ensure consistency across builds.
Through this discovery and the use of PIP

testing, Renishaw is now able to get a fuller
picture of the integrity of parts post-heat

treatment than hardness testing alone canprovide, without incurring the time and cost

penalties associated with tensile testing. ForRenishaw printing and testing this Inconel

718 material, the cost for a PIP test is aroundhalf that of a conventional tensile test and aquarter of the sample preparation time. PIP-derived results equip Renishaw with valuable

insights for enhancing heat treatment processdevelopments and understanding their impact.
PIP also offers a means to gather actionable
information about mechanical properties
quickly and easily without needing large

volumes of material. At Renishaw, the findingspresented in this case study were used to8 ] Baked to Perfection: Renishaw’s Insights Into AM Heat Treatment Sensitivity Using PIP

inform the decision to limit the number of builds
or built parts from being simultaneously heat
treated in the same furnace, preventing any

variation in mechanical properties from being
introduced due to furnace position.

Performing PIP studies on small witness

coupons also offers potential benefits during
production and could be used as a fast andinexpensive way to validate part performancewith high spatial resolution. Using PIP as part ofa quality assurance process can demonstratecompliance with minimum performancethresholds, measure property variation andprovide confidence in results to internal

stakeholders, customers and regulatory bodies.

Conclusion

This study has enabled Renishaw to
identify variation in gas flow conditions
when multiple builds are in the furnace,
allowing for the implementation of heat
treatment best practices to ensure
consistency across builds.

This case study, in collaboration with Renishaw,

result in significant variation in part properties.

PIP can deliver in rapidly identifying heatprovide some insight to furnace conditions anda significantly reduced cost compared withand less costly, as well as offering an advantage

to identify inconsistent conditions during heatproperties over builds. This provides Renishaw

the furnace resulted in significant variations intheir AM parts, as well as requiring significantly

during heat treatment of AM samples canfor printing.

has been able to demonstrate the value

Whilst uniaxial testing of witness coupons cantreatment associated property variation for

the effect on mechanical properties, PIP is fastertraditional tensile testing. Renishaw were able

in providing finer scale resolution for mapping

treatment and exposed how the position withinwith a competitive advantage in characterisingmechanical properties. This variable coolingless material for testing, saving time and cost

9 ] Baked to Perfection: Renishaw’s Insights Into AM Heat Treatment Sensitivity Using PIP

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