metals

Article
Functionally Graded SS 316L to Ni-Based Structures
Produced by 3D Plasma Metal Deposition
Johnnatan Rodriguez 1, * , Kevin Hoefer 2 , Andre Haelsig 2 and Peter Mayr 2
1 Department of Mechanical Engineering, EIA University, 055428 Envigado, Colombia
2 Chair of Welding Engineering, Chemnitz University of Technology, Reichenhainer Straße 70,
09126 Chemnitz, Germany; kevin.hoefer@mb.tu-chemnitz.de (K.H.);
andre.haelsig@mb.tu-chemnitz.de (A.H.); peter.mayr@mb.tu-chemnitz.de (P.M.)
* Correspondence: johnnatan.rodriguez@eia.edu.co




Received: 29 April 2019; Accepted: 23 May 2019; Published: 28 May 2019


Abstract: In this investigation, the fabrication of functionally graded structures of SS316L to Ni-based
alloys were studied, using the novel technique 3D plasma metal deposition. Two Ni-based alloys
were used, a heat resistance alloy Ni80-20 and the solid-solution strengthened Ni625. Different
configurations were analyzed, for the Ni80-20 a hard transition and a smooth transition with a region
of 50% SS316L/50% Ni80-20. Regarding the structures with Ni625, a smooth transition configuration
and variations in the heat input were applied. The effect of the process parameters on the geometry
of the structures and the microstructures was studied. Microstructure examinations were carried
out using optical and scanning electron microscopy. In addition, microhardness analysis were made
on the interfaces. In general, the smooth transition of both systems showed a gradual change in the
properties. The microstructural results for the SS316L (both systems) showed an austenite matrix with
δ-phase. For the mixed zone and the Ni80-20 an austenite (γ) matrix with some M7 C3 precipitates
and laves phase were recognized. The as-built Ni625 microstructure was composed of an austenite (γ)
matrix with secondary phases laves and δ-Ni3Nb, and precipitates M7 C3 . The mixed zone exhibited
the same phases but with changes in the morphology.

Keywords: additive manufacturing; austenitic stainless steel; Ni-based alloy; 3DPMD; functionally
graded; multi-material

1. Introduction
The layer-by-layer production of components with locally variable properties describes the process
of multi-material additive manufacturing (MMAM). The variation of the component density depending
on the location is an example of this and can be seen as an analogy to human bone formation [1–3].
Further examples are the variation of the grain size [4] and changed materials [5] within the component.
The continuous change of the component properties results in advantages such as: improved strength
values [6], reduced internal stresses [7–11], and a lower crack propagation potential [12].
3D plasma metal disposition (3DPMD) is a further development of the classical plasma transferred
arc process (PTA) with regard to the requirements of additive manufacturing (AM) and belong to the
category of directed energy deposition processes [13]. In addition, applications of micro-PTA for the
production for MMAM are known [14,15]. In comparison, the higher build rates (up to 10 kg·h−1 ),
the lower demands on powder quality and the possibility to handle up to four different powders in
parallel, are to be emphasized. Further information on the process and the materials to be processed
can be found elsewhere [16–18].
Due to its good weldability, corrosion resistance and adequate high temperature mechanical
properties, austenitic stainless steel (ASS) is used in different industries, such as petrochemical,

Metals 2019, 9, 620; doi:10.3390/met9060620 www.mdpi.com/journal/metals

Metals 2019, 9, 620 2 of 19

shipbuilding, and nuclear industries. These steels are non-magnetic, stable at room temperature,
and cannot be hardened by heat treatment [19–21]. ASS is generally manufactured by casting and
their components can be joined by different fusion welding processes. In recent years, additive
manufacturing has also been used for the fabrication of ASS components. Different AM processes have
been employed such as laser process [22–24], electron beam [25], and tungsten inert gas welding [26].
Austenite and vermicular ferrite are the main phases found in components fabricated with high
cooling rates characteristic of AM. In addition, the mechanical properties of the components could be
comparable to the components manufactured by conventional fusion process [19].
Depending on the alloy composition, the microstructure and the solidification path of the austenitic
stainless steel can change significantly. Therefore, with the aim to predict the microstructure after
the fabrication of ASS components several diagrams have been developed [27]. The diagrams are
based on the elements that stabilize ferrite and austenite. To group the elements, two equations were
created, the first one is the chromium equivalent (Creq ) and the second one the nickel equivalent (Nieq ).
The equations developed by Kotecki and Siewart [28], that correspond to the diagram WRC-1992
(Welding Research Council) are shown below:

Creq = %Cr + %Mo + 0.7 Nb (1)

Nieq = %Ni + 35%Cr + 20%N + 0.25%Cu (2)

By using the Creq /Nieq ratio, it is possible to identify the primary solidification mode of the
stainless steels. With ratios below 1.2 the steel solidifies in the primary austenite mode (A), where
at the beginning of the solidification the nucleation of austenite grains occurs. Then, the ferrite
stabilizers elements are enriched in the liquid as the austenite grains grow, leading to the solidification
of more austenite or δ-ferrite. Creq /Nieq ratios between 1.2 and 1.5 favor the austenite-ferrite mode
(AF). Therefore, at the end of the primary solidification process some ferrite is formed via a eutectic
reaction. This occurs due to the presence of ferrite-promoting elements (like Cr and Mo) at the subgrain
boundaries to promote the formation of ferrite at the end of the process. Finally, the ferrite-austenite
(FA) mode occurs by the formation of primary ferrite and the austenite forms at the end of the process.
The austenite forms via a peritectic-reaction [27–29].
In industries like aeronautical, aerospace, chemical, petrochemical, and marine one of the most
important materials are the nickel-based alloys. High corrosion resistance, high strength at both ambient
and elevated temperatures, and good ductility and toughness at low temperatures are among the main
mechanical properties of the nickel-based alloys [30]. Ni-based alloy 625 (Ni625) is a solid-solution
strengthened alloy with strengthened refractory metals such as niobium and molybdenum in the
nickel-chromium matrix. Precipitation hardening is mainly derived from the formation of metastable
phases and carbides. The main secondary phases found in these alloys are MC, M23 C6 , M6 C, and
M7 C3 carbides, γ”-Ni3 (Al,Ti), η-Ni3 Nb, laves, and σ and δ-Ni3 Nb [31]. On the other hand, the alloy
Ni80Cr20 is a heat-resistance alloy with an austenitic nickel-chromium matrix. High resistivity, good
oxidation resistance, ductility, and weldability characterize this alloy.
Dissimilar joints of stainless steels to Ni-based alloys also play an important role in the modern
industries. In thermal and nuclear plants the joint of Inconel 625 to AISI 304L is used in cryogenic
applications [32], in sub-sea manifolds dissimilar joints of Inconel 625 and duplex stainless steel are
employed [33], NASA manufactured the sub-scale boilers using dissimilar Inconel 625 to austenitic
stainless steel [34]. In addition, the union of Inconel 625 to ASS is used in calcining process involving
chromic acid which contains diluted acidic environment [35]. Another typical application of the
bimetallic components is the sub-scale boiler employed for Advanced Stirling Conversion System
project, where the sub scale boiler was built with AISI 316 for the condenser and Inconel 625 with 304L
stainless steel wick for the boiler [36]. Cladding is another widely used application for the Ni-based
alloy and stainless steels, e.g., hot section components in thermal power plant boilers, gas turbines
and chemical industries are exposed to corrosive environments. In general, ASS are employed in
Metals 2019, 9, 620 3 of 19

those applications due to their good weldability, corrosion resistance, mechanical properties, and costs.
However, to extend the life of the components weld overlays of Ni-based alloys are used. This method
allow an increase of the protection against corrosion, hot oxidation, wear, and erosion [37].
When a component is in service, only some parts of its structure might be exposed to extreme
service conditions, such as corrosive environments or high temperatures. Therefore, it is not essential
to build the component with a mono-composition. In these cases, the use of functionally graded
materials (FGMs) would be more appropriate. FGMs are characterized by the variation in composition
across the volume [38] and their properties can be tailored for a particular application [39]. Several
processes have been used to build FGMs like material extrusion [38], laser metal deposition [39,40],
laser cladding [41], or laser rapid forming [42].
Functionalized coatings are also applied by plasma spraying and applications for laser-bound
powder bed processes are also known [43]. Different Ni-based graded components have been studied,
e.g., functional graded Inconel 718 components were produced using select laser melting (SLM) to
target different textures optimizing the process parameters [4]. Pulugurtha et al. [40] used laser metal
deposition to study FGMs of Fe-82%V and Ni-based 625 deposited in Ti6Al4V and SS316L substrates.
Cracks of both materials were found with the Ti6Al4V substrate and the geometric dilution increased
with increase in laser power, travel speed, and substrate heating. Xu et al. [42] investigated laser
rapid forming of a Ti-Ni alloy structures with a continuous compositional gradient from pure Ti
to Ti-50%Ni and analyzed the effect of the gradient composition on the microstructural evolution.
Lin et al. [44,45] studied the graded SS316-Rene88DT, Ti/Rene88DT and Ti6Al4V/Rene88DT alloys
fabricated by laser rapid forming (LRF) and performed analysis of the phase evolution with the
variation of the composition. Abboud et al. [41] studied functionally graded Ni-Al and Fe-Al coatings
(up to 4 mm in total thickness) and examined the microstructure and composition based on the
aluminides coatings. Khor et al. [43] fabricated functionally graded thermal barrier coatings of the
system yttria stabilized zirconiar/NiCoCrAlY by plasma spraying and studied, based on the variations
of the chemical composition, the mechanical properties and the microstructure.
In this study, functionally graded structures SS316L to Ni-based alloys were manufactured using
the novel process 3D plasma metal deposition. Two Ni-based alloys were employed, heat resistant alloy
Ni80-20 and solid-solution strengthened Ni625. Different configurations were studied, for the Ni80-20;
a hard transition and smooth transition with a region of 50% SS316L/50% Ni80-20. Regarding the
structures with Ni625, a smooth transition configuration and variations in the heat input were applied.
Moreover, the effect of the process parameters on the geometry of the structure and microstructure
characterization were investigated.

2. Materials and Methods

Functionally graded structures of austenitic stainless steel on the bottom and Ni-based alloy on
the top were fabricated using 3D plasma metal deposition. Structures in two configurations were
fabricated, SS316L to Ni80-20 and SS316L to Ni 625. The chemical composition of the alloys is displayed
in Table 1. A t = 10 mm thick plate of SS316L was employed as base platform (BP) for the structures.
For the SS316L to Ni80-20 systems two configurations, transition between the steel and the Ni-based
alloy, were tested. The first one, a hard transition (HT) between the SS316L (bottom) and Ni80-20 (top).
Fifteen layers of each material were deposited for the HT configuration. The second one, a smooth
transition (ST) between both pure materials was built with 50% of steel and 50% of Ni-based alloy.
In this case, ten layers of each material were deposited, 30 in total. Regarding the SS316L to Ni625,
the smooth transition configuration was chosen and three heat inputs (HI) were employed to build
the structures. For the selection of the HI, a standard welding current was found, then a variation of
∆I = 15 A was used to obtain the low and high input. Like the system with Ni80-20, ten layers of each
material were deposited. The build structures were cuboids with a length of l = 120 mm, width of
w = 30 mm and height of h = 30 mm, with an average wall thickness of t = 6 mm. One cuboid of each
system was built using the final process parameters.
Metals 2019, 9, 620 4 of 19

Table
Metals 2019, 9, x FOR PEER 1. Chemical
REVIEW composition of the powders used (wt%). 4 of 19

Alloy Table 1.
C Chemical
Si composition
Nb Mn Cr
of the powders Mo Ni
used (wt%). Fe
SS316L 0.03 0.7 - 0.5 16.5 2.1 13.0 Bal.
Alloy C Si Nb Mn Cr Mo Ni Fe
Ni80-20 - 1.2 - - 22.0 - Bal. 0.6
SS316L 0.03 0.7 - 0.5 16.5 2.1 13.0 Bal.
Ni625 0.03 0.5 3.2 0.4 21.1 8.5 Bal. 4.2
Ni80-20 - 1.2 - - 22.0 - Bal. 0.6
Ni625 0.03 0.5 3.2 0.4 21.1 8.5 Bal. 4.2
A power source PlasmaStar 500 (Imax = 500 A) in combination with the welding torch PlasmaStar
A power source PlasmaStar 500 (Imax = 500 A) in combination with the welding torch PlasmaStar
MV230 Imax = 230 A was used to fabricate the structures. A meander disk feeder (two separate disk
MV230 Imax = 230 A was used to fabricate the structures. A meander disk feeder (two separate disk
powder feeder) fed the powders with a particle size between 50–150 µm. High-purity argon was
powder feeder) fed the powders with a particle size between 50–150 μm. High-purity argon was used
−1 ), carrier gas (3 L·min−1 ) and plasma gas (1.5 L·min−1 ). A six axis
used as
asshielding
shielding gas (12 L·min
gas (12 L·min ), carrier gas (3 L·min–1) and plasma gas (1.5 L·min–1). A six axis articulated
–1

articulated arm robot REIS RV20-16

arm robot REIS RV20-16 was employedwas employed as a manipulation
as a manipulation system
system (see Figure 1).(see Figure 1).

Figure
Figure 1. 1. Experimentalsetup
Experimental setup for
for 3DPMD.
3DPMD.Adapted
Adaptedfrom [15].
from [15].

A constant
A constant welding
welding speed
speed υ=
of of mm·s−1
υ =1010mm·s −1 was used to build each layer of the structures. In the
was used to build each layer of the structures. In the
case of the system SS316L to Ni80-20, the
case of the system SS316L to Ni80-20, the current was current was II =
= 120
120AAforforthe stainless
the steel,
stainless I = 105
steel, I =A forAthe
105 for the
Ni-based alloy and I = 110 A for the powder mix (50% of steel and 50% of Ni-based alloy), the welding
Ni-based alloy and I = 110 A for the powder mix (50% of steel and 50% of Ni-based alloy), the welding
parameters are shown in Table 2. Regarding, the system SS316L to Ni625, the standard current of I =
parameters are shown in Table 2. Regarding, the system SS316L to Ni625, the standard current of
120 A for the stainless steel, I = 105 A for the Ni-based alloy and I = 110 A for the powder mix were
I = 120 A for the stainless steel, I = 105 A for the Ni-based alloy and I = 110 A for the powder mix
chosen. As mentioned before, a variation of ΔI = 15 A was used to obtain the samples for low and
were high
chosen.
heat As
input mentioned before,
(Table 3). For a variation
the heat input, an ofarc∆I = 15 A of
efficiency was0.72used
was to obtain
used. the samples
According for low
to Haelsig
and high
et al. [46], the plasma arc welding efficiency is between 0.69 and 0.80 with a mean value of 0.75. to
heat input (Table 3). For the heat input, an arc efficiency of 0.72 was used. According
Haelsig et al. [46],
However, the
for the plasma
PTA processarcthere
welding efficiency
is a small decreaseisofbetween 0.69efficiency
the process and 0.80caused
with by a mean
the usevalue
of of
0.75. powder.
However, for the PTA process there is a small decrease of the process efficiency caused by the use
of powder. Flowrate of powder for each material in both systems was set to 𝑚 = 25.52 g·mm−1 for the
stainless steel, and 𝑚 for
= 27.09 . of 𝑚
eachg·mm
−1 for the Ni-based alloy. For the powder mix, a flowrate =
Flowrate of powder material in both systems was set to m = 25.52 g·mm −1 for the stainless
12.32 g·mm and 𝑚 =−1
. −1 25.52 g·mm of SS316L and Ni625 was employed, respectively. .
−1
steel, and m = 27.09 g·mm for the Ni-based alloy. For the powder mix, a flowrate of m = 12.32 g·mm−1
.
and m = 25.52 g·mm−1 Table of SS316L and Ni625 was employed, respectively.
2. Welding parameters used for the system SS316L to Ni80-20.

Configuration
TableLayers Powder
2. Welding TypeusedWelding
parameters Current
for the system (A) toHeat
SS316L Input (KJ·mm−1)
Ni80-20.
1–15 SS316L 120 0.197
HI
Configuration Layers
16–30 Powder
Ni80-20Type Welding 0.173 (KJ·mm−1 )
105 Current (A) Heat Input
1–10
1–15 SS316L
SS316L 120 120 0.197
0.197
HI
16–30 50%Ni80-20
SS316L 105 0.173
ST 11–20 110 0.181
1–10 50% SS316L
Ni80-20 120 0.197
21–30 Ni80-20
50% SS316L 105 0.173
ST 11–20 110 0.181
50% Ni80-20
For the metallographic
21–30 examinations,
Ni80-20 cross-section specimens
105 from the fabricated
0.173structures
were obtained. Three specimens of each system were cut, two for microstructural analysis, and one
for microhardness measurements. Standard metallographic preparation was used based on the
For the metallographic examinations, cross-section specimens from the fabricated structures
were obtained. Three specimens of each system were cut, two for microstructural analysis, and
polishing with 1 μm diamond paste. Then, the final step was carried out using vibration polishing
with colloidal silica. To revel the microstructure of deposited materials, 10% oxalic acid was used at
2 V for 30 s. The microstructure was observed using optical microscopy (OM, ZEISS, Chemnitz,
Germany) and scanning electron microscopy (SEM, Tescan, Chemnitz, Germany). For the
Metals 2019, 9, 620of the precipitates, energy dispersive X-ray spectroscopy (XEDS, Bruker, Chemnitz,
identification 5 of 19

Germany) was employed. Vickers microhardness tests were performed on the interfaces between the
materials
one using an automatic
for microhardness hardness indenter
measurements. (Emco-Test,
Standard Chemnitz,
metallographic Germany)
preparation waswith
used0.98 N load
based on
and 15 s dwell time. The microhardness test were performed according to the standard
the ASTM E3-95 [47]. First, the samples were prepared by grinding using SiC papers, followedASTM E384-
by
99 [48].
polishing with 1 µm diamond paste. Then, the final step was carried out using vibration polishing
with colloidal silica. To revel the microstructure of deposited materials, 10% oxalic acid was used
Table 3. Welding parameters used for the system SS316L to 625.
at 2 V for 30 s. The microstructure was observed using optical microscopy (OM, ZEISS, Chemnitz,
Germany)HI and scanning
Layers (#) Powder
electron Type Voltage
microscopy (V) Welding
(SEM, Tescan, Current
Chemnitz, (A) Heat
Germany). ForInput (KJ·mm−1)
the identification
of the precipitates, 1–10 SS316L X-ray spectroscopy
energy dispersive 22.73 (XEDS, 105 0.176
Bruker, Chemnitz, Germany) was
employed. 50% SS316L
Low (L)Vickers11–20
microhardness tests were performed
23.04 on the interfaces
95 between the materials
0.162 using
an automatic hardness indenter 50% Ni625
(Emco-Test, Chemnitz, Germany) with 0.98 N load and 15 s dwell time.
21–30 Ni625 22.87 90 0.151
The microhardness test were performed according to the standard ASTM E384-99 [48].
1–10 SS316L 22.82 120 0.200
50% SS316L
Standard (S) Table 3. Welding parameters
11–20 used for the system
22.84 SS316L to 625.
110 0.181
50% Ni625
21–30 Ni625 22.90 105 Welding Heat0.177
Input
HI Layers (#) Powder Type Voltage (V)
1–10 SS316L 23.30 135 Current (A) 0.241−1 )
(KJ·mm
50% SS316L
High (H) 11–20 1–10 SS316L 23.57 22.73 125 105 0.176
0.214
50% Ni62550% SS316L
Low (L) 21–30 11–20 Ni625 23.73 23.04 120 95 0.162
0.204
50% Ni625
21–30 Ni625 22.87 90 0.151
3. Results and Discussion1–10 SS316L 22.82 120 0.200
50% SS316L
Standard (S) 11–20 22.84 110 0.181
3.1. 3DPMD Process 50% Ni625
21–30 Ni625 22.90 105 0.177
• SS316L–Ni80-20 structures
1–10 SS316L 23.30 135 0.241
Figure 2 shows a top view of the 50% hybrid components manufactured. The external shape showed
SS316L
High (H)layer structure
a homogeneous 11–20 without delamination or 23.57 125
cracks. The surface of the0.214
components is
50% Ni625
wavy, but negligible due21–30to the intendedNi625
post-processing.
23.73 120 0.204
The macro photographs in Figure 3 also confirm the uniform layer structure. Figure 3 shows the
abrupt
3. transition
Results for the HT sample of the properties within one layer. A wall thickness of t = 6.7 mm
and Discussion
and a component height of h = 30.4 mm were determined. The average layer thickness was z = 1.08
3.1.
mm.3DPMD Process in properties is only damped by the process-typical dilution zone. The mixed
The transition
zoneSS316L–Ni80-20
• (ST sample) forstructures
the soft transition was 10 layers (Figure 3). The smooth transition part is
characterized by a higher wall thickness (Δt = 19%) and a lower component height/layer thickness
(Δh =Figure
6.5%) 2compared
shows a top view
to the of the
hard hybrid components
transition. manufactured.
The higher proportion The external
of Ni80-20 shape
in the ST showed
variant can
a homogeneous
explain layer structure
the differences without
in component delamination
geometry. or cracks.
The lower thermalThe surface of of
conductivity thethe
components is
nickel-based
wavy, but negligible due to the intended post-processing.
alloy leads to heat accumulation and thus to a larger weld pool with effects on the geometry.

SS316L to
Figure 2. SS316L to Ni80-20
Ni80-20 structure
structure manufactured by 3DPMD, top view.

The macro photographs in Figure 3 also confirm the uniform layer structure. Figure 3 shows the
abrupt transition for the HT sample of the properties within one layer. A wall thickness of t = 6.7 mm
and a component height of h = 30.4 mm were determined. The average layer thickness was z = 1.08 mm.
The transition in properties is only damped by the process-typical dilution zone. The mixed zone
(ST sample) for the soft transition was 10 layers (Figure 3). The smooth transition part is characterized
by a higher wall thickness (∆t = 19%) and a lower component height/layer thickness (∆h = 6.5%)
Metals 2019, 9, 620 6 of 19

compared to the hard transition. The higher proportion of Ni80-20 in the ST variant can explain the
differences in component geometry. The lower thermal conductivity of the nickel-based alloy leads to
heat accumulation
Metals
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2019, 9, x FOR PEER
2019, 9, x FOR PEER thus to a larger weld pool with effects on the geometry.
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Metals 2019, 9, x FOR PEER REVIEW 6 of 19

Figure 3.
Figure 3. Macro
Macro view
viewofof
thethe
graded structures
graded of SS316L-Ni80-20
structures of built. built.
SS316L-Ni80-20 HT: hard
HT: transition,
hard ST: smooth
transition, ST:
Figure 3.
Figure 3. Macro
Macro view
view of
of the
the graded
graded structures
structures ofof SS316L-Ni80-20
SS316L-Ni80-20 built.
built. HT:
HT: hard
hard transition,
transition, ST:
ST:
transition.
smooth Marked
transition. regions
Marked represent
regions the areas
represent analyzed
the areas by XEDS
analyzed and
by XEDShardness.
and hardness.
smooth transition. Marked regions represent the areas analyzed by XEDS and
smooth transition. Marked regions represent the areas analyzed by XEDS and hardness. hardness.
• SS316L–Ni625 structures
• SS316L–Ni625 structures
•• SS316L–Ni625 structures
SS316L–Ni625 structures
The
The effect
effect
effect of
of
of modified
modified
modified heat
heat inputs
heatinputs
inputs on
inputson
ononthe
the
the part
part
part geometry
geometry
geometry is shown
is shown
is shown
shown in Figure 4.
4. ItIt
in Figure
in Figure
Figure can
It4.can be
It be
can can
be seen that
be seen
seen that
The effect of modified heat the part geometry is in 4. seen that
the
the
thatheight
height
the of
of
heightthe
the component
component
of the decreases
decreases
component with
with
decreases increasing
increasing
with heat
heat input.
input.
increasing heatConsidering
Considering
input. the
the
Consideringmacro
macro images
images
the in
in
macro
the height of the component decreases with increasing heat input. Considering the macro images in
Figure
Figure 5, an
5,inan
an increase
increase in wall
inincreasethickness
wall thickness
thickness can also
can also
also bebe
be observed.
observed. These changes
These changes
changes are
are caused
caused by a higher
bycaused
images5,
Figure Figure
increase 5, anin wall in wall can
thickness can also be observed.
observed. These Theseare changes
caused are
by aa higher
higher
by
part
part
a temperature.
temperature.
higher part This
This
temperature. leads
leads to
to
Thisaa reduction
reduction
leads to a of
of the
the melt
melt
reduction pool
pool
of theviscosity
viscosity
melt and
and
pool thus
thus
viscosity to
to modified
modified
and thus geometries.
geometries.
to modified
part temperature. This leads to a reduction of the melt pool viscosity and thus to modified geometries.
Table
Table 4 summarizes the geometric properties. Effects on component quality could not be determined
Table 44summarizes
summarizes
geometries. thegeometric
Table 4 summarizes
the geometric properties.
the Effects
geometricEffects
properties. oncomponent
properties.
on component quality
Effects onquality
component couldquality
could notbe
not becould
determined
not be
determined
macroscopically.
macroscopically.
determined macroscopically.
macroscopically.

Figure
Figure 4. Macro
4. Macro view
Macro view
viewofof the
ofthe graded
thegraded structures
gradedstructures
structures of
of SS316L-Ni625
SS316L-Ni625 built.
built. (a)
(a) Low,
Low, (b) standard,
(b) standard,
standard, and
and (c)
(c)
Figure
Figure 4.
4. Macro of the graded structures ofof SS316L-Ni625
SS316L-Ni625 built.
built. (a)(a)
Low,Low,
(b) (b)
standard, and and
(c) (c)
high
high
high HI.
HI. Marked
Marked regions
regions represent
represent the
the areas
areas analyzed
analyzed by
by XEDS
XEDS and
and hardness.
hardness.
high HI. Marked
HI. Marked regions
regions represent
represent the areas
the areas analyzed
analyzed by XEDS
by XEDS and hardness.
and hardness.

Figure 5.
Figure5. SS316L
5.SS316L
SS316L to
to
SS316Lto Ni625
Ni625
toNi625 structures
structures
Ni625structures manufactured by
manufacturedby
structuresmanufactured 3DPMD.
by3DPMD.
3DPMD.
Figure

Table 4.
Table4. Influence
4.Influence of
Influenceof different
ofdifferent HI
differentHI on
HIon the
onthe geometry.
thegeometry.
geometry.
Table
HI
HI Wall Thickness
Wall Thickness (mm)
Thickness (mm)
(mm) Part Height
Part Height (mm)
Height (mm)
(mm) Layer Thickness
Layer Thickness (mm)
Thickness (mm)
(mm)
HI Wall Part Layer
Low
Low 6.5
6.5 27.5
27.5 0.92
0.92
Low 6.5 27.5 0.92
Standard
Standard 5.8
5.8 30.5
30.5 1.02
1.02
Standard 5.8 30.5 1.02
Metals 2019, 9, 620 7 of 19

Table 4. Influence of different HI on the geometry.

HI Wall Thickness (mm) Part Height (mm) Layer Thickness (mm)

Low 6.5 27.5 0.92
Standard 5.8 30.5 1.02
High 4.8 35.2 1.17

3.2. Microstructural Characterization

• SS316L–Ni80-20 structures

The microstructural characterization of the HT and ST samples is shown in Figures 6 and 7,

respectively. The as-built structures presented a microstructure composed of austenite and δ-ferrite.
The austenite showed a dendrite morphology while the δ-ferrite presented a vermicular morphology.
At the interface between the SS316L and the BP segregation of a second phase was also observed in
both structures showing a vermicular morphology (Figures 6f and 7h). No carbide formation was
noted due to the low C content. The equivalent contents of Cr and Ni were calculated using the
chemical
Metals composition presented
2019, 9, x FOR PEER REVIEW in Table 1 (Equations (1) and (2)). 8 of 19

FigureFigure 6. Microstructurecharacterization
6. Microstructure characterization of of the
theHTHTsample,
sample, (a)(a)
macro
macroof the 3D structure;
of the (b,f) (b,f)
3D structure;
interface with the base material; (c,g) stainless steel (316L); (d,h) interface between SS316L and
interface with the base material; (c,g) stainless steel (316L); (d,h) interface between SS316L and Ni80-20; Ni80-
20; (e,i) Ni-based alloy (Ni80-20). (b–e) OM images; and (f–i) SEM images.
(e,i) Ni-based alloy (Ni80-20). (b–e) OM images; and (f–i) SEM images.
Metals 2019, 9, 620 8 of 19
Metals 2019, 9, x FOR PEER REVIEW 9 of 19

Figure
Figure 7. Microstructure
7. Microstructure characterization
characterization of ST
of the thesample,
ST sample, (a) macro
(a) macro of theof
3Dthe 3D structure;
structure; (b,h)
(b,h) interface
interface
with with
the base the base (c,i)
material; material; (c,i) stainless
stainless steel (316L);
steel (316L); (d,j) interface
(d,j) interface betweenbetween SS316L
SS316L and and mixed
mixed zone;
zone;
(e,k) (e,k)zone;
mixed mixed(f,l)
zone; (f,l) interface
interface betweenbetween the mixed
the mixed zonezone
andand Ni80-20;
Ni80-20; (f,m)
(f,m) Ni-based
Ni-based alloy(Ni80-20).
alloy (Ni80-
20).OM
(b–g) (b–g) OM images;
images; and (h–m)
and (h–m) SEM images.
SEM images.
Metals 2019, 9, 620 9 of 19

The results showed a Cr equivalent (Creq ) of 18.6% and Ni equivalent (Nieq ) of 14.1%.
The Creq /Nieq is about 1.32. This ratio indicated that the solidification mode was austenite-ferrite (AF).
The solidification sequence for the AF mode is L → L + A → L + A + (A+F)eut → A + Feut , showing
ferrite at cell and dendrite boundaries. The solidification occurs via a eutectic reaction due to the
presence of ferrite-promoting elements, like Cr and Mo. The ferrite-promoting elements migrate to
the subgrain boundaries during the cooling of the microstructure promoting the formation of ferrite
as terminal product [27]. Comparing both configurations (HT and ST), no morphological changes
of the δ-ferrite were observed due to the use of the same heat input during the fabrication of the
samples. In addition, at the interface between the SS316L and the BP, also SS316L, the formation of
a second phase was observed in both structures showing a vermicular morphology (Figures 6f and 7h).
Therefore, based on the morphology and the solidification sequence, the second phase was identified
as δ-ferrite.
The microstructure of the Ni80-20 was characterized as an austenite (γ) matrix with some
M7 C3 precipitates and laves phase. The γ displayed a dendritic morphology with intergranular and
intragranular second phases for both 3D systems. Figures 6e and 7g show the dendrite structure
observed for the HT and ST samples, respectively. The M7 C3 precipitates exhibited a roughly round
morphology. The precipitate were characterized using an XEDS analysis and classified as Cr7 C3 ,
as shown in Figure 8a. In this precipitates the main stabilizer is the Cr. M7 C3 precipitates are formed at
intermediate temperatures, however, in some cases the formation of M7 C3 at high temperatures is
noticed. The intermetallic compound laves with an A2 B type structure was also observed, where A =
Fe, Ni, Cr and B = Nb, Mo, Si [30]. Figure 8b shows the XEDS analysis of the laves phase classified as
(Ni, Fe, Cr)2 (Mo, Si). The laves phased exhibited blocky morphology roughly continuous at the grain
boundaries,
Metals 2019, 9, xas depicted
FOR in Figure 6h,i and Figure 7m for HT and ST samples, respectively.
PEER REVIEW 10 of 19

Figure 8. SEM images and XEDS analysis of the

Figure 8. the second
second phases
phases in
in Ni80-20.
Ni80-20. (a,b) Secondary
Secondary electron
(SE)
(SE) images
images in
in the
the Ni80-20;
Ni80-20; the
the numbers
numbers represent
represent the
the second
second phases
phasesanalyzed
analyzedbybyXEDS.
XEDS.

Regarding the mixed zone of the ST sample, an austenite (γ) matrix with dendritic morphology
was observed. Similar to the Ni80-20, the mixed zone showed M7 C3 precipitates and laves phase.
However, a change in the laves morphology was observed. Figure 9 displays a detailed view of the
γ/laves eutectic type morphology [30]. In addition, the mixed zone showed a refined microstructure
when compared with the Ni80-20 region. The variation in the laves morphology for this zone can be
explained by the changes in the chemical composition and the dilution during fabrication.

Figure 9. (a,b) SEM images of the laves phase in the mixed zone of the ST sample.

An XEDS line scan across the interface between the SS316L and Ni80-20 for the HT sample
(Figure 3, rectangle a) is shown in Figure 10. Significant variations of the Fe and Ni content through
the layers are observed. However, for Cr, Si, and Mo, no appreciable variations are noticed. An
MetalsFigure
2019, 9, 8.
620SEM images and XEDS analysis of the second phases in Ni80-20. (a,b) Secondary electron
10 of 19
(SE) images in the Ni80-20; the numbers represent the second phases analyzed by XEDS.

Figure 9. (a,b) SEM images of the laves phase in the mixed zone of the ST sample.

An
An XEDS
XEDSlinelinescan
scanacross
acrossthethe
interface between
interface the SS316L
between and Ni80-20
the SS316L for the HT
and Ni80-20 forsample
the HT(Figure
sample 3,
rectangle a) is shown in Figure 10. Significant variations of the Fe and Ni content
(Figure 3, rectangle a) is shown in Figure 10. Significant variations of the Fe and Ni content throughthrough the layers are
observed.
the layers However, for Cr,However,
are observed. Si, and Mo, fornoCr,
appreciable
Si, and Mo, variations are noticed.
no appreciable An approximation
variations are noticed. to the
An
dilution across the interfaces was made based on the classical equation proposed
approximation to the dilution across the interfaces was made based on the classical equation by DuPont [49], where
the concentration of any alloy element, i, in the layerofofany interest i ) is determined by the dilution and
proposed by DuPont [49], where the concentration alloy(C
element,
l i, in the layer of interest (𝐶 )
concentrations
is determined by of element i in the
the dilution and previous layer (Ciplof) and
concentrations the theoretical
element composition
i in the previous layerof(𝐶the) powder
and the
(Ci ) by Equation 3. Therefore, the dilution percentage (%D) across the layers 11 and 12 was calculated.
theoretical
t composition of the powder (𝐶 ) by Equation 3. Therefore, the dilution percentage (%D)
The
acrossNi80-20 composition
the layers 11 and 12waswasreached afterThe
calculated. theNi80-20
deposition of layer 13.
composition wasFor the Niafter
reached content, the %D is
the deposition
61% and13.
of layer 60% forthe
For theNilayers 11 and
content, the 12,
%Drespectively.
is 61% and 60%The for
HTthe
configuration could
layers 11 and be a disadvantage
12, respectively. in
The HT
the mechanicalcould
configuration and corrosion properties of
be a disadvantage inthe
thestructure dueand
mechanical to the rapid change
corrosion in the microstructures:
properties of the structure
due to the rapid change in the microstructures:
Cil = DCipl + (1 − D)Cit (3)
𝐶 = 𝐷𝐶 + (1 − 𝐷)𝐶 (3)
Metals 2019, 9, x FOR PEER REVIEW 11 of 19

Figure 10. XEDS line scan across the transition zone between SS316L and Ni80-20. Region (a) in
Figure 10. XEDS line scan across the transition zone between SS316L and Ni80-20. Region (a) in Figure
Figure 3.
3.

Figure 11 depicts the XEDS line scan across the interface between the SS316L and the mixed zone
Figure 11 depicts the XEDS line scan across the interface between the SS316L and the mixed zone
(Figure 3, rectangle b). Similar to the HT sample, only significant variations in the Fe and Ni content
(Figure 3, rectangle b). Similar to the HT sample, only significant variations in the Fe and Ni content
were observed. The %D for the layer 11 is 54%. For the other interface (Figure 12), between the mixed
were observed. The %D for the layer 11 is 54%. For the other interface (Figure 12), between the mixed
zone and the Ni80-20 (Figure 3 rectangle c), the variation in the chemical composition is less abrupt,
zone and the Ni80-20 (Figure 3 rectangle c), the variation in the chemical composition is less abrupt,
therefore, the %D is 24%.
therefore, the %D is 24%.
Hardness maps across the interfaces for both samples are shown in Figure 13 (taken from regions
marked in Figure 3). The hardness analysis for the HT sample shows a drastic change in the mechanical
properties, increasing from 150 to 230 HV0.1, approximately. Regarding the ST samples, a subtle
transition of the hardness was observed (Figure 13b,c). When compared with the HT sample, small
increases of the hardness values were noticed in both interfaces. The changes were around 30 HV0.1.
Even though no great difference in the microstructure of both samples are observed, the degree of
 Figure 10. XEDS line scan across the transition zone between SS316L and Ni80-20. Region (a) in Figure
Figure 10. XEDS line scan across the transition zone between SS316L and Ni80-20. Region (a) in Figure
3.
3.
Figure 11 depicts the XEDS line scan across the interface between the SS316L and the mixed zone
Metals Figure 11 depicts the XEDS line scan across the interface between the SS316L and the mixed 11zone
(Figure 3, 9,rectangle
2019, 620 of 19
b). Similar to the HT sample, only significant variations in the Fe and Ni content
(Figure 3, rectangle b). Similar to the HT sample, only significant variations in the Fe and Ni content
were observed. The %D for the layer 11 is 54%. For the other interface (Figure 12), between the mixed
were observed. The %D for the layer 11 is 54%. For the other interface (Figure 12), between the mixed
zone and the Ni80-20 (Figure 3 rectangle c), the variation in the chemical composition is less abrupt,
dilution
zone andandthethe hardness
Ni80-20 change
(Figure across the
3 rectangle c),interfaces. Thein
the variation hard
the variations in the HT sample
chemical composition is lesscould be
abrupt,
therefore, the %D is 24%.
atherefore,
disadvantage for industrial
the %D is 24%. applications of these systems.

Figure 11.
Figure 11. XEDS
XEDS line
line scan
scan across
across the
the interface
interface between
between SS316L
SS316L and
and the
the mixed
mixed zone
zone region.
region. Region
Region (b)
(b)
Figure 11. XEDS line scan across the interface between SS316L and the mixed zone region. Region (b)
in Figure
in Figure 3.
3.
in Figure 3.

Metals 2019, 9, x FOR PEER REVIEW 12 of 19

Hardness maps across the interfaces for both samples are shown in Figure 13 (taken from regions
marked in Figure 3). The hardness analysis for the HT sample shows a drastic change in the
mechanical properties, increasing from 150 to 230 HV0.1, approximately. Regarding the ST samples,
a subtle transition of the hardness was observed (Figure 13b,c). When compared with the HT sample,
small increases of the hardness values were noticed in both interfaces. The changes were around 30
HV0.1. Even though no great difference in the microstructure of both samples are observed, the
degree of dilution and the hardness change across the interfaces. The hard variations in the HT
Figure
sample XEDS
12. be
could line scan across
a disadvantage theindustrial
for interface between and the
applications ofmixed
these zone and Ni80-20. Region (c) in
systems.
Figure 12. XEDS line scan across the interface between and the mixed zone and Ni80-20. Region (c) in
Figure3.12. XEDS line scan across the interface between and the mixed zone and Ni80-20. Region (c) in
Figure
Figure 3.
Figure 3.

13.Hardness
Figure 13. Hardnessmaps
mapsacross the the
across interfaces for SS316L
interfaces to Ni80-20
for SS316L samples.
to Ni80-20 (a) HT sample,
samples. (a) HT interface
sample,
between
interface SS316L
betweenand Ni80-20,
SS316L (b) ST sample,
and Ni80-20, interfaceinterface
(b) ST sample, betweenbetween
SS316L SS316L
and mixed zone, and
and mixed (c)and
zone, ST
sample, interface
(c) ST sample, between
interface mixed mixed
between zone andzoneNi80-20.
and Ni80-20.

• SS316L–Ni625 structures
The macro analysis of the three samples fabricated for this system showed the presence of pores
at the mixed zone and Ni625. In addition, lack of fusion of the SS316L on the BM and cracks in Ni625
were noticed for the low HI sample (Figure 14). Regarding the microstructural characterization for
the stainless steel, no variations of the microstructure were observed with the decrease or increase of
the HI. Similar to the samples built of SS316L to Ni80-20, a microstructure composed of austenite and
Metals 2019, 9, 620 12 of 19

• SS316L–Ni625 structures

The macro analysis of the three samples fabricated for this system showed the presence of pores
at the mixed zone and Ni625. In addition, lack of fusion of the SS316L on the BM and cracks in Ni625
were noticed for the low HI sample (Figure 14). Regarding the microstructural characterization for
the stainless steel, no variations of the microstructure were observed with the decrease or increase
of the HI. Similar to the samples built of SS316L to Ni80-20, a microstructure composed of austenite
and δ-ferrite was observed. Figure 15b,h depicts the interface between the SS316L and the BM
where precipitation of the δ-ferrite is noticed. The γ matrix and the δ-ferrite displayed dendritic and
vermicular morphology, respectively.
The microstructure of the as-built Ni625 was composed of an austenite (γ) matrix with secondary
phases laves and δ-Ni3 Nb, and precipitates M7 C3 . Figure 15g,h shows the dendritic morphology of the
matrix and a detail view of the microstructure, respectively. The second phases were characterized by
XEDS analysis and their morphology. No MC-type precipitates were observed here, mainly due to
the lack of MC stabilizers like W, Ta, and Ti. The precipitates were classified as Cr7 C3 , as discussed
for the Ni80-20 alloy (Figure 16a). According to DuPont et al. [30] the high temperature precipitates
are usually replaced by intermediate temperature precipitates during thermal processing and/or
high temperature service. In addition, according to the XEDS analysis the laves phase is classified
as (Ni, Cr)2 (Nb, Mo) and presented an irregular and blocky morphology at the grain boundaries
(Figure 16b) [30]. Figure 17 depicts a needle-like phase observed in the as-deposited Ni625. Based
on the morphology, this phase was identified as δ-Ni3 Nb. Xu et al. [50] also reported the formation
of the δ-phase during the deposition of Ni625 using a pulsed Plasma Arc Deposition Process. The
δ-Ni3 Nb has an orthorhombic structure (ordered Cu3 Ti) and forms usually by cellular reaction at
low aging temperatures and intergranular precipitation at high aging temperatures [51]. Due to the
lack of coherence with the Ni matrix, therefore, not an effective strengthener, this phase is generally
undesirable in the Ni-based alloys. Moreover, a loss in ductility related with embrittlement could
be observed.
Based on the previous observations, the solidification sequence was analyzed for the as-deposited
Ni625. The solidification process starts with the primary reaction L → γ, causing the segregation of
Cr, Nb, and Mo to interdendritic spaces and grain boundaries. Thus, the precipitation of secondary
phases like M7 C3 and δ-Ni3 Nb is promoted in these regions. Then, the subsequent formation of M7 C3
consumes most of the carbon available until the reaction L → γ + laves + δ occurs, finishing the
solidification process [52].
The interfaces of the mixed zone with the SS316L and the Ni625, are shown in Figure 15d,f,
respectively. Similar to the Ni-based alloy on the top, the microstructure of the mixed was composed
of an γ matrix with laves, M7 C3 and δ-Ni3 Nb. However, no significant variations in the microstructure
of the mixed zone was noticed, but changes in the morphology of the phases. The mixed zone matrix
exhibited a dendritic morphology with secondary phases at the grain boundaries (Figure 15j,k,l). The
laves2019,
Metals phase
9, xpresented a more continuous morphology at the grain boundaries, as shown in Figure
FOR PEER REVIEW 13 of 15l.
19

Figure 14. Fabrication defects in the low HI sample (SS316L to Ni625). (a) Lack of fusion, and (b) crack.
Figure 14. Fabrication defects in the low HI sample (SS316L to Ni625). (a) Lack of fusion, and (b) crack.

Based on the previous observations, the solidification sequence was analyzed for the as-
deposited Ni625. The solidification process starts with the primary reaction L → γ, causing the
segregation of Cr, Nb, and Mo to interdendritic spaces and grain boundaries. Thus, the precipitation
of secondary phases like M7C3 and δ-Ni3Nb is promoted in these regions. Then, the subsequent
Metals 2019, 9, 620 13 of 19
Metals 2019, 9, x FOR PEER REVIEW 14 of 19

Figure
Figure 15. Microstructure characterizationofofthe
Microstructure characterization the SS316L
SS316L to Ni625
to Ni625 sample
sample withwith the standard
the standard HI, (a)HI, (a)
macro
macro of the
of the 3D 3D structure;
structure; (b,h) interface
(b,h) interface with thewith
basethe base material;
material; (c,i) stainless
(c,i) stainless steel(d,j)
steel (316L); (316L); (d,j)
interface
interface
between between
SS316L andSS316L
mixedand mixed
zone; zone;
(e,k) (e,k)
mixed mixed
zone; (f,l) zone; (f,l)between
interface interfacethebetween
mixed the
zonemixed zone
and Ni625;
and
(f,m)Ni625;
Ni625.(f,m) Ni625.
(b–g) (b–g) OM
OM images; andimages;
(h–m) and
SEM(h–m) SEM images.
images.
Metals 2019, 9, 620
x FOR PEER REVIEW 15
14 of
of 19
Metals 2019, 9, x FOR PEER REVIEW 15 of 19

Figure
Figure 16.
16. SEM
SEM images
images and
and XEDS
XEDS analysis
analysis of
of the
the secondary
secondary phases
phases in
in Ni625.
Ni625. (a,b)
(a,b) Secondary
Secondary electron
electron
Figure 16.
(SE) imagesSEM
images in images
in the and
the Ni625; XEDS
Ni625; the analysis
the numbers of the
numbers represent secondary
represent the phases
the secondary in
secondary phases Ni625. (a,b)
phases analyzed
analyzed by Secondary
XEDS. electron
by XEDS.
(SE)
(SE) images in the Ni625; the numbers represent the secondary phases analyzed by XEDS.

Figure 17. (a) Needle-like

Needle-like structure
structureininthe
theNi625
Ni625for
forthe
the high
high HIHI sample,
sample, (b)(b) detail
detail view
view of the
of the needle-
needle-like
Figure
like 17. SEM
(a) Needle-like
structure.
structure. SEM images.
images. structure in the Ni625 for the high HI sample, (b) detail view of the needle-
like structure. SEM images.
Regarding the difference in the microstructure of the three samples, the change in the heat input
did not cause significant variations either in the type of phases nor in their morphology. Table 5
shows the dilution percentage of the interfaces for the three samples of the SS316L to Ni625 system.
The dilution percentage was calculated using the chemical composition obtained by XEDS. Similar
results were observed for all the samples, with high dilution percentage for the interface between the
SS316L and Ni625. However, due to the decrease of the heat input for one sample, a drop in the %D
was observed. Moreover, for the three samples the percentage of dilution of the mixed zone to Ni625
interfaces is lower than the interfaces SS316L to mixed zone. The XEDS profile for the standard HI
sample is shown in Figure 18, where the interfaces SS316L to mixed zone and mixed zone to Ni625 are
located on the left and right, respectively. Noteworthy variations are observed in the Fe, Ni, and Cr
profiles. However, the Cr profile was roughly constant throughout the interfaces. Similar results were
noticed for the Mo, Si, and Nb. For the other two samples, similar XEDS profiles were obtained.
The hardness measurements for the three samples of the system SS316L to Ni625 are shown in
Figure 19 (taken from regions marked in Figure 3). It is possible to see the changes in the material for
the interfaces SS316L to mixed zone Figure 19a,e; except for the standard heat input where some zones
presented low hardness
Figure values
18. XDS line Figurethe
scan across 19c. The values
interfaces of theare between
regions in the120 andto180
SS316L HV0.1.
Ni625 On the other
structure.
hand, theFigure 18. XDS
interface line scan
between the across
mixedthezoneinterfaces
and theofNi625
the regions in the
depicted a SS316L
higher to Ni625 structure.
hardness values, between
180 and
The270 HV0.1.measurements
hardness The as-deposited for Ni625
the threeof the standard
samples heat
of the input SS316L
system showedtoa Ni625
higherare
hardness.
shown in
The hardness measurements for the three samples of the system SS316L to Ni625
Figure 19 (taken from regions marked in Figure 3). It is possible to see the changes in the material are shownfor
in
Figure 19 (taken from regions marked in Figure 3). It is possible to see the changes
the interfaces SS316L to mixed zone Figure 19a,e; except for the standard heat input where some in the material for
the interfaces
zones presented SS316L to mixedvalues
low hardness zone Figure
Figure 19c.
19a,e;
The except
valuesfor
arethe standard
between 120heat
and input whereOn
180 HV0.1. some
the
zones presented low hardness values Figure 19c. The values are between 120 and
other hand, the interface between the mixed zone and the Ni625 depicted a higher hardness values, 180 HV0.1. On the
other hand, the interface between the mixed zone and the Ni625 depicted a higher hardness values,
MetalsFigure
2019, 9,17.
620(a) Needle-like structure in the Ni625 for the high HI sample, (b) detail view of the needle-
15 of 19
like structure. SEM images.

Metals 2019, 9, x FOR PEER REVIEW 16 of 19

between 180 and 270 HV0.1. The as-deposited Ni625 of the standard heat input showed a higher
hardness.
Figure 18.
Figure XDS line
18. XDS line scan
scan across
across the
the interfaces
interfaces of
of the
the regions
regions in the SS316L to Ni625 structure.

The hardness measurements for the three samples of the system SS316L to Ni625 are shown in
Figure 19 (taken from regions marked in Figure 3). It is possible to see the changes in the material for
the interfaces SS316L to mixed zone Figure 19a,e; except for the standard heat input where some
zones presented low hardness values Figure 19c. The values are between 120 and 180 HV0.1. On the
other hand, the interface between the mixed zone and the Ni625 depicted a higher hardness values,

Figure 19.
Figure 19. Hardness maps for the the SS316L
SS316L to to Ni625
Ni625 samples
samples across
across the
theinterfaces.
interfaces. (a,c,e)
(a,c,e) Interface
Interface
between the
between the SS316L
SS316L and
and mixed
mixed zone;
zone; (b,d,f)
(b,d,f) interface
interface between
between the
the mixed
mixed zone
zone and
and Ni625.
Ni625. (a,b) Low
Low
HI;
HI; (c,d)
(c,d) standard
standard HI;
HI; and
and (e,f)
(e,f) high
highHI.
HI.

4. Conclusions
Functionally graded structures of SS316L to Ni80-20 and SS316L to Ni625 were fabricated by 3D
Plasma Metal Deposition. The structures exhibited a homogeneous layer without any external
delaminations or cracks. However, some pores were observed at the Ni-based.
Regarding the SS316L to Ni80-20 structures, the hard transition and smooth transition
Metals 2019, 9, 620 16 of 19

Table 5. Percentage of dilution for the system SS316L to Ni625 fabricated with three heat inputs, low,
standard, and high.

Interface %D
Low (L) HI
SS316L to mixed zone 38.9
mixed zone to Ni625 26.3
Standard (S) HI
SS316L to mixed zone 55.2
mixed zone to Ni625 25
High (H) HI
SS316L to mixed zone 51.1
mixed zone to Ni625 23.1

4. Conclusions
Functionally graded structures of SS316L to Ni80-20 and SS316L to Ni625 were fabricated by
3D Plasma Metal Deposition. The structures exhibited a homogeneous layer without any external
delaminations or cracks. However, some pores were observed at the Ni-based.
Regarding the SS316L to Ni80-20 structures, the hard transition and smooth transition
configurations were investigated. The smooth transition showed a better performance due to
a continuous change in the chemical composition of the structures. The SS316L to Ni625 configuration
did not show significant variations on the microstructure with the change of the heat input.
Different microstructures were recognized in the 3D structures. In the austenitic stainless steel an
austenite (γ) matrix with δ-phase, product of the AF solidification mode was observed. The Ni80-20
exhibit austenite (γ) matrix with some M7 C3 precipitates and laves phase. On the other hand, the
as-deposited Ni625 showed an austenite (γ) matrix with laves, M7 C3 and δ-Ni3 Nb. In the mixed zones
for both configurations similar microstructures were noticed, however, changes in the morphology
were observed.

Author Contributions: Conceptualization: J.R.; K.H.; A.H.; P.M.; methodology: J.R.; K.H.; A.H.; validation:
J.R.; K.H.; A.H.; formal analysis: J.R.; K.H.; investigation: J.R.; K.H.; writing—original draft preparation:
J.R.; K.H.; writing—review and editing: J.R.; K.H.; A.H.; P.M.; visualization: J.R.; supervision: P.M.; project
administration: P.M.
Funding: This research received no external funding.
Acknowledgments: The authors would like to thank all the members of the Institute of Joining and Assembly for
the support during the development of this work. J.R. would like to thank the financial funding of the German
Academic Exchange Service (DAAD) for the scholarship Research Stays for University Academics and Scientists,
2018 (57381327). In addition, special thanks to Gökhan Ertugrul and Kevin Abstoß for the support during the
experimental work.
Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the
study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to
publish the results.

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manufacture porous and functionally graded structures for load bearing implants. J. Mater. Sci. Mater. Med.
2009, 20, 29–34. [CrossRef]
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