Page 58 - MSAM-3-2

Materials Science in Additive Manufacturing Heat treatment on bimetallic parts

A B

Figure 9. The X-ray diffraction (XRD) patterns of 17-4PH/IN625 bimetallic samples under different heat treatment conditions: (A) Overall XRD patterns;
(B) The XRD peak shift at specific location. (A) The XRD patterns and (B) the XRD peak shift at the specific locations of 17-4PH/IN625 bimetallic samples
under different treatment conditions.

whether they underwent heat treatment, the reaction in hardness can be attributed to the reduction in
layers predominantly consist of oxides and precipitates porosity and the diminution of pore size. However,
exhibiting a high concentration of Nb and Mo elements. over time, an enlargement in grain size is discernible, as
In addition to the Ni matrix, peaks indicative of MC indicated in Figure 3, adversely affecting hardness. The
carbides were also identified. Similarly, MC carbides predominant phases in 17-4PH consist of ferrite and
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were found in IN625 treated under 1150°C. The heat martensite. Slower cooling rates or extended heating
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treatment process appears to diminish the presence of periods facilitate the migration of carbon and other
carbides, as evidenced by Figure 4, showing a reduction alloying elements, leading to a reversal from martensite
in both the size and quantity of carbides. Simultaneously, to ferrite. An increase in the proportion of ferrite, at the
as the duration increases, the peak position shifts due to expense of martensite, typically results in a diminished
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stress relaxation and precipitate formation, as depicted hardness, given the inherent softness of ferrite compared
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in Figure 9B. As IN625 alloy and 17-4PH possess different to martensite. A comparable trend was observed in the
coefficients of thermal expansion, the stress level escalates microhardness of 17-4PH manufactured through PBF
with prolonged time. Previous literature has reported as time progressed, manifesting a decrease. This trend is
similar observations regarding the behavior of IN625 attributed to the presence of samples consisting of fine
under various heat treatment conditions, especially when lath martensite lacking distinct preferred orientations,
compared to stress-free powder materials. 50 coupled with the elimination of delta-ferrite following
homogenization. Meanwhile, through aging followed
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3.4. Effects of heat treatment on microhardness by homogenization, fine particles (precipitates) form
The impact of heat treatment on the microhardness and impede dislocation movement, thereby augmenting
of 17-4PH/IN625 bimetallic parts was determined hardness compared to the material devoid of aging, as
through microhardness testing. Figure 10 presents the illustrated in Figure 10. For IN625, the microhardness
microhardness distributions along the bimetallic interfaces initially experiences a marginal increase from 208
under both as-sintered and heat-treated states. HV to 209 HV , before subsequently decreasing to
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1.0
205 HV . The strengthening of IN625 alloy mainly
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The microhardness of the bimetal improves ensues through solid solution hardening facilitated by
uniformly across all areas, including both the 17-4PH Cr, Mo, and Nb. However, during heat treatment, the
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side and the IN625 side, as well as the interface, after segregation of these elements within the transition zone
undergoing heat treatment. In the case of 17-4PH forms precipitates, thereby reducing the strengthening
steel, there is an initial increase in hardness, peaking effect in IN625. In alternative bimetallic structures
at 367 HV , followed by a subsequent decline. This consisting of IN625 and Ti6Al4V, the absence of Cr- and
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observation aligns with other studies wherein a peak Mo-enriched phases within the Ni matrix on the IN625
hardness value of approximately 360 HV was noted side culminates in a reduced hardness gradient.
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1.0
following solution heat treatment. The enhancement With prolonged holding time, the enlargement of
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Volume 3 Issue 2 (2024) 11 doi: 10.36922/msam.3281