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Development of a sulfur-based self-terminating process for support removal and surface finishing of additively manufactured Ti-6Al-4V
Raikar, Subbarao
Raikar, Subbarao
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2022
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Abstract
The advent of Additive Manufacturing (AM) has widened the design space and
complexity of Ti−6Al−4V (Ti64) components. Its adoption is continually increasing in the
AM industry due to its benefits over conventional manufacturing techniques. However, the
complex geometry designs fabricated using AM often result in high post-processing costs
due to challenges associated with support removal, trapped powder removal, and improving
surface finish. Traditional means of support removal and surface finishing involve using
labor-intensive and expensive mechanical tools. Furthermore, the need for mechanical
access to supports and surfaces hinders the AM design space.
To address some of these issues, this work introduces a novel sulfur-based
self-terminating etching process for support removal, trapped powder removal, and surface
smoothing of Ti64 alloys. In this process, the part is first sulfidized in a furnace with
elemental sulfur to convert the outer 50 − 150 μm of the part and supports to sulfides. This
sulfide layer is then selectively dissolved in a solution of sulfuric acid (H2SO4) and sodium
molybdate (Na2MoO4). Etching is limited to the sulfide layer because the underlying
unsulfidized Ti64 is protected from the acidic environment. The inherently self-terminating
nature of this process enables geometry-agnostic post-processing of Ti64 components with
a specific amount of material removed from all the surfaces. As a result, support structures
and trapped powder can be removed with ease at the same time that the surface finish is
improved. To make this process more time-efficient, sulfidation in the presence of iodine is
explored. Sulfidation with iodine increased the amount of Ti64 consumed for a given
sulfidation temperature and time. However, the Ti64 consumed is uneven and the surface
rougher than earlier. Fatigue performances of the as-printed and the post-processed Ti64
specimen are evaluated. The sulfur-based post-processing technique increased the average
fatigue life by 3× from about 7,000 cycles to 30,000 cycles.
In this work, a material removal predictor is developed to predict the amount of Ti64
consumed for a given sulfidation temperature and duration by studying the sulfidation
kinetics of Ti64. The role of H2SO4 and Na2MoO4 is also studied to establish their effect
on the dissolution of titanium disulfide (TiS2), which led to the formulation of a ∼ 20%
more efficient etching recipe. In summary, a novel sulfur-based process for support removal
and surface finishing is introduced to reduce post-processing costs while expanding the
design space for AM Ti64 parts.
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