Page 54 - ESAM-1-1

Engineering Science in
Additive Manufacturing Additive manufacturing of EH36 steels

retained austenite content in the steel. The presence of pores, which typically vary in size from a few microns to
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retained austenite was particularly pronounced at higher several tens of microns, are often induced by inadequate
cooling rates, resulting in a core-shell dendritic structure, laser power and improper scanning speed. Such porosity
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with martensite α’ in the core and retained austenite γ in compromises the fatigue resistance and tensile strength
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the shell. Nitrogen absorption during processing further of as-print components, particularly under dynamic or
stabilized the austenite, contributing to this segregation cyclic loading conditions. Stress concentration around
phenomenon. This precise thermal control contributes these voids accelerates crack initiation and propagation,
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to the high strength and dimensional accuracy of PBF- reducing the overall durability of the component. In
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LB-built components. In contrast, DED-LB generally DED-LB, the incidence of gas porosity is lower than in
produces a significantly larger melt pool, ranging from 1 PBF-LB due to the larger melt pools and slower cooling
to 3 mm in width and 0.5 – 2 mm in depth. The slower rates, which allow trapped gases to escape more effectively.
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cooling rates associated with the larger melt pools lead However, inconsistencies in powder delivery or improper
to broader HAZs, typically measuring several hundred shielding gas flow can still result in localized porosity. The
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micrometers. Similarly, segregation in DED-LB might be gas pores are normally a size of a few microns for AMed
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comparatively less pronounced than in PBF-LB. The HAZ EH36 steel produced by DED-LB. These localized pores
in DED-LB is characterized by a mixture of acicular ferrite, can degrade elongation at break and reduce ductility by
bainite, and martensite, where the melt pool boundaries serving as stress concentration sites, especially under cyclic
are delineated by short, elongated ferrite grains, which loading conditions. DED-Arc, which uses continuous
are transformed phases from the prior austenite grains. wire feedstock, inherently minimizes the risk of gas
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This microstructural coarsening in the HAZ necessitates porosity by avoiding issues related to powder handling.
careful optimization of process parameters to mitigate However, shielding gas coverage during deposition may
toughness reductions caused by heat accumulation. DED- still cause pores, thus damaging fatigue resistance and
Arc produces the largest melt pools, approximately 3 – fracture toughness of DED-Arc fabricated EH36 steel.
10 mm in width and 2 – 5 mm in depth, due to its extensive Mitigation strategies for gas porosity involve optimizing
thermal input. This results in wide HAZs often exceeding energy density, improving shielding gas flow, and ensuring
several millimeters, with significant grain coarsening consistent material feed. In PBF-LB, a balanced volumetric
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observed near the fusion line. The microstructure in energy density ensures sufficient melting and degassing of
DED-Arc contains a mixture of acicular ferrite, bainite, the melt pool. For DED-LB and DED-Arc, maintaining
and martensite-austenite phases, while the EBSD map can proper shielding gas flow and optimizing feed rates are
effectively illustrate the transition from fine ferrite grains critical to minimizing gas porosity
in the melt pool center to coarse equiaxed grains in the Lack-of-fusion defects occur when adjacent layers
HAZ. Effective interpass temperature management and or scanning tracks fail to bond fully due to insufficient
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optimized travel speeds are essential to control thermal energy input or improper material feed. In PBF-LB, these
stresses and maintain the mechanical performance of defects are often caused by low laser power, high scanning
DED-Arc fabricated components. In summary, PBF-LB speeds, or inadequate hatch spacing. These defects are
produces the smallest melt pools and narrowest HAZs, characterized by irregularly shaped voids along layer
resulting in fine microstructures and high precision. interfaces, typically ranging in size from a few microns
DED-LB offers larger melt pools and broader HAZs with to hundreds of microns, depending on the energy density
moderately coarse microstructures, balancing deposition and scanning strategy. Such voids create weak interfacial
rates, and mechanical properties. DED-Arc, with its regions, which are prone to crack propagation, significantly
extensive melt pools and wide HAZs, is best suited for lowering the tensile strength and ductility of AMed EH36
large-scale components but requires careful thermal steel fabricated using PBF-LB. In addition, under fatigue
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management to mitigate grain coarsening and residual loading, these voids also act as stress concentrators, serving
stresses. These differences emphasize the need for tailored as initiation sites for fatigue cracks and accelerating failure
process parameters to achieve optimal microstructure and during cyclic stress. Lack of fusion generally arises from
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mechanical properties in AMed EH36 steel. inconsistent powder delivery or suboptimal laser power
in DED-LB. Incomplete melting and bonding along layer
3.2. Defects in AMed EH36 steel
boundaries lead to a reduction in fracture toughness and
Gas porosity arises from entrapped gas within the melt elongation at break. These defects reduce effective nominal
pool during solidification. In PBF-LB, the rapid cooling stress, facilitate multiple crack initiations, and significantly
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rates and improper inert gas flow creates small, spherical shorten fatigue life. The larger melt pool in DED-LB
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pores distributed uniformly throughout the matrix. These mitigates the extent of these defects compared to PBF-LB,

Volume 1 Issue 1 (2025) 5 doi: 10.36922/ESAM025060005