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Engineering Science in
Additive Manufacturing Multi-material additive manufacturing of metals

and traction in the AM community, especially for MMAM; moderate material. Generally, material pairs that have the
hence, it is presented separately here and in subsequent most contrasting thermal properties are reported to have
discussions. In WAAM, a wire of heterogeneous a weaker metallurgical bond at the interface. For example,
characteristic materials is fed into the melt pool created a moderately conductive steel alloy, which is commonly
by the welding arc along the designed path to form a paired with a highly conductive Cu alloy (Table 1), is
component (Figure 3C). WAAM is compatible only with usually reported as having poor bonding, where local
wire feedstock, like the feeding rods used in most welding cracking is the most common defect near the interface.
processes. As a result, material restrictions include only Reports show that cracking occurs in these Cu alloys for
materials that are ductile enough to be pulled into the two main reasons: (i) A mismatch of the thermo-physical
wire. Weldable materials such as SS, Ni, Ti, and Al alloys material properties and (ii) an infiltration of Cu to the grain
are commonly used in WAAM. 71,101 Like MM-LDED, boundaries in steel, which provides micro-cracks during
multi-material WAAM (MM-WAAM) follows the same melting due to thermal mismatch. Optimizing process
procedure as the dissimilar materials deposited through a parameters, namely those related to the temperature
wire-fed nozzle. Building upon the basic WAAM process, distribution (e.g., laser speed, power, and scanning
the major advantages and challenges associated with strategy), can help mitigate the severity of the thermal
MM-WAAM are outlined below. gradient across the interface and improve metallurgical
bonding.
The advantages of WAAM coincide with those of
LDED. While WAAM offers a higher deposition rate than The dilution effect is another common phenomenon
PBF, it comes at the expense of increased surface roughness observed in MMAM processes and describes the
and reduced dimensional accuracy. To improve the gradual decrease in alloy blending as a part is built in the
surface finish and dimensional accuracy, components are vertical direction. Dendritic cracking commonly occurs
often required to undergo post-processing or subtractive perpendicularly to the boundary of the fusion zone and
machining to produce near-net shape geometry. Alongside extends gradually into the material of the higher thermal
a higher deposition rate, WAAM is associated with an stress, which is usually the less conductive material.
inexpensive machine cost, simple configuration, high Dendritic cracking is influenced by the thermomechanical
efficiency, and large-scale component fabrication. 71,102-105 stresses that arise due to the temperature gradients during
the solidification processes. The presence of a secondary
2.4. Mechanisms of melt pool formation alloy in the melt pool can exacerbate these stresses and
The formation of a bi-metallic interface using MMAM increase the likelihood of crack formation. Cracking from
processing involves thermal and fluidic interactions dissimilar metal mixing is further discussed in Section
between the solid substrate and the unmelted feedstock of 2.5. On a related note, element diffusion at the interface
the dissimilar metal. A key challenge in MMAM processing describes the causal mechanisms behind the blending of
two dissimilar alloys within the melt pool. Diffusion is
is mitigating the defects that tend to form at the interface aided by the Marangoni convective forces, driven by the
of the dissimilar metals, mainly caused by the mismatch
in thermal properties. This mismatch may induce macro- surface tension gradients of the melt pool’s molten fluid.
strains near the interface and cause defects such as This leads to a non-homogeneous distribution of elements,
cracking and porosity. Understanding the mechanisms of commonly observed by the solidified heat-affected zones
through energy dispersive spectroscopy (EDS). The
melt pool formation and the subsequent cooling process difference in material density may also play a role in how
is a necessary foundation for mitigating the formation of the metals mix and interact under gravitational forces.
defects in that region.
All factors considered in this section play a crucial role
In all metal MMAM processes, many studies in identifying the alloy compatibility between dissimilar
characterize the interface as having grain refinement and materials in MMAM.
report an increase in hardness across the transition. For
processes such as MM-LPBF, a high cooling rate (10 K/s) 2.5. Alloy compatibility in MM mixtures
7
compared to other laser or arc-welding processes (10 K/s) Achieving compatibility between dissimilar alloys in
3
may cause additional grain refinement. Grain refinement bimetallic structures is a critical challenge, as differing
may be further exaggerated for all processes while printing physical, chemical, and mechanical properties of each alloy
bi-metallic structures, which pair highly conductive can significantly influence the performance and longevity
materials (such as Cu alloys) with a moderate alloy (such of the components. The alloys are designed precisely to
as steel), wherein the highly conductive material may act tailor the elemental proportions and thermomechanical
as a heat-sink and expedite the cooling rate of the more processing conditions to generate the ideal microstructural

Volume 1 Issue 2 (2025) 6 doi: 10.36922/ESAM025180010