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

While the array of current applications of MMAM (ii) powder recyclability, (iii) AM in-process monitoring,
demonstrates its versatility, a deeper understanding of the (iv) MM process monitoring, (v) MM mechanical testing
underlying microstructures, mechanical behavior, and standardization, and (vi) thermal- and thermo-mechanical
modeling and simulation of materials used in MMAM is modeling as presented in Figure 1.
crucial to ensure wider adoption. The goal of this review
article is to identify a potential road map for advancing the 2. Overview of MMAM processes
field of MMAM by providing a detailed insight into the 2.1. Laser powder bed fusion (LPBF)
current state of microstructure, mechanical characteristics,
and modeling and simulation of MMAM structures. In LPBF is a metal-AM process that utilizes a high-powered
contrast to previous review articles (Table A1), which scanning laser beam to selectively melt a region of a
have primarily focused on FGM, this article gathers and powder bed onto a metal substrate in a layer-by-layer
synthesizes the results reported in empirical studies fashion to produce three-dimensional (3D) solid metal
that investigated the P-S-P relationships of MMAM parts. Melting occurs on metal powder fabricated through
structures, with a roadmap toward further development various powder processes (e.g., gas atomization, plasma
of MMAM. Prior literature has explored various aspects, atomization, plasma rotating electrode process, hydride-
including applications, 60-66 challenges in FGM-LDED, 60,67 dehydride, and wire atomization) inside a sealed inert
challenges in thermal properties and creation of harmful gas build chamber, usually filled with argon or nitrogen,
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compounds, challenges in steel- and metal-based which is pumped throughout the build chamber to
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FGM, 69,70 progress in structures and functionality of FGM, maintain a low oxygen content. Previous studies indicate
manufacturing techniques, 60,64,72-75 experimental studies that an oxygen content of 300 – 1,000 ppm is required to
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on metal-metal, metal-ceramic and metal-intermetallic prevent oxidation during the manufacturing process.
gradient, and numerical studies on material science and In addition to maintaining a low oxygen content, inert
engineering, 67,76,77 and practical applications. 62,63,67,70,71,74,75,78 gas is used to reduce the likelihood of defects associated
While these review articles provide abundant information with high oxygen levels, which may include irregular melt
on MMAM, a critical research gap and future direction track morphology, irregular melt pool surface tension, and
(applicable to MMAM) concerning structures with discrete spattering. 80-83 The build plate, as shown in Figure 3A, is
transitions remain unexplored. This article aims to address made of a material similar to the feedstock metal and can
that gap in the following sections. be preheated to minimize thermal gradients and reduce the
buildup of thermally induced residual stresses or thermally
To systematically address these research gaps and induced part distortion. LPBF imposes process-specific
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advance the understanding of structures with discrete design constraints on part geometry and material selection
transitions, this review article is organized as follows. but offers an elevated level of design complexity compared
This review article is divided into five main sections, to traditional subtractive manufacturing methods. The
followed by a discussion and future trends. The first metal alloys that are compatible with LPBF include Ti,
section includes an overview of the processing principles Al, Fe (steels), cobalt–chromium, Ni, and Cu-based alloys
of the three main metal MMAM processes, along with (analogously, any metal that can be welded). Similar
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a discussion on MM melt pool formation mechanisms to the single-material LPBF process, in MM-LPBF, the
and alloy compatibility observed across the discussed powder that is not melted is retained in the powder bed
processes. The second section consists of a detailed review while dissimilar material is deposited over it, following a
of macro- and micro-structural characteristics observed process analogous to single-material LPBF. The dissimilar
at bimetallic interfaces (e.g., microstructural growth, material powder spreading mechanism in MM-LPBF
defects, metallurgical bonding, intermetallic phases) comprised various methods, such as (i) blade-based
from reported studies. The third section focuses on the dissimilar material spreading, (ii) ultrasonic-based dual
available data on the mechanical properties of MMAM powder dispenser, (iii) electrophotographic-based dual
(e.g., microhardness, tensile strength, flexural strength,
compression, fatigue, etc.). The fourth section focuses on powder dispense, and (iv) “blade + ultrasonic” hybrid
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the modeling and simulation (e.g., phase transformation, powder spreading technique, which were used.
melt pool formation, computer coupling of phase diagrams Advantages of MM-LPBF include (i) the ability to
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and thermochemistry [CALPHAD], finite element analysis manufacture intricate 3D structures monolithically, (ii)
[FEA]) approaches for bimetallic structures. Finally, high resolution and rigorous build accuracy with dimension
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the review article closes with a discussion on current error lower than 100 μm, (iii) better processing accuracy
technological roadblocks in advancing the development and compared to other metal-AM due to smaller powder size
adoption of MMAM, specifically: (i) Alloy compatibility, (10 – 50 μm), larger laser spot diameter (50 – 80 μm),
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Volume 1 Issue 2 (2025) 4 doi: 10.36922/ESAM025180010