A few degrees at the nozzle can decide whether a 3D printed aluminum part comes out sound or riddled with pores. That is the central finding of a new study from The University of Manchester, published in Materials & Design, which pins down how the thermal conditions of molten metal deposition (MMD) govern defects and grain structure in aluminum alloy 4043.
The MMD process was developed by ValCUN BV, a Belgium-based firm focused on making metal 3D printing affordable and easy to deploy. The paper, released on 25 June 2026, gives engineers something they have largely lacked for this emerging process: a physics-based set of levers for controlling part quality. And the levers turn out to be small. Modest changes in nozzle and substrate temperature reshaped the internal microstructure and strongly influenced how many voids formed inside each component.
Cooler runs, cleaner metal
The mechanism the team documented is a chain reaction rooted in cooling rate. Higher nozzle and substrate temperatures slowed the rate at which deposited aluminum solidified. That slower cooling coarsened the grain structure and drove up porosity, the tiny internal voids that undermine a part’s strength and fatigue life. Run the process cooler, and the opposite held: faster solidification produced finer grains and noticeably fewer defects.
The researchers also found that grain size and porosity move together, a tight coupling that lets a single process parameter influence both structure and defect count at once. Just as telling, defect levels and grain size generally fell as the build climbed through successive layers, evidence that thermal conditions do not stay fixed during a print but evolve as the part grows and sheds heat differently layer by layer.
Crucially, none of this came at the cost of performance. Despite the presence of some porosity, the printed samples returned hardness and elastic modulus values inside the expected band for aluminum 4043, on par with parts made by conventional routes.
“Understanding how processing conditions affect the internal structure of a printed component is essential if additive manufacturing technologies are to be used more widely in demanding industrial applications,” said co-authors Dr Fan Wu and Dr Wajira Mirihanage of Manchester’s Department of Materials. “Our study shows that relatively small adjustments in manufacturing temperatures can have a major impact on defect formation and microstructural development.”
Filling a data gap
To map the process, the team printed 4043 samples across a range of nozzle and substrate temperatures, then interrogated them with advanced microscopy, examining grain structure, crystallographic orientation and the distribution of pores, before mechanical testing measured how the material actually performed. The result is a clear line drawn from process settings through microstructure to defect formation.
That chain has been missing. As Wu and Mirihanage noted, MMD remains young and “there is currently limited understanding of how processing conditions affect the final material.” By tying parameters to microstructure and defects, the study hands manufacturers a foundation for tuning builds deliberately rather than by trial and error, the groundwork, the authors argue, for more reliable aluminum components and for pushing MMD toward applications where quality, consistency and efficiency are non-negotiable.
Turning defect formation into something engineers can predict
ValCUN sells MMD as a simpler, cheaper route to aluminum parts, but a process this young only reaches production once its metal behaves predictably. The metallurgy has lagged the machine, with little data on how MMD’s settings translate into grain structure and porosity. That gap is what the Manchester study begins to close and what lets MMD be qualified for demanding work rather than sold on cost alone.
The wider field has been closing the same gap for laser-based metal AM. One 2026 effort engineered a new aluminum alloy specifically for the extreme cooling of directed energy deposition, building the chemistry from the process up to reach sub-5-micron grains and suppress the cracking that thermal mismatch usually causes.
Others are attacking the measurement side: a POSTECH and KIMS team unveiled an AI framework that reads a part’s internal microstructure and pore distribution to predict its strength before any physical test.
Control is the next frontier. Fraunhofer’s UltraGRAIN project set out to tailor microstructures during the build rather than accept whatever it produces, and RMIT University has shown that ultrasonic vibration can refine grain structure mid-print. Each mirrors the shift the Manchester work embodies for MMD: treating grain size and defect count as outputs an operator can steer, not fixed consequences of the process.
Manchester’s contribution is to run that same playbook on the aluminum process built for cost and deployability. If MMD is to earn industrial trust, its defect physics had to be mapped and now they are starting to be.
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Featured image shows visualization of the EBSD scan region on the 2D model. Image via The University of Manchester.
