• Al 3 Zr L1 2 / θ' co-precipitates form faster in additively manufactured Al-Cu-Mn-Zr (ACMZ) alloys • Rapid solidification allows for higher matrix solute contents • Interfacial Zr segregation rate is dependent on the Zr matrix content • Theta prime precipitates are perfect templates for planar Al 3 Zr L1 2 formation Controlled Mn and Zr additions to Al-Cu alloys have allowed for the improved retention of mechanical properties after extended 350°C exposures by stabilizing the main strengthening θ' (Al 2 Cu) phase. Ultimately, θ' /L1 2 (Al 3 Zr) co-precipitate formation stabilizes θ' most effectively; however, Zr diffuses sluggishly and has low solubility in aluminum castings. Increasing the Zr segregation rate would allow for faster and more effective θ' /L1 2 co-precipitation. It is demonstrated that the Zr segregation rate is faster when the Zr matrix content is higher. A much higher Zr matrix content facilitated by rapid cooling during additive manufacturing (AM) was achieved that produces faster θ' /L1 2 co-precipitation, which is shown by scanning transmission electron microscopy and atom probe tomography experiments. It was also found that Zr continuously segregates to θ' interfaces up to the most aggressive heat treatment studied such that planar L1 2 precipitates remain after the metastable θ' dissolves. In this manner, we demonstrate that θ' coherent interfaces serve as perfect templates to form stable planar L1 2 precipitates that can provide strength at higher temperatures than traditional θ' strengthened AlCu alloys. This work introduces an alloy design strategy that uses metastable precipitates to quickly nucleate and grow co-precipitates with a desired geometry that contain slow diffusing elements. These ideas can be applied to engineer more heat resistant alloys by taking advantage of high solute matrix contents enabled by rapid cooling during additive manufacturing.

• Powder delivery systems for directed energy deposition processes can exhibit oscillatory variation in mass flow rates, matching the characteristic speed of individual hopper components. • Flow rate variation can be observed at the nozzle exit, both by offline weight measurement and by imaging the powder flow. • Flow rate variation is observed to have a measurable effect on melt track height, with the potential to cause build quality issues. • Melt pool imaging shows a correlating pattern, indicating that flow rate variation can affect melt pool image brightness, and may influence the behaviour of image based control approaches. Powder flow rate is a key parameter in Directed Energy Deposition (DED) processes. During a typical build, if powder flow rate is reduced for just 1 second, 30 mm of melt track is affected. Consequently, even a small variation in powder flow rate can have significant implications on build quality. In this work, the powder flow stability for different types of 316 L steel powders was quantified using a combination of methodologies including offline weight measurements, flow imaging, in-situ build data and coaxial melt pool imaging. Flow rate oscillation was observed, correlated with the periodicity of powder hopper turntable rotation, at sufficient magnitude to cause build quality effects and be identifiable in coaxial melt pool imaging. The implications of flow rate variation on the use of melt pool imaging for closed-loop control are discussed.

• Demonstration that polymer melting and coalescence lags behind the location of laser – material interaction. This suggests that absorption of the laser energy and subsequent melting is slow relative to the scan speed. • This instrument has captured phenomena which would otherwise not be possible with slower camera frame rates, less magnification, and in a non-heated build environment. The phenomena responsible particle fusion during selective laser sintering (SLS) have been modeled, but attempts at experimental validation have been limited to unrealistic conditions compared to those used in commercial SLS processes. We have designed, implemented, and tested an instrument that is capable of performing in situ microscopy to image particle-scale dynamics of the SLS process at realistic scan speeds, temperatures, and gas environments. The instrument uses a combination of purchased and manufactured components to capture video at length scales of ∼1 µm and timescales of ∼1 ms. Spatial and temporal calibration is performed so that the laser position can be mapped onto the recorded video. Results show that laser melting of polymer is accompanied by particle rearrangement that occurs over an extended time (∼10 ms) after the laser scans a particular location. The findings highlight the importance of performing in situ microscopy under realistic conditions for SLS.

Cold Spray Additive Manufacturing (CSAM) is a well-established technology that has recently attracted interest for forming 3D shapes in a fast and scalable fashion. Nonetheless, the resulting surface of cold sprayed parts normally requires post-deposition machining to achieve the desired surface finish. In this work, a convolution-based digital framework for CSAM yield and surface finish prediction able to calculate the optimal interline distance to reduce surface waviness was developed. The aim is to minimise post-deposition treatments, thereby reducing production time, material waste and costs. This method is applicable beyond CSAM and can be of interest for other additive manufacturing techniques.

Whilst aluminum alloys are widely used in military aircraft, space vehicle, and satellites, most of the work on the durability and damage tolerance (DADT) assessment of additively manufactured (AM) parts has focused on other types of materials. AM Scalmalloy®, which has a yield strength greater than 450 MPa and an elongation greater than 10%, appears to have the potential to meet the certification requirements needed for load bearing “aluminum alloy” aerospace parts. However, the ability to characterize crack growth is central to certification. As a result the present paper addresses crack growth in the AM Scalmalloy®. It is shown that when expressed as per the Hartman-Schijve crack growth equation, the curves for AM Scalmalloy® essentially collapse onto a single curve that is associated with the growth of both long and short cracks in the aluminum alloy AA7075-T7351. It is suggested that this finding has the potential to simplify the process for determining the upper bound growth curves required in NASA HDBK-5010 for the certification of AM space parts.