Laser directed energy deposition (LDED) is an effective route for fabricating NiTi shape memory alloys, but the surface quality and tribological response of LDED-fabricated NiTi alloys are sensitive to the matching between laser power and scanning speed. An improper power-speed combination may lead to unstable molten pool spreading, insufficient powder melting, surface defects, composition deviation, and weakened resistance to sliding contact. In this work, pre-alloyed NiTi powder is used to fabricate bulk NiTi specimens by LDED at laser power of 500 W and 540 W and scanning speed of 260, 300, and 340 mm/min. The effects of power-speed matching on surface formation, defect characteristics, phase transformation response, local mechanical behaviors, and dry sliding wear behaviors are investigated.
Surface height distribution is characterized by three-dimensional ultra-depth-of-field microscopy, and surface defects are observed by scanning electron microscopy. The chemical composition and worn surface element distribution are analyzed by energy dispersive spectroscopy. X-ray diffraction and differential scanning calorimetry are used to evaluate phase constitution and thermally induced transformation behaviors. Compression tests, microhardness testing, nanoindentation, and reciprocating dry sliding wear tests are performed to clarify the relationship among surface defects, local mechanical response, and wear behaviors.
The results show that all specimens are mainly composed of B2 austenite at room temperature, accompanied by weak B19′ martensite-related peaks. However, surface defect distribution, B2/B19′ phase characteristics, transformation response, local hardness, and wear behaviors vary significantly with process parameters. At 500 W, increasing the scanning speed from 260 mm/min to 340 mm/min deteriorates the surface formation quality. The surface roughness parameters Sa and Sq increase from 2.8 μm and 3.5 μm to 5.8 μm and 7.1 μm, respectively. The surface defect area fraction increases from 0.78% to 4.35%, and the pore number density reaches 47.1 mm-2. This indicates that excessive scanning speed under relatively low laser power shortens the interaction time between the laser, powder, and molten pool, thereby weakening powder melting, molten pool spreading, and inter-track bonding. Meanwhile, the relative B2 peak-area fraction decreases from 95.4% to 93.1%, and the nanohardness decreases from 4.268 GPa to 2.987 GPa. The average wear scar width increases from 642.18 μm to 705.84 μm, and the steady-state friction coefficient increases to 0.878 6, indicating reduced surface bearing stability during sliding.
At 540 W, the surface formation quality and wear resistance first increase and then decrease with increasing scanning speed. The specimen fabricated at 540 W and 300 mm/min shows the most uniform surface height distribution, with Sa and Sq of 2.3 μm and 2.9 μm, respectively. Its surface defect area fraction is only 0.36%, and no obvious adhered particles are counted. Although this parameter does not correspond to the highest nominal energy input, it provides a more suitable balance among powder melting, molten pool spreading, inter-track bonding, and heat accumulation control. This balance helps reduce surface defects and maintain a relatively stable phase transformation response. The specimen also exhibits the highest relative B2 peak-area fraction of 96.7%, with cooling and heating transformation enthalpies of 9.6 J/g and 9.1 J/g, respectively. Its nanohardness reaches 4.921 GPa, suggesting improved resistance to local contact deformation. Correspondingly, the average wear scar width and steady-state friction coefficient decrease to 622.36 μm and 0.845 8, respectively.
The worn surface analyses further reveal the difference in wear mechanisms. For the specimen fabricated at 500 W and 340 mm/min, obvious delamination, compacted debris, Fe-containing transferred material, and oxygen enrichment are observed on the worn surface. The wear process involves abrasive wear, adhesive transfer, oxidative wear, and local delamination. In contrast, the worn surface of the 540 W and 300 mm/min specimen is relatively smooth, with shallow and continuous grooves, limited debris accumulation, and weak Fe and O signals. This indicates that material transfer from the GCr15 steel ball, oxide debris adhesion, and local delamination are effectively suppressed. The lower defect level, higher nanohardness, and weaker Fe/O enrichment together make the sliding process more stable, and the wear mechanism is dominated by relatively mild abrasive wear.
Key words
laser directed energy deposition /
NiTi alloy /
surface morphology /
phase transformation behavior /
microhardness /
friction and wear
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Funding
Key Research and Development Project of Jilin Provincial Science and Technology Development Plan (20240302115GX)
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