Laser additive manufacturing (LAM) technology uses a focused high-energy laser beam as the heat source to achieve integrated forming of complex metal components, avoiding the complex post-processing steps of traditional processing techniques and achieving high forming efficiency, which makes it have broad application prospects in aerospace, automotive, medical and other fields. The metal additive manufacturing process based on laser and powder mainly includes two types: selective laser melting (SLM) and laser directed energy deposition (LDED). LAM has been widely used in the forming of various metal materials, including aluminum alloys, titanium alloys, copper alloys, nickel-based superalloys, magnesium alloys, steel, and so on.
Due to the current widespread use of Gaussian laser in laser additive manufacturing technology, the peak intensity generated in the focusing area is very high. When laser interacts with metal powder, the width-to-depth ratio of the melt pool is small and there are large temperature gradient and cooling rate. The instability caused by complex melt flow dynamics and the accumulation of repeated heating and cooling cycles are prone to keyholes, splashing, spheroidization, residual stress, cracks, and anisotropic microstructures, which seriously affect the strength, toughness, and fatigue resistance of formed components in turn. Modifying the alloy composition or adding strengthening phase particles can effectively eliminate the cracks and anisotropic columnar crystal structure in metal samples. It should be noted that component modification may cause pitting corrosion and reduce the corrosion resistance of the alloy, and the addition of strengthening phase particles to the alloy may lead to particle agglomeration and poor bonding between the strengthening phase and the matrix interface. Heat treatment is an effective method to eliminate cracks and defects in LAM-prepared samples. However, heat treatment further prolongs the preparation time and increases the complexity of the forming process.
Laser shaping and external field matching can regulate the LAM process from the source, solving the problem of defects in formed components. Revealing the in-situ function mechanism and influence law of organization and performance between laser/thermal/magnetic/acoustic fields and materials provides reference for future research on metal LAM technology, promoting its widespread application in multiple fields.
The distribution of laser energy can affect the spatial shape of the melt pool, thereby affecting the thermal gradient and metal cooling and solidification process. Flat-top laser, anti-Gaussian laser, Bessel laser, and defocusing laser all weaken the peak energy in the center of the traditional Gaussian laser beam to different degrees, reduce the temperature gradient in melt pool, suppress powder evaporation and splashing, limit the keyhole effect, and thus reduce the number of defects such as pores and cracks within a wider process window, obtaining almost dense samples. From the perspective of the influence on grain orientation, flat-top laser, anti-Gaussian laser, and defocusing laser can increase the width-to-depth ratio of the melt pool, and research has shown that they can promote the epitaxial growth of columnar crystals. Especially, flat-top laser has a uniform energy distribution, which can obtain almost complete <001> oriented textures, which is beneficial for the preparation of single crystals and β-type medical titanium alloys. In contrast, the elliptical beam profile has a significant impact on the solidification microstructure. By reducing the temperature gradient of the melt pool and increasing the undercooling zone of the composition, the nucleation tendency is improved. It is beneficial for equiaxed crystal formation and achieving grain refinement and tissue densification.
The thermal field reduces the temperature gradient of the melt pool during LAM through heat transfer, prolongs the solidification time of the melt pool, reduces element segregation, and eliminates residual stress and cracks in formed components. The thermal field formed by substrate preheating is gradually transmitted to the surface of the formed part through contact from bottom to top. The process is simple, but it can cause uneven microstructure and properties of the formed component along the building direction. Moreover, due to the high integration of LAM equipment, it is difficult to achieve high preheating temperature. Electromagnetic induction heating can uniformly preheat the entire formed component, avoiding tissue anisotropy caused by uneven preheating, and can achieve high preheating temperature. However, the process is complex and puts forward high requirements for LAM equipment. The static and alternating magnetic field-assisted metal LAM technology has proven to have excellent effects on different forming materials. By generating Lorentz forces and thermoelectric convection to regulate the flow of molten metal in the melt pool, grain refinement is achieved, isotropic equiaxed crystal zone formation is promoted, harmful phase precipitation is suppressed, and mechanical properties are improved. In the process of ultrasonic assisted LAM, the acoustic flow effect can promote the uniform distribution of solutes in the melt pool and reduce segregation. Cavitation effect causes dendrite fragmentation to increase nucleation sites, which promotes grain refinement and epitaxial to equiaxed transformation, and improves the mechanical properties of the prepared samples. In addition, ultrasonic fields can also alleviate residual stresses in formed samples and suppress the generation of defects such as cracks.
Research at home and abroad has confirmed that laser shaping and thermal/magnetic/ultrasonic fields can regulate the shape of the melt pool and the cooling solidification process in situ during the forming process, so as to overcome the defects that affect the quality of formed parts in metal LAM technology. The mechanism of action of different forms of fields on the solidification process of melt pool metal is different. In the future, further exploration can focus on the effect of multi-field synchronously assisted LAM technology on different forming materials, and broaden the application prospects of metal LAM technology in different fields.
