Advances in Laser Materials Processing

1. Introduction and Scope

Today, laser processing is becoming more and more relevant due to its fast adaptation to the most critical technological tasks, its ability to provide processing in the most rarefied and aggressive mediums (e.g., vacuum conditions), a wide field of potential applications, and green aspects related to the absence of industrial cutting chips and dust. With the development of 3D technologies in production, laser processing has received a new round of interest associated with its abilities in selective high-precision powder melting or sintering [1,2,3,4] that are now available even for oxide ceramics without a binder for remelting [5,6]. New technologies and equipment, which improve and modify laser optic parameters [7], contribute to the better absorption of laser energy by metals or powder surface [8] and allow an increase in laser power up to a few kilowatts. This can positively influence the industrial spread of the laser in mass production and advance existing manufacturing methods.

The latest achievements in laser processing have become a relevant topic in the most authoritative scientific journals and conferences over the last half-century. Advances in laser processing have received multiple awards in the most prestigious competitions and exhibitions worldwide and at international scientific events.

This Special Issue is devoted to the most recent achievements in the field of the laser processing of metals and innovative manufacturing methods based on lasers.

2. Contributions

Eleven research articles and reviews are published in the presented Special Issue. The touched upon subjects cover the most critical topics in laser materials processing. Among them are questions regarding:

• Distributing/redistributing energy in the laser beam spot (round and quadratic) in laser additive manufacturing (experimentally and numerically) and surface treatment for various types of metals alloys, such as cast iron and steel [9,10,11,12];
• Laser surface treatment to repair surface defects and cracks in irons, steels, and composites [13,14,15,16];
• The post-processing (mechanical, heat and plasma treatment) of laser welds and parts produced by laser additive manufacturing [17,18,19].

In particular,

• Beam profiling in the laser-additive manufacturing of metals and alloys [9],
• The influence of specific energy on the microstructure and properties of laser cladded high entropy alloy [10];
• Spacing distribution on the surface of gray cast iron by laser remelting using a biomimetic design model [11];
• A square, top-hat (flat-top) shaped intensity distribution in the laser micro polishing tool steels [12];
• Laser scabbling to remove the defect surfaces of cement mortar composites [13];
• Laser melting to bionically repair thermal fatigue cracks in ductile iron [14];
• Biomimetic laser treatment on five types of low and medium carbon steels [15];
• The heat treatment and ultrasonic peening of the laser-welds on metals and alloys [17];
• The heat treatment, vibratory tumbling, and ultrasonic cavitation finishing of the anticorrosion steels of the austenite and martensitic class [18].

These subjects can be highlighted as the main, crucial research directions in laser materials processing. The topics of the laser surface treatment of cast irons [16] and defect minimization in the laser powder bed fusion of metals and alloys [19] are reviewed.

Beam profiling or shaping in the laser additive manufacturing of metals and alloys [9] provides the redistribution of the laser energy flux in a laser beam spot. It leads to a decrease in material losses through evaporation by more than 2.5 times when switching from the classical Gaussian mode (TEM₀₀, laser beam spot of 109 µm) to the inverse Gaussian (donut) distribution (airy distribution of the first harmonic, TEM_01* = TEM₀₁ + TEM₁₀, laser beam spot of 310 µm) during the dynamic simulation of a CoCr type of alloy. The calculation of the Péclet number (a similarity criterion characterizing the relationship between convective and molecular processes of heat transfer (convection to diffusion) in a material flow in the liquid phase) shows that the cylindrical (top-hat or flat-top) distribution (TEM_FT = TEM_01* + TEM₀₀ mode, laser beam spot of 210 µm) is effective in a narrow temperature range (oxide ceramics).

Experimental research of a critical laser cladding parameter, such as the specific energy in processing high entropy alloys (HEA, 24.44% of Fe, 26.18% of Co, 22.43% of Cr, Ni in balance) [10], showed significant differences in the microstructure and properties of the coatings. The increase of specific energy plays a positive role in

• The microstructure evolution that is mainly composed of the face-centered and body-centered cubic phases, precipitating a small amount in the Fe-Cr phase and Laves phase;
• Promoting the diffusion of Ti from the substrate of Ti6Al4V to the HEA coating, mainly composed of columnar crystal and shrinkage cavities;
• Subsequently affecting the microhardness of samples up to 1098HV, which is ~200% higher than for the substrate.

In order to improve the wear resistance and thermal fatigue resistance of gray cast iron (3.41% of C, 1.61% of Si, 0.96% of Mn, Fe in balance) surfaces, the original uniform distribution laser melting strengthening model was designed as a nonuniform distribution model [11]. The adjacent melting zones affect each other, resulting in heat preservation and a tempering effect; simultaneously, the area of the melting zone and grain size increase, and the hardness decreases from 765–820 HV to 570–620 HV. The phase transformation law of the microstructure in the melting zone was determined at the later stage of thermal fatigue. The maximum residual tensile stress was 204 MPa in the melting zone and 103.4 MPa in the phase transformation zone. The surface wear morphology of different unit combinations after 600 thermal fatigue cycles showed that different degrees of wear appeared on the material surface, caused by microcrack peeling with an increase in fatigue times.

The influence of three different quadratic top-hat (flat-top) laser beam sizes (100 µm, 200 µm, 400 µm side length) and fluences from 3.0 up to 12.0 J/cm² on the resulting surface topography and roughness of the 1.2379 tool steel product (AISI D2, 1.56% C, 0.4% Si, 11.86% Cr, 0.83% Mo, Fe in balance) in laser micro polishing showed [12] that chromium carbides are the source of undesired surface and dimples. The laser process parameters were: the frequency of 20 kHz, scanning speed of 200, 400, 800 mm/s, track offset of 10, 20, 40 µm, and laser power of 8–24, 32–96, 128–384 W, correspondingly. Particularly for high laser fluences, a noticeable stripe structure was observed, which is typically a continuous remelting characteristic. The micro-roughness was significantly reduced from 32 to 3 nm when the macro-roughness was increased from 0.002 to 0.200 µm. The results show that smaller laser polishing fluences are required for larger laser beam dimensions. Additionally, the same or even a lower surface roughness and less undesired surface features were created for larger laser beam dimensions. Therefore, a potential path for the industrial applications of laser micro polishing, where area rates of up to several m²/min might be achievable with commercially available laser beam sources.

In laser scabbling with a pulsed laser with a power density of 1.45 × 10⁵ W/mm², the effects of silica sand proportion in color changing and penetration depth samples were studied for the five types of cement mortar samples, mainly composed of CaO and SiO₂ [13]. Increasing the silica sand proportion resulted in a decrease in scabbling penetration depth and fewer surface cracks on the top and bottom surfaces. The evaporation of material was the dominant mechanism of this scabbling due to the high-power density laser. The chemical changes in the cement mortar were an increase in Si and a decrease in Ca. The difference is explained by the boiling point of silicon dioxide (3220 K) being higher than for calcium oxide (3120 K).

The thermal fatigue cracking of ductile iron (3.65% C, 2.42% Si, 0.60% Mn, Fe in balance) machine parts can be a severe problem in abrasive wear conditions when the presence of graphite in the material complicates the repair of crack defects [14]. A novel method for remanufacturing ductile iron brake discs based on coupled bionics to repair thermal fatigue cracks discontinuously, using bioinspired crack blocking units fabricated by laser remelting, showed that the crack could be fully closed at a laser energy of ${165.6}_{-15}^{+19}$ J/mm² with a pulse duration of 8 ms, an electric current of 155 A, laser beam spot of 1 mm, and laser power of 347.2 W to a sufficient depth of 0.59 mm. The microhardness was 680HV0.2, comparing 298HV0.2 of the untreated sample. The sample treated at the mentioned energy level exhibited the highest tensile force of 40.68 kN, which was 37.11% higher than the unrepaired one. After 2000 thermal fatigue cycles, the crack width of the unrepaired specimen increased by 499.21 μm, while the crack width of the repaired specimen increased between 118.31 and 412.34 μm. The result of 118.31 µm was shown at the laser energy level of ${165.6}_{-15}^{+19}$ J/mm², which was 23.70% of the unrepaired sample. Research has shown the beneficial effects of reducing the spacing of units from 7 to 3 mm by inhibiting thermal fatigue crack propagation from 150.62 to 57.68 μm, which was 61.70% smaller than that of the unrepaired sample.

Five kinds of low and medium carbon steels with different carbon element contents (Steel 1—0.15%, Steel 2—0.25%, Steel 3—0.37%, Steel 4—0.45%, Steel 5—0.58%) were studied by laser remelting [15]. Laser process parameters were: an energy level of 16.88 J, pulse a duration of 8 ms, a frequency of 5 Hz, a scanning speed of 1 mm/s, and a beam diameter of 1.59 mm. Compared with the untreated samples, when the carbon content is 0.15–0.45%, the tensile strength of the laser biomimetic samples of Steel 2 is 532 MPa, which is higher than for the untreated samples (440 MPa) by 20.91%. For other groups of steels, the difference ranged from 3.14 to 1.26%, or even to 5.21% for Steel 5. With an increase in carbon content, the tensile strength increases first and then decreases, while the plasticity of the biomimetic samples decreases continuously from 19 to 2% compared with untreated samples (from 39 to 15%). The difference in plasticity degree between untreated and treated samples for low carbon samples (Steel 1) was 34.48% and for medium carbon samples (Steel 5) was 86.67%. The bionic samples have better wear resistance than that of the untreated samples. For bionic specimens with different carbon elements, wear resistance increases with an increase of carbon element content. The difference in weight loss reduction percentage was 31.01% for Steel 1, 31.72% for Steel 2, 36.08% for Steel 3, 48% for Steel 4, and 67.76% for Steel 5.

The review discusses the main experimental aspects of the laser surface treatment of four cast-iron groups: gray (lamellar) cast, pearlitic ductile (nodular), austempered ductile, and ferritic ductile iron [16]. The review summarizes the typical laser types used in surface treatments, i.e., CO₂, Nd:YAG, or fiber (diode); the coating is graphite or manganese/zinc phosphate; shielding gas is argon, nitrogen, or helium; the power distribution is Gaussian, uniform; and the spot geometry is circular, elliptical, or rectangular. The use of diode and Nd:YAG lasers without coatings allows for greater energy absorption than when using conventional CO₂ lasers, which is responsible for a cooling rate that ensures the formation of a more resistant martensitic structure. Lasers with circular geometry and Gaussian energy distribution generate a parabolic heat-affected zone in the transverse section, where properties are not uniform within a constant layer depth. The heat-affected zone’s hardness is 700–800 HV, which decreases as the fraction of retained austenite increases in favor of the martensitic phase. The compressive residual stresses in the hardened zone are explained by the volumetric increase associated with the transformation from austenite to harder martensite. Single-pass with rectangular-shaped and uniform energy distribution lasers and adjacent discrete laser spots can be used to avoid a tempering effect of overlapping that reduces the austempered ductile iron wear resistance by 10–100 times. Ductile irons and austempered ductile irons present lower wear damage under comparable conditions than gray cast irons since graphite flakes act as stress raisers, favoring crack nucleation and growth during friction, as in dry sliding tests with a SiC platform (frequency of 2.5 Hz, a load of 5 kg). Regardless of the initial microstructure of the cast iron, linear energy is the critical parameter since it considers the combined effect of experimental parameters, such as laser power, absorption layer thickness, and scanning velocity. It is suggested to apply surface hardening without melting on ductile irons and austempered ductile irons to achieve higher wear resistance because of the residual stresses created during phase transformations (compressive in laser surface hardening and tensile in laser surface melting).

The effect of laser offset and defocusing on microstructure, geometry, and mechanical property responses of 2 mm-thick dissimilar AA6061/Ti-6Al-4V laser welds were studied in [17]. In order to reduce residual stresses, the joints were both stress relaxation heat-treated (at 530 °C for 2 h followed by air cooling) and mechanically treated by ultrasonic peening (for 1 min/side, the diameter of the peening needle was 3 mm, and the vibration frequency was 20 kHz). The welds microstructure was martensitic in the Ti-6Al-4V fusion zone, columnar dendritic in the AA6061 fusion zone, and partially martensitic in the Ti-6Al-4V heat-affected zone. Intermetallic compounds of the Al–Ti system were detected at the AA6061/Ti-6Al-4V interface and in the aluminum fusion zone. Both negative defocusing and a higher laser offset of 0.3 mm compared to 0.1 mm improved the tensile strength of the welds (from 110.2 ± 6 to 158.1 ± 8 MPa or by 43.6% for top weld and from 100.0 ± 3 to 172.7 ± 8 or by 72% for bottom weld), mainly by reducing the amount of brittle intermetallic compounds. The heat treatment, leading to the aging of the martensite and the increasing the intermetallic compound size, reduced the tensile strength and ductility of the joints. The average growth of the intermetallic layer was 2.60 ± 0.6 μm and 1.30 ± 0.3 μm for 0.3 mm-offset top and bottom joints correspondingly. On the contrary, for dissimilar Al–Ti welds, the mechanical treatment effectively increased joint ductility and corrosion resistance in the 3.5% NaCl solution (corrosion current density for 0.3 mm-offset bottom joint was reduced from 16.12 to 9.28 μAχμ⁻²).

The samples produced by laser additive manufacturing of two types of anticorrosion steels—20kH13 (X20Cr13, 0.16–0.25% of C, 12–14% of Cr, powder fraction of ~40 µm) and 12kH18N9T (X10CrNiTi18-10, ≤0.12% of C, 17–19% of Cr, 8–9.5% of Ni, ≤2.0% of Mn, powder fraction of 20–63 µm) steels—of the martensitic and austenitic class were subjected to cavitation abrasive finishing and vibration tumbling to research the various effects on surface quality and physical and mechanical properties [18]. The laser process parameters were a laser power of 80 and 100 W and a scanning speed of 390 and 100 mm/s, for X20Cr13 and X10CrNiTi18-10 steels, respectively, and a layer thickness of 20 µm. The roughness parameter Ra was reduced by 4.2 times for the X20Cr13 sample after cavitation-abrasive finishing when the roughness parameter Ra for the X10CrNiTi18-10 sample was reduced by 2.8 times after vibratory tumbling. The tensile strength was increased by 15% for X20Cr13 steel (1584.09 ± 3.58 MPa for additively manufactured sample comparing 1485 MPa of cast sample after quenching and low tempering) and 20% for X10CrNiTi18-10 steel (657.03 ± 3.32 MPa for additively manufactured sample comparing 540 MPa of cast sample after quenching at 1050–1100 °C with cooling in water) than for cast and traditionally heat-treated cast samples. The wear resistance of 20kH13 (X20Cr13) steel correlated with measured hardness (44.2 HRC_z/46.2 HRC_xy for tempering at 240 °C in air, 38.7 HRC_z/39.1 HRC_xy for tempering at 680 °C in oil, 33.4 HRC_z/35.8 HRC_xy for annealing at 760 °C in air compared with 43.0 HRC and 22.3 HRC of cast samples after quenching at 1030 °C and tempering at 240 °C in air, 680 °C in oil, correspondingly) and decreased with an increase in tempering temperatures.

The compressive review [19] observes the defects in the additively manufactured machinery products of the anticorrosion steels of the martensite and austenite classes; difficult to process materials such as pure titanium, nickel, and their alloys; super and high entropy alloys; and triple fusions. Studies were conducted on the structural defects observed in such products to improve their quality by eliminating residual stress, reducing porosity, and improving surface roughness. Electrophysical and electrochemical treatment methods (ultrasound, plasma, laser, spark treatment, induction cleaning, redox annealing, gas flame, plasma beam, and plasma spark treatment) for removing oxide phase formed during the melting and remelting of deposed tracks in layers are considered. Surface plasma cleaning methods demonstrate their ability to remove it, smooth the surface, and recrystallize (grain size decrease) by heat treatment in metal products. The proposed type of cleaning can be used for laser powder bed fused products to provide conditions for reliable product growing that can be even more durable in extreme conditions. Special attention is focused on the atmospheric plasma sources based on a dielectric barrier and other discharges as a part of a production setup that presents the critical value of the conducted review in the context of the novelty for transition to the sixth technology paradigm associated with the Kondratieff’s waves [20,21,22].

3. Conclusions and Outlook

Topics such as various types of laser processing, including additive manufacturing, ablation, polishing, and micromachining, are covered by this Special Issue, presenting recent developments and achievements in the laser processing of traditional and the most-advanced metal-based materials. However, it should be noted that there are still many issues related to transferring some of the most outstanding ideas to applications in real production conditions. The Guest Editors of the Special Issue hope that the most outstanding results will contribute to and accelerate the switch to the sixth technological paradigm in the near future.