Ultra-high-power laser welding of thick-section steel: Current research progress and future perspectives

Laser-welded forged steel pistons can meet the needs of the new era of heavy truck engines. 42CrMo4 and 38MnVS6 are widely used as piston materials due to the good mechanical properties. This study investigates the influence of preheating on microstructure and mechanical properties of 42CrMo4/38MnVS6 laser welding joint. The experimental results show preheating increases the laser absorption capacity of the metal, which can lead to an increase in weld width. The microstructure of weld is the high-hardness and poor toughness twin martensite without preheating. As the temperature of preheating increases, the twin martensite in the weld begins to transform into lath martensite and regenerates ferrite and bainite. As the preheating temperature increases, the plane fracture toughness (K1C) of the weld increases and then decreases, reaching the highest value of 2322.94 MPa·mm−1/2 at 150 °C. Compared with no preheating conditions, the tensile strength of the welded joint after preheating is improved. The fracture mode of welded joints changes from brittle fracture to ductile fracture. When the preheating temperature is 100–200 °C, the tensile strength of the welded joint reaches 1018.1–1032.5 MPa; when the preheating temperature is 250 °C–300 °C, the tensile strength decreases.

Effect of Preheating Temperature on Microstructure and Properties of 42CrMo4/38MnVS6 Heterogeneous Laser Welded Joint

In this work study, a comparative analysis was undertaken to investigate investigation into the cavitation erosion (CE) and corrosion behavior of laser powder bed fusion (LPBF) TC4 and as-cast TC4 in 0.6 mol/L NaCl solution. Relevant results indicated that LPBF TC4 revealed a rectangular checkerboard-like pattern with a more refined grain size compared to as-cast TC4. Meanwhile, LPBF TC4 surpassed its as-cast counterpart in CE resistance, demonstrating approximately 2.25 times lower cumulative mass loss after 8 h CE. The corrosion potential under alternating CE and quiescence conditions demonstrated that both LPBF TC4 and as-cast TC4 underwent a rapid potential decrease at the initial stages of CE, while a consistent negative shift in corrosion potential was observed with the continuously increasing CE time, indicative of a gradual decline in repassivation ability. The initial surge in corrosion potential during the early CE stages was primarily attributed to accelerated oxygen transfer. As CE progressed, the significant reduction in corrosion potential for both LPBF TC4 and as-cast TC4 was attributed to the breakdown of the passive film. The refined and uniform microstructure in LPBF TC4 effectively suppresses both crack formation and propagation, underscoring the potential of LPBF technology in enhancing the CE resistance of titanium alloys. This work can provide important insights into developing high-quality, reliable, and sustainable CE-resistant materials via LPBF technology. 

Cavitation erosion-corrosion properties of as-cast TC4 and LPBF TC4 in 0.6 mol/L NaCl solution: A comparison investigation

Invar 36 alloy manufactured via the laser powder bed fusion (LPBF) process have attracted interest for their excellent low expansion properties. The properties of Invar 36 alloy, particularly thermal expansion, are highly sensitive to variations in process parameters. However, the effect of process parameters on microstructure and properties has not been systematically studied. This study examines the microstructure, mechanical properties, and coefficient of thermal expansion (CTE) of Invar 36 alloy under various processing parameters with different volumetric energy densities (VED). The results demonstrate that VED variations significantly impact the microstructure, mechanical properties, and CTE. Increasing VED leads to larger grain size and higher high angle grain boundaries, while geometric dislocation density and Taylor factor do not exhibit a linear relationship. At 89.29 J/mm3 VED, the Invar 36 alloy displays the optimal yield strength (405.4 MPa), tensile strength (489.3 MPa), and elongation (43.3%). The CTE meets the ASTM F1684 standard for all specimens. Lower relative density corresponds to smaller CTE, with the lowest CTE achieved at VED of 41.67 J/mm3. Furthermore, the influence of porosity on CTE diminishes with rising temperature. This research opens up the possibility of process parameters optimization for improved CTE and mechanical properties in as-printed Invar parts. 

Effect of processing parameters on the microstructure, mechanical properties and thermal expansion behavior of Invar 36 alloy manufactured by laser powder bed fusion

To reveal the effect of laser specific energy on the mechanical properties of Fe60 coatings, and determine the laser specific energy as well as the corresponding process parameters for the optimal mechanical properties of the coatings, Fe60 coatings were prepared on the surface of 304 stainless steels by laser cladding. The microhardness, tensile properties, and impact properties of the coatings were investigated at different laser specific energies by changing the laser power and scanning speed. The results show that the Fe60 coatings are mainly composed of cryptocrystalline martensite, carbides, a small amount of ferrite and residual austenite. Compared with the substrate, the microhardness of Fe60 coatings is significantly increased, the tensile properties of Fe60 coatings are reduced, the brittle cleavage fracture characteristics are presented, and the impact toughness of Fe60 coatings is slightly reduced. With the increase of laser specific energies, the microhardness, tensile properties, and impact properties of the coatings show a trend of increasing and then decreasing. When the laser power is 2300 W, the scanning speed is 11 mm/s, and the laser specific energy is 105 J/mm2, the coatings with the optimal mechanical properties can be obtained. The results can provide a reference for preparing Fe60 coating of excellent performance by adjusting the laser specific energy through synthetically coordinating the laser cladding process parameters. 

Effect of laser specific energy on mechanical properties of Fe60 coatings by laser cladding

Many factors can affect the formability of TC4 alloy in laser additive, and the effects of their interaction are complex. The essence lies in the energy input of the molten pool. In this study, the energy input of the molten pool was characterized by the laser volume energy density under the coupling of multiple process parameters. The effects of volume energy density on the microstructure evolution and mechanical properties of TC4 alloy were studied, and the fracture mechanism was discussed. The results showed that the microstructure of TC4 alloy prepared by selective laser melting (SLM) was composed of α phase, metastable α′ phase, and a small amount of equiaxed nano-β phase. When the TC4 powder was irradiated by low volume energy density, fine columnar β grain with low texture intensity and random grain orientation distribution appear. Meanwhile, a large number of fine acicular martensite α′ phases generated inside the β grains, but fewer nano-β phases precipitated. At the same volume energy density, compared with increasing laser power, reducing the scanning speed could lead to the coarsening of α′ martensite, and more nano-β phase precipitated at the martensite boundary. The volume energy density greatly influenced the elongation (EL) but had little effect on the ultimate tensile strength (UTS), and the UTS could reach up to 1507 MPa. The decreasing volume energy density was beneficial to improve the plasticity, mainly attributed to the increasing deformation coordination ability caused by the refinement of α/α′ martensite and the reduction of the nano-β phase. When the volume energy density was 36 J/mm3, the fine columnar β grain precipitated, and a ductile fracture displayed with high-density dimples shown in fracture morphology. The mapping relationship between process, microstructure and properties was established, which provided a theoretical basis for process, microstructure optimization and performance improvement. 

Effect of volume energy density on microstructure and mechanical properties of TC4 alloy by selective laser melting

The hot cracking at circular welding joints of quenched and tempered 42CrMo steel were studied. The flow of the molten pool and the solidification process of weld were observed with a high-speed video camera. The information on the variations in the weld temperature was collected using an infrared (IR) thermal imaging system. The metallurgical factors of hot cracking were analyzed via metallographic microscope and scanning electron microscope (SEM). The result shows that leading laser laser-metal active gas (MAG) hybrid welding process has a smaller solid–liquid boundary movement rate ( V SL ) and a smaller solid–liquid boundary temperature gradient ( G SL ) compared with leading arc laser-MAG hybrid welding process and laser welding process. Additionally, the metal in the molten pool has superior permeability while flowing toward the dendritic roots and can compensate for the inner-dendritic pressure balance. Therefore, leading laser laser-MAG hybrid welding process has the lowest hot cracking susceptibility.

Experimental study of hot cracking at circular welding joints of 42CrMo steel

The nickel-based superalloys processed by additive manufacturing are very promising structural materials in aircraft engines as high-pressure turbine discs. In this work, a nickel-based superalloy with good mechanical performance and few defects was manufactured by optimized selective laser melting (SLM) processing. We then investigated the influences of post heat treatments on its microstructural characteristics and mechanical performance. The results indicated that a fine grain size with uniform grain orientation was presented in the as-printed nickel-based superalloy sample. After heat treatments, the grains were slightly grown and grain orientation was also changed. Under transmission electron microscopy, fine subgrains with an approximate size of 0.5 μm were found in the as-printed sample which accompanied massive dislocations and discontinuous Laves phases. After the post heat treatments, fine subgrains and less dislocations were retained. On the other hand, massive γ′ and γ″ precipitates with an orientation relationship of (001)[100]γ′//(100)[001]γ or (001)[100]γ″//(100)[001]γ were formed. As a result, the yield stress and tensile strength increased to 1362 and 1410 MPa, respectively, in a heat-treated sample, which retained the identical elongation of the as-printed specimen.

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Microstructural Characteristics and Mechanical Properties of an Additively Manufactured Nickel-Based Superalloy

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Papers

Effect of volume energy density on microstructure and mechanical properties of TC4 alloy by selective laser melting

Experimental study of hot cracking at circular welding joints of 42CrMo steel

Microstructural Characteristics and Mechanical Properties of an Additively Manufactured Nickel-Based Superalloy