Mechanical properties and microstructure of 3D-printed high Co–Ni secondary hardening steel fabricated by laser melting deposition

The mechanical properties and microstructure of the 3D-printed high Co–Ni secondary hardening steel fabricated by the laser melting deposition technique was investigated using a material testing machine and electron microscopy. A microstructure investigation revealed that the samples consist of martensite laths, fine dispersed precipitates, and reverted austenite films at the martensite lath boundaries. The precipitates are enriched with Co and Mo. Because the sample tempered at 486°C has smaller precipitates and a higher number of precipitates per unit area, it exhibits better mechanical properties than the sample tempered at 498°C. Although the 3D-printed samples have the same phase constituents as Aer Met 100 steel, the mechanical properties are slightly worse than those of the commercial wrought Aer Met 100 steel because of the presence of voids.

Microstructure and properties of hybrid additive manufacturing 316L component by directed energy deposition and laser remelting

Arc additive manufacturing is a high-productivity and low-cost technology for directly fabricating fully dense metallic components.However,this technology with high deposit rate would cause degradation of dimensional accuracy and surface quality of the metallic component.A novel hybrid additive manufacturing technology by combining the benefit of directed energy deposition and laser remelting is developed.This hybrid technology is successfully utilized to fabricate 316L component with excellent surface quality.Results show that laser remelting can largely increase the amount ofδphases and eliminateσphases in additive manufacturing 316L component surface due to the rapid cooling.This leads to the formation of remelting layer with higher microhardness and excellent corrosion resistance when compared to the steel made by directed energy deposition only.Increasing laser remelting power can improve surface quality as well as corrosion resistance,but degrade microhardness of remelting layer owing to the decrease inδphases.

Effect of thermal deformation on microstructure and properties of TC18 titanium alloy produced by laser additive manufacturing

Grain boundary of α phase damaged ductility of laser melting-deposited TC18 titanium alloy and grain boundary of α phases were difficult to break by nominal heat treatment. An extra thermal deformation was introduced to break the grain boundary of α phase with the improved mechanical property of TC18 titanium alloy fabricated by laser melting deposition technique.Results indicated that after thermal deformation, β grains in alloy seriously elongated. When sample was deformed at temperatures from 750 to 850 ℃, α phase exhibited both rod and irregular morphologies with discontinuous distribution at grain boundary, and the subsequent heat treatment would lead to spheroidization of the α phase. However, after deformation at 900 ℃, α phase transferred into β phase and the subsequent heat treatment would make continuous grain boundary of α phase reappear. The suitable hot deformation can effectively break the continuous grain boundary in laser melting-deposited TC18 alloy with respected improved ductility.

Effects of annealing temperature and cooling rate on microstructures of a novel titanium alloy Ti-6Al-2V-1.5Mo-0.5Zr-0.3Si manufactured by laser additive manufacturing

The microstructure changes of Ti-6Al-2V-1.5Mo-0.5Zr-0.3Si alloy manufactured by laser additive manufacturing （LAM） are systematically investigated with statistical analysis of primary α phase （αp） and secondary α phase （αs） under different annealing conditions. Results indicate that, with the increase in holding temperature, the content of αp lamellas decreases with the increasing αs content, maintaining the total α phases concentration stabilized. The width of αp lamellas and the nominal specific surface area of α phase both exhibit positive correlation with the temperature, while the increment of αp and the widths of αs lamellas show an increase-decrease tendency. Besides, with the decrease in cooling rate, the total content of α and the width of αp lamellas increase, while the nominal specific surface area of α phase shows no significant change. The results indicate that, in the annealing process, the holding temperature determines the surplus and growth interfaces of % lamellas, and the cooling rate influences the nucleation quantity of c^s in unit time. During the cooling stage, the αp lamellas grow initially, and then, the nucleation and crab-like structure growth occur followed by those of the αs lamellas. The time intervals among them are influenced by cooling rate. The mechanism of microstructure formation of the LAMed titanium alloy during annealing stage was discussed, which would guide for the heat treatment method to achieve required microstructure.

Solid-state phase transformation of TC11 titanium during unstable thermal cycling in laser melting deposition process

Thermal cycling procedure during laser additive manufacturing (LAM) process causes the appearance of bright and dark patterns on the etched surface of TC11 alloy components. The formation mechanisms of these patterns and the solid-state transformation related to LAM process are systematically investigated with the predication of temperature fields using the finite element software ABAQUS. The results indicate that by increasing subsequent thermal cycles, the peak temperatures for every cycle decrease. When peak temperatures are above Tβ(phase transition temperature of β phase), which is 1010℃ in TC11 alloy, no pattern is observed. Meanwhile, a decrease in peak temperature leads to appearance of an ultrafine basket-weave α+β microstructure (dark contrast) with gradually increased amount of α colonies in the alloy. A special bimodal microstructure with ‘fork-like'α lamella appears in the layer when the peak temperatures of thermal cycles firstly fall into α+β dual-phase region. And this special bimodal microstructure gives a bright contrast and only appears at the region where the peak temperatures are below 970℃, leaving the rest region with a dark contrast. With the continuous increase in thermal cycles in α+β dual-phase region,α lamella gradually coarsens. After five thermal cycles in α+β two-phase region, no further changes in microstructure are observed, and the morphologies of α lamella in dark and bright regions are almost the same but with different amounts of α phase.

Effect of Hot Isostatic Pressing on Fatigue Properties of Laser Melting Deposited AerMet100 Steel

The ultra-high strength steel AerMetl00 was fabricated .by laser melting deposition （LMD） process. The effect of hot isostatic pressing （HIP） on high-cycle fatigue properties of the LMD AerMet100 steel was investigated, and the influence of defects on fatigue behavior was discussed. Results showed that the LMD AerMetl00 steel had fine directionally solidified cellular dendrite structure and coarse columnar prior austenite grains. Metallurgical de fects such as gas pore and lack-of-fusion porosity were produced during the laser deposition process. After HIP treat- ment, the number and size of metallurgical defects had remarkably decreased. Moreover, high-cycle fatigue proper ties of the alloys after HIP treatment were superior to the as-deposited alloys.

Microstructure and mechanical properties of hybrid fabricated 1Cr12Ni2WMoVNb steel by laser melting deposition

Laser melting deposition was carried out to deposit a 1Cr12Ni2WMoVNb steel bar on a wrought bar of same material. Room-temperature tensile properties of the hybrid fabricated 1Cr12Ni2WMoVNb steel sample were evaluated, and microstructure, fracture surface morphology, and hardness profile were analyzed by an optical microscope （OM）, a scanning electron microscope （SEM）, and a hardness tester. Results show that the hybrid fabricated 1Cr12Ni2WMoVNb steel sample consists of laser deposited zone, wrought substrate zone, and heat affected zone （HAZ） of the wrought substrate. The laser deposited zone has coarse columnar prior austenite grains and fine well-aligned dendritic structure, while the HAZ of the wrought substrate has equiaxed prior austenite grains which are notably finer than those in the wrought substrate zone. Besides, austenitic transformation mechanism of the HAZ of the wrought substrate is different from that of the laser deposited zone during the reheating period of the laser deposition, which determines the different prior austenite grain morphologies of the two zones. Microhardness values of both the laser deposited zone and the HAZ of the wrought substrate are higher than that of the wrought substrate zone. Tensile properties of the hybrid fabricated 1Cr12Ni2WMoVNb steel sample are comparable to those of the wrought bar, and fracture occurs in the wrought substrate zone during the tensile test.

Improvement of proof-ultimate strength difference in laser additive manufactured Ti-6Al-2V-1.5Mo-0.5Zr-0.3Si alloy by tuning basketweave structure

After annealed at 1000 ℃, a special basket-weave structure is obtained in laser additive manufactured Ti-6Al-2V-1.5Mo- 0.5Zr-0.3Si alloy. The unit of the special basket-weave structure is a lamellas clusters, which consist of lamellar primary ot (otp), crab-like structures at the edges of otp and lamellar secondary a (as) on both sides of otp. As the units of basket-weave structures, the width of the clusters is much larger than that of a lamellas in as-deposited alloy. The formation temperature and process of the special basket-weave structure are studied, and the room temperature properties are tested and compared with the as-deposited alloy. The results show that the formation of the special basket-weave structure finishes within about 30 s and crab-like structures form earlier than lamellar as. The yield strength of the alloy is decreased by about 75 MPa compared to that of the as-deposited alloy. Besides, the proof-ultimate strength difference of the alloy is two times higher than that of the as-deposited alloy with about 34% improvement for the impact toughness. It is because ot colony size shows a positive correlation to the width of the unit forming basket-weave structure. The enhancement in proof-ultimate strengthdifference could significantly improve the toughness of the alloy, and thus effectively increase the safety of the alloy.