1. Availability of material property databases
2. Process-model compensation to account for distortion and accuracy 3. Process sensing, control, and nondestructive evaluation
4. Clear and affordable paths for certification and quantification 5. Bigger, faster, and more capable equipment
Financial investments to support such endeavors include the recent initiation of the National Additive Manufacturing Innovation Institute (NAMII), funded jointly by the U.S. Department of Commerce, Department of Defense, Department of Energy, National Science Foundation, and National Aeronautics and Space Administration, along with a few other partners from industry, nonprofit organizations, and academia.
Because the mechanical properties and resultant performance of a product are greatly influenced by the internal characteristics of the chosen material, an understanding of the microstructural evolu- tion of the various additive manufacturing processes is essential. A few studies have been conducted on selected additive manufactured metallic materials.95–100 In these studies, the microstructure of the additive manufactured part was found to be noticeably different when compared to the conven- tionally manufactured materials. These differences included the presence of distinct layer patterns, heat-affected zones (HAZs), directional grains, and other fine or intrinsic microstructural features within each grain.
1.8.1 alloySoF titaniuM
For the titanium alloy (Ti6Al4V) fabricated using Laser Engineered Net Shaping (LENS), the grains parallel to the direction of deposition were found to be columnar in nature as a consequence of heat extraction from the substrate. At the interfaces between the LENS deposit and the substrate, macroscopic heat-affected zones were easily observed. Depending on the specific AM process used, the microscopic heat-affected zones had observable coarse characteristics between the lay- ers. This finding was attributed to the reheating of previous layers as a consequence of subsequent and additional deposition.95 It is the presence of fine microscopic heat-affected zones that give the additive manufactured microstructure a layered appearance. The presence of directional grains in conjunction with both the macroscopic heat-affected zones and microscopic heat-affected zones did result in an overall non-uniform microstructure of the additive manufactured materials. This was responsible for differences in properties along the different orientations.95,97–100 In one independent study, it was observed that the yield strength and ultimate tensile strength of the additive manufac- tured processed titanium alloys exceeded the typical values for a wrought Ti6Al4V alloy. Further, the ductility of the additive manufactured alloy was found to be slightly lower than the wrought counterpart, and anisotropy in yield strength, ultimate tensile strength, and ductility was associated with the build direction. The static properties of additive manufactured Ti6Al4V was found to be comparable with the wrought product.101,102 Some of the values reported for both the wrought and hot-isostatic processed (HIP) titanium alloys are summarized in Table 1.5.
In a recent study, it was found that the properties and microstructure of the Ti6Al4V alloy that was produced using electron beam powder bed (EBPB) and laser beam powder bed (LBPB) revealed appreciable differences.101,102 The morphology and severity of porosity induced in the final product by the two techniques differed significantly. The researchers observed that the LBPB-processed material exhibited irregularly shaped pores, while porosity in the EBPB alloy was essentially spher- ical. The presence of surface defects was found to be detrimental to the high-cycle fatigue resistance of the alloy.102 In terms of fatigue crack growth performance, the presence of porosity was found not to be the dominant influencing factor but the microstructure of the alloy was. The use of hot isostatic pressing (HIPing) to induce closure of the porosity was observed not to have a significant influence on fatigue crack growth resistance.102
1.8.2 nickel-baSed SupeRalloyS
In the as-fabricated condition, the microstructure of nickel-based alloys was found to be columnar with grains up to 20 àm in width. Subsequent to hot isostatic pressing, the columnar grains tended to recrystallize, while the metastable γʹ precipitates (Ni3Nb) gradually dissolved.
Overall, depending on the additive manufacturing technique used, unusual microstructures and tAble 1.5
static Mechanical properties of processed titanium Alloy ti-6Al-4v101
property
typical Wrought
orientation hot Isostatic pressed (hIp) +
solution heat treated (sht)
hot Isostatic pressed (hIp)
X–Y Z X–Y Z
Yield strength (MPa) 828 887 946 848 841
Ultimate tensile strength (MPa) 897 997 1010 946 946
Elongation (%) 15.0 11.4 13.9 13.2 13.9
microstructural architectures were generated, which opened the door to microstructural design.
The static mechanical properties of an additive manufactured nickel-based superalloy IN625 are summarized in Table 1.6.103 In the as-fabricated condition, the ductility of IN625 was equivalent to that of the wrought and annealed counterparts, and the yield strength was only marginally lower. Upon hot isostatic pressing, the yield strength was observed to decrease by a good 26%, with a concurrent improvement in ductility of well over 57%. The mechanical properties of the shaped metal deposition (SMD) IN718 alloy were found to be superior to those of the as-cast counterpart, but were noticeably inferior to those that were engineered using the additive manu- facturing technique. The tensile strength and elongation for the entire additive manufactured processed materials were found to exceed that of the as-cast counterpart.104 The mechanical prop- erties of this alloy are summarized in Table 1.7.
Compared to the parts produced by conventional manufacturing processes, additive manufac- tured components demonstrate promising properties for most metallic materials;93,105–108 however, the presence of observable amounts of shrinkage, porosity, and residual stresses and their mutu- ally interactive influences can affect both the static and dynamic properties, depending on the additive manufacturing techniques used and the parameters governing the processing.105,106 This finding has led to an examination of post-AM processed materials. Both hot isostatic pressing and heat treatments are considered to be effective methods for relieving residual stress, eliminat- ing porosity, and recovering ductility. As a direct consequence of the specific processing condi- tion used and the resultant characteristic microstructure, conventional post-processing treatments may not always result in the expected properties and behaviors for the materials prepared using any one of the additive manufacturing techniques.108,109 Overall, achieving the desired properties and performance of additive manufactured materials and the resultant components requires the following factors:
1. Judicious selection of the appropriate additive manufacturing process
2. Tailoring the fabrication condition, post-processing conditions, and key control parameters The surface finish given to an additive manufactured part is influenced by the type of equipment used, the direction of build, and the process parameters. Martukanitz and Simpson109 reported that both the build data and feature definitions can be linked to the overall quality of the surface. In gen- eral, as the build rate increases, the feature quality or resolution decreases; thus, for fatigue-critical tAble 1.6
static Mechanical properties of Additive Manufactured nickel-based superalloy In625103,104
process yield strength (Mpa) tensile strength (Mpa) elongation (%)
Wrought (annealed) 450 890 44
As-fabricated (EBM) 410 750 44
Electron beam melt + hot isostatic pressed 330 770 69
Wrought (cold worked) 1100 — 18
tAble 1.7
Mechanical properties of Additive Manufactured nickel-based superalloy In718104
process yield strength (Mpa) tensile strength (Mpa) elongation (%)
Shaped metal deposition 473 828 28
As-cast 488 786 11
Laser 552 904 16
Electron beam 580 910 22
parts that are fabricated using high-deposition-rate additive manufacturing processes, the need for post-process surface finishing becomes necessary. When properly processed, the static mechani- cal properties of additively manufactured metallic materials are comparable to the conventionally fabricated metallic components or counterparts. The relatively high cooling rates achieved dur- ing additive manufacturing tend to reduce partitioning while concurrently favoring reduced grain size. Most additive manufacturing fabricated materials tend to exhibit both microstructure and mechanical property anisotropy along the Z direction, or through the thickness. This direction was found to be the weakest. The dynamic properties of additive manufactured materials are notice- ably influenced by the presence of macroscopic, fine microscopic (microporosity), and surface finish defects. However, when properly processed, hot isostatic pressed, and even finish machined, the additive manufactured alloys can exhibit better fatigue properties as compared to their wrought counterparts.20,109