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From the materials perspectives, akin to fusion welding in many respects, AM involves a multitude of complex and interacting physical phenomena such as heat transfer, fluid flow, discrete and continuum mechanics, sintering, melting, solidification, solid state transformations, grain growth, diffusion, textures etc.
Other considerations that need careful thoughts are relevant to ensuing cooling rates that would affect the grain structures, phases and mechanical properties.
From process perspectives itself, additive manufacturing brings in to foresight a number of complex interacting physical phenomena such as: Heat transfer, Fluid mechanics, continuum mechanics, melting, solidification, solid state phase transformations, grain growth, textures, diffusion.
Further complexity arises as the number of process variables that include input materials interact to control the deposit characteristics such as: deposit dimensions, stress state, extent of lack of fusion, gas porosity, structure & chemical gradients.
The role of feedstock powders needs careful investigations as the grain homogeneity and shape are known to affect the profile of the deposit bead and the extent of projections that diminish the surface quality as well as the process stability.

Hard materials based on Alumina (AN), Silicon Carbide (SC), Boron Carbide/Nitride (BC/N), Zirconia (ZN), and alike are often used to produce abrasive grains and coat cutting tools.
The variants of AN, SC, BN, BC, and ZN not used for producing abrasive grains and coatings are excluded from the search.
Range compliance provides the compliance in the interval 0,1 of a given range with respect to a given fuzzy number.
Table 1 lists the fuzzy numbers used in this study to define VL, L, M, H, and VL.
Fuzzy numbers used in this study.

In case of grain size measurement, Lizhen Lu et al.
In this experiment, as a large number of failures, such as material firing, vacuum pumping, air leakage, argon atmosphere, have been meet in the first processing progress.
After then, the picric acid is used for etching grain boundary.
Figure 3 shows there exists a large number of impurities along grain boundary.
In the case of grain boundary in LA141-0.3La alloy, it composes of bright field and dark field inside grain boundaries.

Stepwise cracking occurs along grain boundaries or within grains.
A large number of gaps and cavities within the material’s structure were detected, having their origins in internal pressure increment.
These stresses were mainly concentrated at grain boundaries and other areas of structural weakness.
Comparing uncharged and charged specimens fracture surfaces, the number of splits increased for the hydrogen-charged specimens.
It occurred within individual grains of the material, and the cracks propagated through the grain boundaries rather than around them.

Composite of two phases with crystal grains of titanium nitride (TiN) and amorphous of silicon nitride (Si3N4) had shown an improvement of mechanical properties as shown by composite layer of nickel (Ni) and TiN.
The change diffraction peak of Ni crystals is due to the decrease of nuclei number [8].
It was observed that at low current density (2 mA/cm2), the morphology is not compact with large separation between grains and it became more compact when the current was increased.
At current density of 2.5 mA/cm2, the large in sizes and number of agglomerations were appeared on the surface and were reduced for further increased current density up to 3 mA/cm2.
However, in a composite system, the high hardness was achieved by not only crystallite size or grain refinement but also particles dispersion strengthening within the metal matrix [15].

This can be attributed to the grain refinement due to stirring.
The relation between number of strands and maximum stress is shown in Fig. 9.
As the number of slits increases, the tensile strength tended to decrease.
Material hardening as a result of grain refinement or softening due to over aging during FSF was observed in the stirring part. 2.
Inhomogeneous grain distribution after FSF is considered to be the reason of the mentioned fact. 4.

The Young & Cutler´s model for the initial stage (IS, [7, 8]) in solid stage sintering (SSS), for grain boundary diffusion is also plotted in Fig. 1.
Young & Cutler's models for grain boundary diffusion for agglomerates A and B are shown in the inset.
The driving force for this process would be the reduction of the grain boundary energy by rotation of adjacent grains [11].
Acknowledgements The authors greatly acknowledge the financial support of DFG under the project "Gradient materials" - project number: Mu 959/5. and DFG Graduiertenkollegs: IIIGK-GRK 232/2.
Material Science under number JMS 11204-03

Good weld quality and grain refinement in the weld area were obtained for steels with low and medium carbon volume fraction.
Dynamic tool action leads to breaking down the grains to nanosizes in subsurface layers.
In the subsurface layer of the steel, there was observed grain size reduction in the structure to the sizes of 20-50 nm.
Experimental curves of crack length against number of cycles for treated and untreated S235 steel at the stress of smax =115 MPa in the air and in the corrosive environment.
Comparative curves of crack growth rate behaviour against number o cycles for treated and untreated S235 steel at the stress of smax =115 MPa in the air and in the corrosion environment.

Moreover, only a comparatively small number of processed composites will be defect-free [6].
The molten Si, which penetrates into the diamond skeleton, reacts with the diamond and promotes cracks along the grain boundaries, Fig. 3(b).
Changes in the graphite plane orientation occur (cracking or folding) and large numbers of microcracks appear.
The adhesion between diamond grains depends on the contact surfaces between them.
After forming the grain structure of the binder phase in the graphite-Si disk, the SiC begins to infiltrate the diamond skeleton, forming the Si-layers microstructure and the overall geometry of the distribution of the binder phase between the diamond grains, as illustrated in Fig. 4(a).

The influence of particle size has been studied and reported over a number of decades including a diversity of minerals and has not as yet been fully understood [2, 3, 4, 5, 6, 7, 8].
Most of the results observed in relevant literature have shown that for a given system of flotation there is an optimum grain size for the process.
For the different grain size ranges analyzed, the respective grain size distributions were determined using the laser diffraction technique, with the employment of the Malvern Mastersizer Microplus (MAF 5001) equipment.
Thus, twenty-eight trials were carried out resulting from seven two-level full factorials experiments, one for each grain size range.
In addition, although the probability of collision between particles and bubbles in these grain size ranges is higher, the adhesion efficiency is lower [5].