*6.2.4 Oxidation*

Oxidation (Redox) is a process when a chemical reaction happens in metal and leads to change in the structure of atom by losing electrons. Oxidation happens with the availability of oxygen and it is a defect that generates cracks and distortion failure in the printed metal part. It creates a weak insulation material between the printed layers, mostly in DED MAM category. Oxidation is the main reason for variation on mechanical properties. It causes minute or unstable printing gaps between the torch and the bed surface or the previous printed layer. Not enough flow of gas during the process might also lead to oxidation.

#### *6.2.4.1 Proposed solution*

The best way to prevent oxidation is by eliminating oxygen from the printing environment. That can be obtained by using air vacuumed chamber 3D metal printer. However, if the vacuumed environment is difficult to be adjusted, optimal stable gap of the torch should be used with optimal gas flow.

**253**

*An Investigation of the Metal Additive Manufacturing Issues and Perspective for Solutions…*

One of the main defects that interrupt the metal 3D printing part is a heat accumulation. This issue happens when the average temperature of the workpiece increases continuously. That will cause many other defects, such variation in mechanical properties and microstructure evolution, and variation in grain size which will, consequently, lead to undesirable part quality, such structural collapse [14]. This issue is also mostly inherent to DED category of the MAM. According to the author analysis, it happens in DED category mostly because in DED process, metal is melted. As a new layer is created over the previous hot one, it will initiate the heat accumulation. Thus, this issue depends on the philosophy of metal printing process. As compared to PBF, in which lower powered laser is used with lower heat, a new layer of powder will be poured over the previous one and that will absorb the

The previously printed layer should be cooled completely before printing the new layer. That could be performed by installing a cooling unit with the help of in-situ method. The cooling unit will work simultaneously while recording temperature of in-situ method. Similarly, variant building direction will accumulate heat in different direction and reduce the average heat. Heat treatment can be a traditional solution if the workpiece could not be collapsed during the printing process. According to the author's understanding, another way to solve the issue of heat accumulation or residual stress is by using nucleation after printing. Nucleation is a process of formation of a new crystalline structure during solidification. This process might be applied on the printed parts by heating it to a temperature lower than the melting point and building new crystalline structure with the desired mechanical properties, grain size, free residual stress and heat

Two types of cracks mainly occur in MAM method (DED processes) and they are: grain boundary cracks and solidification cracks. Cracks happen because of the method of solidification. Delamination is a type of failure in metal alloys or composite material. In defective parts, the layers of metal are separated because of repeated fluctuated stress and the material characteristic of the deposited metal. A drawback of delamination is that it cannot be repaired by post-process heat treat-

The only way to fix the problem of delamination and cracks is by preventing them. It can be achieved by optimizing solidification. Preheating the printing environment and substrate can be a good solution as well. Avoid using combined material in the DED processes that might separate the layers between them when the dissimilar material has significant difference in the chemical reaction and solubility. Comprehensive study should be performed on the metals which are difficult to melt and solidify without cracks and delamination such as Inconel. Developing temperature map of the printed surface, that might help to optimize the preheating, can avoid cracking and delamination. The high cooling rate is also a cause of cracks and delamination. However, it is mentioned previously that it is a good solution to

*DOI: http://dx.doi.org/10.5772/intechopen.93630*

*6.2.5 Heat accumulation*

heat of the previous layer.

*6.2.5.1 Proposed solution*

accumulation.

*6.2.6 Cracks and delamination*

ment, but it could be avoided [13].

*6.2.6.1 Proposed solution*

*An Investigation of the Metal Additive Manufacturing Issues and Perspective for Solutions… DOI: http://dx.doi.org/10.5772/intechopen.93630*

## *6.2.5 Heat accumulation*

*Concepts, Applications and Emerging Opportunities in Industrial Engineering*

Analyze the functionality and the geometry of printed parts to specify where the fatigue can happen for a particular temperature and surface finish. This fatigue could be avoided by assigning specific heat treatment and surface finish. Similarly, the geometry of metallic parts which can induce fatigue can be changed after comprehensive analysis to avoid undesirable residual stress in these features. Finally, using different environments with different temperature during the process can

Another common defect in MAM method that needs to be prevented during the process is porosity. The porosity is a void or hole in the solid printed metal. In the MAM method, the porosity happens between the layers and is mostly inherent to Direct Energy Deposition (DED) category of metal printing, such as WAAM. It occurs while the printed layer is in liquid form because of gas or solidification shrinkage. Porosity is a defect that will end in producing parts that have undesirable mechanical properties, such as low strength and cracks. Unstable deposition process and poor path planning, especially with complex path cause porosity as well [13].

The gap between the torch of DED processes and the melting surface should be continuously adjusted to prevent any slags that might happen and lead to porosity. Using inert vacuum environment during the printing process will eliminate the oxidation that will lead to porosity. Keeping the printing surface (substrate) clean from any dirt or slags either on the previous printed layer or the main surface of the bed will also reduce porosity. While using the optimal gas flow for GT-WAAM process, incorrect gas flow may cause a lot of slags during the printing which may lead to porosity. Optimal energy density can play a main role in eliminating the porosity because it will generate fine printed layer without defects. Creating a map for optimal deposition rate and gap for every specific alloy and reducing the complexity of the printing path can be another solution. Similarly, installing inspection system to check the printing

process layer by layer during the process can fix the porosity simultaneously.

flow of gas during the process might also lead to oxidation.

stable gap of the torch should be used with optimal gas flow.

Oxidation (Redox) is a process when a chemical reaction happens in metal and leads to change in the structure of atom by losing electrons. Oxidation happens with the availability of oxygen and it is a defect that generates cracks and distortion failure in the printed metal part. It creates a weak insulation material between the printed layers, mostly in DED MAM category. Oxidation is the main reason for variation on mechanical properties. It causes minute or unstable printing gaps between the torch and the bed surface or the previous printed layer. Not enough

The best way to prevent oxidation is by eliminating oxygen from the printing environment. That can be obtained by using air vacuumed chamber 3D metal printer. However, if the vacuumed environment is difficult to be adjusted, optimal

*6.2.2.1 Proposed solution*

*6.2.3 Porosity*

*6.2.3.1 Proposed solution*

*6.2.4 Oxidation*

*6.2.4.1 Proposed solution*

also be used to eliminate fatigue.

**252**

One of the main defects that interrupt the metal 3D printing part is a heat accumulation. This issue happens when the average temperature of the workpiece increases continuously. That will cause many other defects, such variation in mechanical properties and microstructure evolution, and variation in grain size which will, consequently, lead to undesirable part quality, such structural collapse [14]. This issue is also mostly inherent to DED category of the MAM. According to the author analysis, it happens in DED category mostly because in DED process, metal is melted. As a new layer is created over the previous hot one, it will initiate the heat accumulation. Thus, this issue depends on the philosophy of metal printing process. As compared to PBF, in which lower powered laser is used with lower heat, a new layer of powder will be poured over the previous one and that will absorb the heat of the previous layer.
