*6.2.1.1 Proposed solution*

It is advantageous to use well maintained heat treatment machine (vacuum furnace) to remove the residual stress from the MAM parts. Based on the author's understanding, melting back of the previous layer is the main issue which generates the residual stress. This melting back can be reduced by reducing the pace of heating while building the next layer. Reducing the melting back can be done by installing a cooling unit to reduce the heat between layers, such as air jet to reduce the heat generated when a new layer is being melted and laying down on the previous one. Also, using variable building direction might help in reducing heat accumulation in one direction. More comprehensive study should be conducted to build inspection system rather than in-situ method. This inspection system is used to analyze the melting back spot and figure out where it exists and assign permanent solution for that. Moreover, post process control (a type of heat treatment control) should be applied on the printed metal parts which contain residual stress after inspection.

#### *6.2.2 Fatigue*

Fatigue is a damage in a solid material which causes it to weaken. The main reason for fatigue is fluctuating (cyclic) loads applied on metal parts which causes damage at the end and failure in the parts. Several parameters, such as surface finish, shape and geometry of the part, grain size and direction, functionality of the part, load type, part size, corrosion, temperature, and surface treatment cause fatigue. For the MAM method, the temperature is the most affective parameter to cause fatigue in printed metallic parts. Some literature studies have been performed on this subject, especially on multiaxial fatigue in many industries for small and medium size of metallic parts [1].
