Revisiting fundamental welding concepts to improve additive manufacturing: From theory to practice

TL;DR: In this article, a unified equation to compute the energy density is proposed to compare works performed with distinct equipment and experimental conditions, covering the major process parameters: power, travel speed, heat source dimension, hatch distance, deposited layer thickness and material grain size.

About: This article is published in Progress in Materials Science.The article was published on 2020-01-01 and is currently open access. It has received 369 citations till now. The article focuses on the topics: Welding & Arc welding.

Summary

1. Introduction

• Additive manufacturing allows structures fabrication in a layer-by-layer deposition process and is revolutionizing the manufacturing industry due to its ability to obtain near-net shape products, in a short time span, with almost no material waste.
• An upmost key feature of additive manufacturing based on melting and solidification, is the use of fundamental knowledge generated by decades of research on welding metallurgy and technologies, based on electric arc and high-energy density beams (laser and electron beam).
• Thus, additive manufacturing with electric arc and plasma are dedicated to large parts manufacturing.
• Moreover, a heat affected zone is created in the first deposited layer, which, depending on the material, may promote additional solid-state transformations.

2. Welding

• The need to produce complex parts constituted a major driving force for the development of welding technologies.
• It was just in beginning of the 20th century, that welding became a reality in industry, after major scientific and technological breakthroughs.
• Melting of both electrode and base materials to join is promoted by the heat generated in the electrical arc.
• The gas type, rod diameter, feeding rate and stick-out affect the acting forces and, thus, the drop detachment form.
• Both electron and laser beam welding are commonly referred as high-density energy welding processes, owing to the ability to decrease the beam spot significantly.

2.1. Thermal aspects

• The quantity of heat introduced during welding, the thermal conductivity and specific heat of the base material(s) are fundamental to determine the cooling rate, and this is of paramount importance to control solid-state transformations upon cooling that may occur in the heat affected and fusion zones.
• It is worth to mention that these equations have some limiting assumptions, such as: it considers punctual heat sources, heat dissipation simply by conduction under equilibrium conditions, disregarding convection losses in the dynamic molten pool and surface radiation.
• The physical meaning of the heat input is, thus, the energy [J] introduced by unit of length [m].
• The heat input is fundamental in welding, since it affects the volume and shape of the molten material, the extension of both heat affected and fusion zones and the cooling rate in these regions.
• The concept of power density is not considered in arc welding since the arc/material interaction area is difficult to quantify as it varies during welding, even when automated.

2.2. Chemical and microstructural aspects

• In the melted region, due to high temperatures, different chemical reactions can occur: (1) within the liquid; (2) between the liquid and the solid filler material, if any; (3) between the liquid and the surrounding atmosphere.
• The main consequences of each type of reaction are: (1) volatilization of low vapor pressure elements with corresponding modification of the chemical composition of the molten metal; (2) pores and lack of fusion; (3) oxide inclusions.
• Loss of elements generally leads to poor mechanical properties, as well as, to the formation of defects.
• The microstructures in the fusion zone can have different morphologies, such as, planar, cellular and dendritic and these are controlled by: temperature gradient (T), grain growth rate (R), undercooling (ΔT) and chemical composition.
• After some time the supercooling effect decreases, and solidification tends to be planar, in an epitaxial grain growth mode, where the new grains form over the previous ones, and growth along more compact crystallographic directions perpendicular to isothermal lines, in a competitive grain growth process.

2.3. Mechanical aspects

• Since there is always a thermal gradient along a weld, expansion and contraction upon heating and cooling, respectively, are not uniform and thus, the material undergoes differential deformation.
• The pattern of these stresses depend on the joint configuration, material thermo-physical properties (thermal conductivity and coefficient of thermal expansion) and mechanical properties (elastic modulus, yield strength and Poisson ratio).
• Since there is a sharp temperature gradient near the weld, the material experiences distinct rates of expansion and contraction in adjacent sections.
• Depending on the rigidity of the structure residual stresses may equal or exceed the yield stress of the material.
• Pre- and/or post-heating are typically used to reduce residual stresses, since the thermal cycle is smoother and the cooling rate decreases.

2.4. Multipass welding

• In multipass or multi-run welds, microstructural refinement is observed with improvements of weld toughness and reduction of residual stresses [67].
• – each subsequent thermal cycle refines or “normalizes” the grains of the previous weld metal; – the previous pass has an effect of pre-heating the material and, so, the cooling rate decreases; – subsequent thermal cycles tend to anneal out residual stresses caused by previous passes, also known as The main reasons are.
• The subsequent pass partially remelts the previous one and, if there are no solid-state phase transformations, grains grow by epitaxy.
• Therefore, a graded microstructure is often observed along deposited beads, with characteristics and morphologies that depend on the material solid-state transformations.
• A common procedure adopted in design and in welding procedure specifications (WPS) is to record the heat input for each single pass and calculate an average value that is later used in process qualification.

3. Additive manufacturing

• Several technologies of fusion-based additive manufacturing are currently under development.
• The most important advantage of this new way of manufacturing relies in the freedom to produce, virtually, any shape and geometry, in any material or combination of distinct materials.
• Though solid-state manufacturing technologies are being investigated, fusion-based ones are still the most interesting due to productivity issues.
• As far as heat sources are concerned, laser beam, electron beam, electric arc and plasma are the ones under use, while materials to melt can be in the form of powder or wire.
• The thermal effects of successive melted layers deposited on top, or aside, of each other are of paramount importance to prevent defect formation, microstructural features and mechanical performance of the produced parts [70] and, thus, the next section is devoted to analyze the specific effects of multiple thermal cycles.

3.1. Thermal aspects

• While in welding, the heat input concept is well established and it has a linear dimension, in additive manufacturing this largely varies between research groups, especially those using laser-based systems.
• Firstly, an in-volume energy density concept, ED [J mm−3], was introduced.
• In fact, all these equations are empirical without phenomenological physical demonstration.
• Several research efforts are ongoing, attempting to predict component characteristics that could accomplish process certification [1].

3.2. Thermal analysis of additive manufacturing in light of welding technology

• That is, the physical quantity of heat input based on “energy per length” must be used instead of “power per velocity”.
• Therefore, a simple thermodynamic analysis, based on the identification of: (i) a control volume and (ii) an energy balance, leads to conclude that, the 3D equivalent of heat input for additive manufacturing is “Energy per Volume” and is given in Table 2.
• It is important to notice that both end passes (1 and 4 in Fig. 8) contribute with just one half each to the energy balance; – Dividing this energy input summation by the volume of melted material encompassed in the control volume (Eq. (15)).
• This equation can be simplified resulting in Eq. (18), which provides the 3D generalization of heat input in additive manufacturing, referred as energy density (ED): = =HI Q v·h· z ED,3D (18).

3.3. Microstructural effects in fusion-based additive manufacturing

• The energy density highly affects the amount of heat deposited in a certain point, and so the heat transfer conditions.
• From the examples highlighted above, it is clear that the microstructural control that can be achieved in additive manufacturing is an important characteristic, since it can overcome some of the most intricate problems observed in conventional manufacturing processes.
• Delamination is another defect that is identified during additive manufacturing, but not in welding.
• Since for powder-bed systems it is necessary to selectively solidify the top powder layer, the energy is non-uniformly introduced into the material.
• When it comes to additive manufacturing, online inspection is mostly wanted, since a layerby-layer monitoring is most appropriate to detect defects earlier in production of the parts, allowing its correction and the adjustment of the process parameters [117].

4. Strategies to improve weld fusion zone microstructures and its application to additive manufacturing

• One of the critical issues in fusion-based welding is the development of coarse columnar grains in the fusion zone [61], which, according to the Hall-Petch equation [86], decreases the yield stress of the deposited material and, thus, its mechanical resistance.
• For certain alloys, such as Mg alloys, the existence of a fine grained fusion zone can greatly improve the solidification-cracking resistance [122].
• Several techniques exist to control the microstructure and dilution with the parent base material is, eventually, the simplest one.
• Other operating techniques have been developed for alloys without solid state transformations to improve their mechanical resistance.
• This section describes the most important techniques, how they impact the microstructure and mechanical properties of the fusion zone, and how additive manufacturing can use them to improve the microstructure control in as-built 3D parts.

4.1. Heat source manipulation

• In either welding or additive manufacturing techniques based on melting, the heat source plays a fundamental role in the heat transfer, developed microstructures and residual stresses.
• Therefore, the control and manipulation of the heat source is of major importance and this is typically performed by modulation of the pulse current (for arc or high-power beam sources) or by oscillation of the welding arc induced by a magnetic oscillator (for arc-based heat sources).
• Because manipulation of the heat source can be performed in distinct ways, this section is divided in Sections 4.1.1 and 4.1.2.

4.1.1. Modulation of the heat source

• Modulation of the pulse current has been widely exemplified in both arc-based welding [131–133], and laser and electron beam welding [134].
• With such high frequency pulses, it is possible to decrease the arc length, thus providing a tighter control of the fusion zone width and depth, reducing remelting of previous passes and increasing the deposited material quantity.
• Since it is known that during solidification processes there are preferential growth directions, the continuous change in the direction of maximum thermal gradient restricts grain growth, thus favoring the formation of a refined grain structure, in comparison to joints obtained using continuous welding current.
• Another consequence from the use of pulsed welding current is the variation of the arc force during arc welding, which, in turn, enhances fluid flow, thus lowering the temperature at the solidification front.
• When keyhole mode is achieved during powder-bed additive manufacturing [149–151] these instabilities can contribute to increase the probability for pore formation.

4.1.2. Heat source control via magnetic oscillation

• Another way to manipulate the heat source is trough magnetic oscillation and this can be only applied in arc-based processes.
• Oscillation of the arc during welding is effective in modifying the grain orientation in the fusion zone [125].
• For Mg alloys, the controlled addition of Al increases constitutional supercooling, allowing that small solid particles that exist within the fusion zone are not remelted and grow into an equiaxed structure.
• Similar refinement effect can be achieved, provided that the oscillation amplitude and frequency operate within a suitable range.
• It is likely that the most effective arc movements during arc-based additive manufacturing are transverse and circular oscillation, following the observations made during arc-welding of Mg and Al alloys by Kou et al. [122,125].

4.2.1. In welding

• The adjustment of operating procedures during welding, namely the control of the arc voltage, current intensity and type (AC/DC and/or pulsed/continuous), welding power, welding speed and protective gas are fundamental not only to modify the microstructure of the fusion zone but also its geometry, that is, width and depth of the molten pool.
• Therefore, the control of the heat input during welding is of major importance as referred in Section 2 and in multiple research works [62,163–170].
• It must be noticed however, that the constant heat and cooling experienced during sample build-up and the prolonged times at higher temperatures during wire and arc additive manufacturing can act to promote an in-situ stress relieve heat treatment which can lower previously developed residual stresses.
• The control of the heat input is also of major importance for alloys that are prone to liquation cracking.
• Welding shielding gases also have other roles during welding.

4.2.2. In additive manufacturing

• As described above, there are multiple operating parameters that can be modified to control the microstructure, mechanical properties and defect generation during fusion-based welding.
• Because these aspects are addressed in Section 5 of the paper, the authors refrain from developing the topic here.
• Often, researchers in both the welding and additive manufacturing fields, focus on the microstructural changes induced by using different values of heat input.
• Additionally, the high heat input lead to the development of a large grain size microstructure and the hardness throughout the part height was seen to be less uniform than when low heat input is used.
• For arc-based additive manufacturing the currently available literature on the effect of shielding gases is scarcer, with few works addressing the effect shielding gases.

4.3.1. In welding

• The use of grain refiners is widely used in both welding and casting technologies to decrease the grain size of the solidified metals and/or to promote the columnar to equiaxed transition.
• Typically, in fusion-based welding, the introduction of alloying elements to control the grain structure is done by incorporate them in the filler material.
• As a result of this dependence with the free energy volume, the modification of the composition of the melt pool can be such that this term gains more importance than the liquid/solid surface energy.
• As described before, solidification of the weld pool can occur with very small undercooling at the fusion boundary, and by epitaxy typically large columnar grains are obtained.
• Because Sc is very expensive and small amounts of this element have a significant impact on the grain refinement, researchers focused on the development of Sc-based filler materials that can increase the weldability of 2XXX and 7XXX Aluminum alloys series [229].

4.3.2. In additive manufacturing

• The discussion about the mechanisms in which the use of grain refiners or filler materials can be used in fusion-based additive manufacturing follows the same reasoning as previously detailed for fusion welding.
• In [246], the authors tested the use of Tibor, a commercial titanium-boron grain refiner with a chemical composition, in wt.%, of Al5Ti1B, and Sc to act as grain refiners during selective laser melting of two Aluminum alloys, Al-7Si and 6061.
• Though no mechanical testing was performed on these samples, it is expected that improved mechanical properties are achieved when both the grain refinement and the columnar to equiaxed transition exist in the as-built material sample.
• The strategy to deposit a grain refiner by painting each deposited layer was again followed by the same authors but using LaB6 instead [255].
• It is expected that with the increasingly interest in arc-based additive manufacturing for the creation of large Ti-6Al-4V components with short delivery times, more efforts will be devoted to the production of dedicated wires which can incorporate some of the most potent grain refiners for this class of alloys.

4.4.1. Electromagnetic stimuli in welding and additive manufacturing

• During arc-based welding, the presence of an external magnetic field influences both the arc and weld pool behavior due to the Lorentz force, F .
• The use of electromagnetic stirring during arc-based welding can modify the weld bead shape and appearance, alter the solidification microstructure of the fusion zone and refine it, decrease porosity and promote the redistribution of solute in the molten pool [263].
• In arc-based additive manufacturing it is possible to obtain very high deposition rates, in opposition to those possible for powder-bed systems.
• Another potential application for the use of external magnetic fields during fusion-based additive manufacturing is to promote grain refinement, similar to what is already well-established in welding.
• When the magnetic field was employed, the stronger stirring effect within the melt pool changed the solidification microstructure from columnar dendritic to near-spheroidal, thus resulting in a more refined grain structure.

4.4.2. Ultrasonic stimuli in welding and additive manufacturing

• Another way to control the grain structure and obtain grain refinement during welding by mechanical means can be achieved by ultrasonic vibration.
• Some works [130,161,276] have addressed the use of ultrasonic vibration during fusion-based welding.
• In Dai’s work [130] ultrasonic vibration was used to refine the grain structure of the fusion zone and decrease solidification cracking susceptibility.
• Often, in arc welding using a filler material, there exists the formation of unmixed regions.
• The authors studied the position of the ultrasonic probe relatively to the arc and its influence in the grain refinement.

4.5. Summary and future expectations regarding the microstructural control during additive manufacturing

• There is a series of actions in arc welding that can be used to improve the microstructure of the melted region, refining the grain size, controlling its chemical composition and, thus, its mechanical properties.
• Some works [13] report that more than 5500 different alloys are currently available for additive manufacturing, though only few such as Inconel, Ti-6Al-4V and AlSi10Mg, for example, can actually be produced with appropriate microstructures and defect-free.
• The same effect was previously observed during welding, where a vast range of alloys were considered “unweldable”.
• These, as described in this section, encompass the modulation of the heat source, use grain refiners or adjust operating procedures.
• Some key examples from both welding can additive manufacturing were selected and discussed to highlight the multiple similarities between processes and microstructural evolution.

5. Process parameter optimization criteria for additive manufacturing

• Typically, in both laser and electron beam additive manufacturing research works, process parameters as power, travel speed and hatch distance are varied to obtain fully dense.
• Additionally, in additive manufacturing, it is critical to determine the hatch distance that, for a given set of power, travel speed and layer thickness, can be used to produce parts free of porosities or lack of fusion without sacrificing productivity.
• In powder-bed systems the heat conduction is not as effective as in wire feed systems, owing to potential irregularities in the powder distribution that may prevent optimum heat flow during the interaction between the heat source and the material.
• To guarantee that all powder is completely melted during production and to avoid the presence of lack of fusion defects and pores, it is understandable that the hatch distance should be as small as possible.
• Thus, one objective of this work is to establish criteria to determine the best hatch distance for any given set of process parameters.

5.1. Geometric criterion to determine the hatch distance, h

• Consider, for simplicity, that the fusion zone created in a given deposited track is a semi-circle, as schematically depicted in Fig. 20.
• Such simplification is valid owing to the conduction mode model derived by Eagar et al. [277], which predicts this type of geometry when conduction mode is observed and was experimentally validated for fusion-based additive manufacturing in [149].
• The overlap distance, OL, between two consecutive tracks is given by: =OL R h2 ,m max (20) However, it must be noticed that increasing the overlap distance, decreases the hatch distance below a maximum value, hmax, schematically presented in Fig. 20.
• Thus, the amount of non-melted powder increases when increasing the hatch distance above hmax.
• The above-mentioned criterion allows to determine an optimum hatch spacing simply based on geometric relationships between consecutive deposited tracks considering that the process parameters (power, P, and travel speed, v) are adequate to generate an approximate semi-circular region of melted material with a radius of Rm.

5.2. Energy criterion to determine the melted radius, Rm

• This criterion assumes, for simplicity, an adiabatic system and that all the introduced energy is used to raise the material temperature above its melting temperature but below the vaporization point.
• Cp solid and Cp liquid are the heat capacity [J kg−1 K−1] of the solid and liquid material, respectively; Heatfusion is the latent heat of fusion [J kg−1]; ΔTsolid and ΔTliquid are, respectively, the temperature increase (in [K]) in both the solid and liquid phases owing to the energy introduced in the control volume.
• Owing to the simplifications assumed to determine Rm, this value is higher than in actual practical conditions, thus acting as an upper bound limit.
• That is, this is the maximum radius of melted pool created for a given set of processing parameters.
• After Rm is known it is possible to determine the upper bound value for the hatch distance using Eq. (22).

5.3. Thermal criterion to determine the melted radius, Rm

• The third proposed criterion to determine the Rm value is based on the Rosenthal equation, which is widely used to estimate the thermal cycle experienced in conduction welding mode.
• The thermal diffusivity (α) and the thermal conductivity (K) are related to the powder.
• Therefore, the solution of interest is the pair (liquidus temperature; Rm).
• With the theoretical value of Rm calculated, the maximum hatch distance can be computed using Eq. (22).

5.4. Effectiveness of the proposed criteria

• It is important to emphasize that all the proposed criteria consider some simplifications, namely: it is assumed that the thermophysical properties of the powder and of the as-deposited material are the same, disregarding the effects of porosity and grain boundaries between individual powder grains.
• If the layer thickness is larger than half of the beam diameter, the probability to have regions with incomplete fusion gradually increases with increasing hatching distance.
• Therefore, it is important to take into consideration the powder size distribution.
• In fact, in [148], it was observed that after this point the as-build parts start to have a lower density as a result of the porosity created, which is in good agreement with the fact that the selected hatch distance is higher than the one calculated by the energy criterion.
• Schematically, the methodology to determine the maximum theoretical hatch distance for the three proposed criteria can be schematically presented in Fig. 25.

6. A dimensionless parameter for energy density calculations in additive manufacturing

• Additionally, from the analysis previously presented, two parameters were not considered, and these were the diameter of the heat source and the powder grain size.
• Such would greatly help to replicate experiments and could potentially benefit the additive manufacturing community so that the influence of different process parameters could be fully understood and manipulated to achieve high quality parts.
• This is a common procedure in industrial equipment, where the parameters to control are set to a limited range to prevent damages and reduce setup time.
• Therefore, it is possible to assume that the beam diameter used in those investigations is the one set by the machine manufacturer.
• Therefore, it is proposed an equation for the energy density during additive manufacturing, defined as: =ED P v h d , (29) where β is a dimensionless parameter defined as the ratio between the powder grain size or wire diameter (gs) and the diameter of the heat source : = g d ,s HeatSource (30).

7. Conclusions

• This paper evidences the major similarities between fusion-based multipass arc welding and fusion-based additive manufacturing, especially in terms of thermal effects, solidification mechanisms, and chemical reactions within the melted region, as well as, distortion and residual stresses.
• The major concepts developed in fusion welding were described in detail and applied to different materials that are also being used in additive manufacturing.
• Evaluation of these criteria was performed using actual data reported in the literature.
• It was observed that the theoretical values of the hatch distance correspond to an upper bound limit, which can be of assistance for researchers to tune the process parameters (power, travel speed and layer thickness) to obtain defect-free parts without compromising productivity.
• There are other process parameters, namely the size of the heat source and the grain size of the material to be deposited, that affect the quality of additive manufacturing, besides those contained in the heat input concept.

Figures

Fig. 3. Schematic representation of the effect of temperature gradient and growth rate on the solidification microstructures (from [1]).

Fig. 17. Macro and microstructures of: (a–c) Ti-6Al-4V; (d–f) Ti-3Al-8V-6Cr-4Mo-4Zr; (g–i) Ti-3Al-8V-6Cr-4Mo-4Zr+ La2O3. The red squares on the top row represent the regions analysed by scanning electron microscopy depicted in the middle and bottom rows (adapted from [252]).

Fig. 18. Effect of Si addition during wire and arc additive manufacturing of commercially pure Ti: (a) 0.04 wt.% Si; (b) 0.19 wt.%; (c) 0.75 wt.% (from [260]).

Fig. 11. Inverse pole figure orientation maps on the cross-section of a Ni-based superalloy fabricated using powder-bed electron beam additive manufacturing (from [160]).

Fig. 12. Different types of movements enabled using a magnetic oscillator.

Table 3 Effect of different filler materials in fusion welding of engineering alloys.

Fig. 20. Geometrical profile created by two consecutive melted tracks in additive manufacturing where melting of the powder and partial remelting of part of the previously deposited layer occurs.

Fig. 4. Schematic of a butt welded joint showing a residual stress pattern.

Fig. 5. Example of angular distortion due to a weld deposited bead.

Fig. 6. Schematic representation of a multipass weld. Numbers 1 to 6 represent the sequence of passes. Dotted lines correspond to the heat affected zones created by each pass. Zone A is reheated once (by weld pass 2), whereas zone B is reheated twice (by weld passes 2 and 3) (adapted from [68]).

Fig. 19. Microstructure in the fusion zone of AZ31 welds: (a) without ultrasonic stirring; (b) with ultrasonic stirring and the ultrasonic probe located between the arc and the mushy zone; (c) with ultrasonic stirring and the ultrasonic probe located in the mushy zone. Adapted from [161].

Fig. 14. EBSD maps for two Aluminum alloys processed by selective laser melting: (a) Al-7Si without grain refiners; (b) Al-7Si with 0.33 wt.% Tibor; (c) Al-7Si with 0.4 wt.% Sc; (d) Al 6061 without grain refiners; (e) AA 6061 with 0.33 wt.% Tibor; (f) AA 6061 with 0.4 wt.% Sc (from [246]).

Fig. 24. Use of the thermal criterion to evaluate the hatch distance: comparison between hatch distance used in [279] (for process parameters were the layer thickness was kept constant, while the power and the travel speed were varied) with maximum theoretical hatch distance determined based on Eq. (22). The hatch distance used in all experimental conditions of [279] was of 50 µm.

Fig. 25. Schematic representation of the methodology used to determine the maximum theoretical hatch distance for the three proposed criteria.

Table 4 Effect of different elements during fusion-based additive manufacturing of several engineering alloys.

Table 2 Heat input calculation for welding (1D) and for additive manufacturing (3D).

Fig. 8. Control volume used to determine the in-volume heat input (or energy density) during additive manufacturing.

Fig. 9. Schematic representation of different gradient structures which can be created by additive manufacturing (from [91]).

Fig. 10. Effect of thermal gradient and solidification rate on the columnar-to-equiaxed transition during laser-based additive manufacturing (adapted from [99]).

Fig. 1. Forces acting on a drop hanging from a wire during welding: Fg – gravity; Fd – plasma jet; Fem – electromagnetic forces/Pitch effect; Fts – surface tension; Fv – vaporization.

Fig. 23. Comparison between calculated hatch distance based on energetic criterion and the one used in [148] (power and layer thickness were kept constant and the velocity varied) with maximum theoretical hatch distance determined based on Eq. (22).

Frequently asked questions

Q1. What are the contributions in "Revisiting fundamental welding concepts to improve additive manufacturing_ from theory to practice" ?

This paper aims to review these concepts, highlighting the distinctive characteristics of fusion welding that can be embraced by additive manufacturing, namely the nature of rapid thermal cycles associated to small size and localized heat sources, the non-equilibrium nature of rapid solidification and its effects on: internal defects formation, phase transformations, residual stresses and distortions. This equation enables to compare works performed with distinct equipment and experimental conditions, covering the major process parameters: power, travel speed, heat source dimension, hatch distance, deposited layer thickness and material grain size. 

Q2. What is the effect of the introduction of LaB6 on the microstructure of the asbuilt?

the introduction of CaF2 was also seen to affect the microstructure of the asbuilt parts by increasing the content of β phase. 

Q3. What are the main regimes for drop detachment from the tip of the wire?

Depending on the current voltage and intensity, three main regimes for drop detachment from the tip of the wire can occur and these are: short circuit, spray and globular. 

Q4. What can be used to control the residual stresses and distortion during fusion-based additive manufacturing?

management of the interlayer temperature can be used to control residual stresses and distortion during fusion-based additive manufacturing [207]. 

Q5. What is the important parameter to control the temper bead technique?

Parameters of interest to control the temper bead technique include the heat input, which will control the thermal cycle but also the bead overlap. 

Q6. What are the main techniques used to control the microstructure of fusion-based welding?

Microstructural control during fusion-based welding can be achieved through: (i) manipulation of the heat source, which includes, amongst others, control of the pulse shape [123], by arc pulsation [124] or oscillation [125]; (ii) control the chemical composition of the fusion zone by the introduction of nucleation particles into the melt pool [126]; (iii) adjustment of welding parameters (welding power, welding speed, shielding gas type and flow) [21,47,127]; (iv) through the use of external electromagnetic [128,129] and ultrasonic stimuli [130]. 

Q7. What are the main advantages of using in-situ monitoring techniques in additive manufacturing?

Other in-situ monitoring techniques currently used in additive manufacturing encompass the use of high-speed cameras to control the size of the melt pool and its temperature. 

Q8. What can be used to manipulate the grain orientation in different regions of interest?

Aside from controlling the grain size and morphology, manipulation of the heat source can be used to texture the material in different regions of interest. 

Q9. What is the effect of shielding gases during wire and arc additive manufacturing?

The effect of shielding gases during wire and arc additive manufacturing is quite important since, depending on the shielding gas selection, it can consume or add energy to the arc and thus affect the deposited depth and width, as well as, its chemistry due to liquid/gas reactions in the melt pool. 

Q10. What is the effective way to reduce pore formation during laser welding?

During laser or electron beam welding, the high energydensity of the process can actively contribute to increase the likelihood of pore formation. 

Q11. What are the popular tools for metallurgical understanding and description of the underlying phenomena?

For accurate metallurgical understanding and description of the underlying phenomena, new tools for materials analysis, such as dilatometry [48,49], scanning electron microscopy [39,50–53], transmission electron microscopy [54–56], X-ray diffraction [57–59] and software for thermodynamics, thermal and mechanical behavior [60], have been developed and are now extensively used by both academia and industry. 

Q12. What is the effect of a selected gas on the welding surface?

This effect is of significant importance for both welding and additive manufacturing: if a selected shielding gas promotes an increase in both width and penetration of the weld metal, then low heat inputs can be used, potentially decreasing the risk of distortion. 

Q13. What are the three criteria used to determine the upper bound limit for the hatch distance?

Concerning additive manufacturing process optimization, distinct criteria (geometric, energetic and thermal) are proposed to determine an upper bound limit for the hatch distance. 

Q14. What is the reason for the presence of porosity in parts fabricated by additive manufacturing?

The presence of porosity in parts fabricated by additive manufacturing may be intentional, when it is aimed to obtain porous structures [100,101], or unintentional, when the process is not controlled and undesired pores remain in the part [102]. 

Q15. What is the main reason why the use of low heat input is helpful during additive manufacturing?

Although this was not discussed by the authors, decreasing the heat input results in lower shrinkage upon cooling, which further justifies why the use of low heat input is helpful during additive manufacturing of materials with high cracking susceptibility, such as Inconel. 

Q16. What is the effect of the arc welding on the mechanical properties of the as-built parts?

no columnar to equiaxed transition occurred suggesting that the as-built parts still present a relatively high anisotropy in their mechanical properties. 

Q17. What is the way to control the grain structure during welding?

Another way to control the grain structure and obtain grain refinement during welding by mechanical means can be achieved by ultrasonic vibration.