3.1. WAAM parts characterization
Evaluating the low-alloy thin-walled steel structures manufactured through WAAM and HF-WAAM (Fig. 2), is noticeable the superior spattering presented on the hot-forged component, sustained by the higher measured values of surface waviness (1210 ± 25 µm), when compared to the measured values for the conventional WAAM structure (977 ± 63 µm). Such observation was slightly unexpected, running against the predictions presented by Duarte et al. , where deforming the as-deposited layers at high temperature could result in improved waviness and surface roughness of the produced components.
Isometric micrographs from the centre areas of both structures are presented in Fig. 5. The material is heat treatable, and consequently sensible to thermal cycling effects. Considering the cooling rate as one of the main factors to delineate the grain growth process, the relative higher cooling rates of the WAAM deposition process may be responsible for promoting the formation of larger grains, as well as the development of a quasi-equiaxed structure, i.e without a significant preferential orientation.
As depicted in Fig. 6, both thin-walled structures revealed uneven microstructures along the building direction, changing the grain size across the height, as a consequence of the effect of the different thermal behaviour (cooling rates, substrate effect) experienced by the material during the deposition process. Moreover, the larger grain size is commonly observed on the centre zones of the samples, which results from two different heat effects: the increased difficulty in sinking the heat through the underlying layers, which are progressively warmer; and the repetitive thermal effects caused by the deposition of the overlying layers that causes a temper heat treatment effect.
As supported by Duarte et al. , the hot-forging WAAM variant can generate a microstructure refinement, promoting grain size reduction and so improving the mechanical properties. Similar results were reported by Hönnige et al.  when peening a WAAM deposited Ti-6Al-4V wall. From Fig. 5 and Fig. 6, it is clear the hot forging effects on the microstructure of the HF-WAAM sample, particularly on the smaller grain size of the material.
The hardness measurements registered for the areas of interest of the cutting experiments are shown in Fig. 7. For WAAM samples, the hardness was in the 220-260 HV range, while for HF-WAAM samples higher hardness values were measured, namely 240-300 HV. The low-carbon steel, as expected, registered lower hardness values (160-180 HV range).
Both WAAM and HF-WAAM structures revealed identical hardness profiles along the height, decreasing from the base to the centre, and increasing from the centre to the top, being in good agreement with the aforementioned changes in grain size, following the Hall-Petch equation. The grain size has a direct impact on the hardness levels, since the grain boundaries act as a mechanical barrier to dislocations propagation, validating the acquired data for the superior hardness of the HF-WAAM structures. The higher hardness on the HF-WAAM sample is generated by the hot-forging effects when the material is at the range of hot-working temperatures, promoting the grain refinement effect on the material.
The hot forging effects have revealed major mechanical improvements regarding the yield strength, when subjected to uniaxial compression tests. As depicted in Fig. 8, the yield stress has revealed a very similar profile as the hardness measurements, due to the microstructural changes developed across the height. For the conventional WAAM specimens, the measured yield stress range is in accordance with the values presented in .
The electric conductivity measurements revealed no main differences between samples, and the results were in the 5-6 % IACS amplitude, as shown in Fig. 9. The variation experienced during the tests can be assigned to microstructural differences within the same zone. However, is more evident in the base zone of the HF-WAAM structure. It is possible to conclude that despite the microstructural variations observed along the sample height, the electrical conductivity remains almost constant. Moreover, the hot forging does not significantly affect the material electrical conductivity. Although this technique can identify microstructural changes , the grain size variations observed in each sample, and during the comparison between the conventional WAAM and the HF-WAAM, are within the same order of magnitude, and therefore the electrical conductivity of the material is not affected.
3.2. Cutting force and Specific Cutting Energy
In an orthogonal cutting operation, the total cutting force F can be decomposed into two components: Fc, the cutting force acting in the direction of the tool movement; and Fp, the feed force acting perpendicularly to the cutting force (Fig. 3). Both components produce deflection in the workpiece and the cutting tool. The influence of cutting speed and undeformed chip thickness on the cutting force is depicted in Fig. 10. No major influence of the cutting speed is observed on the cutting force results, as expected. Moreover, increasing the undeformed chip thickness, the cutting force increases, due to the higher amount of material removed during the cut. Such observations are shared in both WAAM, HF-WAAM and low-carbon steel samples. As commonly referred in the literature [28–30], there is no straightforward formula to calculate how machinability is dependent on the hardness of the material. However, the literature refers to the fact that cutting force tends to increase with increasing hardness. Such evidence was not completely observed in this work, as the increased hardness of the HF-WAAM samples (when compared to conventional WAAM and low-carbon steel) has revealed different trend. As would be expected, the cutting force tends to increase with increasing UCT in all samples, and its behaviour was similar in all cases. The measured cutting force values were lower for WAAM samples, on the base (except for lower values of UCT when cutting at lower speed) and centre areas. With respect to the base area, the cutting force for the WAAM samples ranged from being 14% lower (v = 5 m/min; h = 0.3 mm) to 14% higher (v = 7 m/min; h = 0.6 mm) than for the similar area on HF-WAAM samples. Regarding the centre area, the cutting force values for WAAM samples were up to 10% lower. On the top area the cutting force was lower for the HF-WAAM samples (up to 12 %). Nevertheless, the results are in accordance with the literature [31, 32] since the low influence of the mechanical properties and cutting speed on the cutting force is documented.
Figure 11 depicts the comparison of WAAM, HF-WAAM and low-carbon steel experimental results for specific cutting energy (SCE). It is possible to notice that the SCE values, regardless of the cutting parameters and sample type/area of interest, remained within the anticipated range of 1.68 to 2.28 GJ/m3, in accordance with most steel alloys [33–35]. Therefore, it is possible to confirm that neither the mechanical properties (hardness and yield strength) or the cutting speed have largely changed the SCE values for the employed cutting conditions. The increase of the undeformed chip thickness tends to decrease the SCE according to the typical power tendency (SCE = C·h−m). The models obtained for the low-carbon steel samples presented an expected exponent for steel alloys (within 0.252 and 0.358) as the WAAM samples machined using a lower cutting speed. However, whit increased cutting speed, the uncut chip thickness influence diminished, as the power exponent decreased to a range between 0.035 and 0.122. The same occurs for HF-WAAM samples, regardless the used cutting speed, which can indicate that the UCT is not the only variable to consider for the mathematical expression for SCE.
The SCE can also be used as an indicator of the tool wear during the machining operation . Thus, the observed consistent behaviour can sustain the tool integrity throughout the experimental work.
3.3. Shear Angle and Shear Stress
The shear angle is fundamental in chip formation, as increasing the shear angle allows to decrease strain, machining forces, and power requirements. In fact, compared to the cutting force results, the measured shear angle showed inverse behaviour. Analysing the differences between WAAM and HF-WAAM, WAAM samples presented higher values for lower cutting speed, with exception of the top area. The range of shear angle was within 29.1° and 33.7°. It should be noted that WAAM samples showed a very similar behaviour to low-carbon steel when the cutting speed was higher.
The experimental values were compared to the theoretic models, namely the geometrical relationship of orthogonal cutting, based on the cutting ratio or chip thickness ratio, the Lee-Shaffer Maximum Shear Stress Principle (MSSP) and the Merchant’s Minimum Energy Principle (MEP). As shown in Fig. 13, the MSSP is the model that better predicts the experimental shear angle for most of the experimental tests. Similar conclusions were found by Silva et al.  performing orthogonal cutting on AISI 1045 medium carbon steel and Berezvai et al. , when studying the shear angle variation during orthogonal cutting of aluminium, in a stationary cutting regime and without build-up edge (BUE) formation.
The geometrical relationship of the orthogonal cutting model presented the worst prediction on the shear angle, which would be expected, since the chip thickness measurements are greatly operator influenced. Also, as predicted, the MEP has overestimated the values for the shear angle; however, the MEP prediction was the most uniform through the cutting experiments.
Regarding the cutting parameters, both cutting speed and undeformed chip thickness have shown no major influence on the shear angle. However, the value range is similar to that observed by Varga et al.  when simulating, through finite element analysis, the cutting process on AISI-1045 steel. Choi et al. , have reached similar conclusions in a study on the shear angle variation in orthogonal cutting, attesting the independence of the shear angle from the undeformed chip thickness. In addition, the shear angle experimental values are more consistent throughout the height when machining HF-WAAM.
Figure 14 shows the shear stress levels required to allow the chip formation process to occur. Examining the WAAM and HF-WAAM tests, it can be noted that the generated shear stress is close to the material tensile strength (range from 769 to 942 MPa), as should be expected. Moreover, it is also possible to emphasize the similarity behaviour between the WAAM and HF-WAAM samples, when using different combinations of cutting parameters. Despite the known differences in the mechanical properties of the additively manufactured material, the effects on the shear stresses, apparently, only arises when machining at the higher speed (v= 7 m/min). However, the generated shear stress levels for the low-carbon steel shows almost an equal behaviour as for WAAM-produced samples, revealing an apparent independent relation of the shear stress from the UCT. However, for the HF-WAAM, such relation is less noted when cutting at the higher speed.
3.4. Friction Coefficient
Figure 15 depicts, the calculated friction coefficient, µ, for the different experiments. The cutting speed produced minor effects on the friction, which may show that the material did not undergo severe thermal softening effects, as described by Okida et al. , when machining alloy steel. Machining WAAM and HF-WAAM material has reduced the generated friction levels up to 17 %, comparatively to the low-carbon steel, probably due to the refinement effect of the microstructure. Additionally, for the WAAM and HF-WAAM components, the friction coefficient was, for all experiments, in conformity with the 0.5-0.8 range described in the literature  for static friction on steel-on-steel contact, considering dry conditions. However, the friction coefficient was described by the constant friction coefficient of Coulomb, µ = Ff / FN. Still, the values of the friction coefficient experimentally measured in orthogonal cutting tests are usually much higher than those used analytically on metal cutting, due to only be considered the static friction effect. The models typically assume µ = 0.5, whereas experimentally obtained values can be up to 2 . A direct relationship is theoretically established between the friction coefficient and the friction angle, being the processed values being constrained in the 34.2° to 35.5° range.
3.5. Surface Roughness
When machining functional components, the surface quality must respect defined specifications. The surface roughness results for each material, cutting speed, WAAM manufacture process and area of interest were measured. From the experimental measurements, it was possible to confirm that all the results were in accordance with the recommended values of Ra for shaping operations, within the 0.4-25 µm range . Machining additive manufactured HSLA steel components has revealed lower roughness values compared to machining low-carbon steel (1.2-10.9 µm range), despite the improved hardness. The average surface roughness values were lower within the range 5–90% for WAAM samples and from 7–84% for HF-WAAM samples. This is a general result of the lower plastic deformation that harder (and brittle) materials sustain on the machined surface, comparatively to softer materials.
The most uniform and better surface roughness levels were reached when machining at a higher cutting speed, most likely due to the decreasing tendency for BUE formation and increased thermal-softening effects that reduces the adhered material in the rake surface of the tool. However, the higher surface roughness results were reached at higher UCT, probably due to vibrations generated by the greater amount of material removed. Similar conclusions were reported when machining additive manufactured Titanium Alloy .
3.6. Chip Observations
The generated chips were collected, and the morphology analysed for different samples and areas of interest, as shown through the macroscopic images of Fig. 16. No pronounced difference was observed in terms of cutting speed. However, the effect of different UCT on the chip thickness and curvature radius is notorious. As stated by Arshinov et al. , studies shown that the chip curvature radius tends to decrease with increased shear angle and with decreased UCT. Following the chip radius measurements presented in Fig. 16 (for different UCT) and the shear angle results, it is possibly to confirm that such relation was witnessed for almost all the cutting conditions. In comparison, the radius of the chip curvature is significantly lower when machining the HF-WAAM samples relatively to machining both conventional WAAM and low-carbon steel, regardless of the area of interest. Since the curvature radius is related to the plastic deformation of the chip and nonuniform thermal-cooling effects throughout its thickness, the lower values of chip radius of the HF-WAAM samples can possibly be attributed to smaller cutting temperatures and lower friction effects, generated in the tool-chip interface zone.
Also, from Fig. 16, it is possible to recognize that all the cutting operations have generated continuous chips without side-flow tendency. Such chip geometry is in accordance with the obtained surface finish results.
The chip thickness ratio (CTR) is an important machining feature that is used to evaluate the efficiency of the chip formation process, through the degree of plastic deformation generated during the machining operation . It is known from the literature that higher CTR values are desirable, since are followed by higher shear angles and, therefore, less cutting forces, promoting a better cutting operation. As shown in Fig. 17, regarding the WAAM samples, the higher CTR was obtained when cutting the centre zone of the samples, at higher cutting speed and UCT (v = 7 m/min, h = 0.6 mm). Following the HF-WAAM samples, the CTR values become more homogeneous, revealing a more consistent cutting operation. Comparing to WAAM, the CTR values of the HF-WAAM samples were improved when machining the base (up to 38%) and the centre zone (up to 28%) of the structures.
Following the low-carbon steel, higher CTR values were registered comparing to HF-WAAM and WAAM samples. Such results are generally in accordance with the cutting force measurements and the superior mechanical properties observed for the additively manufactured material. However, no substantial differences were observed for the CTR values of the low-carbon steel chips, for the employed cutting conditions.
Figure 18, Fig. 19, and Fig. 20 depict the microscopic images of the generated chips corresponding to the base area of each sample (conventional WAAM, HF-WAAM and Low-carbon steel) when cutting with UCT of 0.6 mm and cutting speed of 7 m/min.
From Fig. 18, regarding the originated chip from the WAAM cutting test, it is possible to point out several details. First, it is relatively easy to identify the primary and secondary shear zones. Originated at the shear plane, the primary shear zone is created by the tool compressive forces during tool penetration into the sample, represented by an elongated effect on the material grains. The secondary shear zone is created between the tool rake surface and the chip, where the friction effects lead to additional plastic deformation, rearranging the material in a parallel direction to the tool rake surface. During the cutting operation, the hardness of the sample acts as an opposite force to the cutting tool movement. Therefore, while the adjacent side to the tool is smooth, a sawtooth-like profile, composed by serrations, is generated in the opposite side of the chip, probably due to thermal-softening effects and uneven strain distribution during the cutting operation. Observing one serration in detail, it is possible to detect the material reorientation (plastic deformation) when subjected to the compressive forces on the shear plane, describing a vortex-style shape.
Figure 19 depicts the originated chip from the HF-WAAM sample. It is also possible to identify the primary and secondary shear zones. However, it is possible to notice more dense and linear shear zones, due to the grain refinement effect created by the hot-forging effect on the material. Additionally, the improved mechanical properties of such samples may have had an impact on the created serrations. As described in the literature , the number of chip segments per unit length is directly related with the microstructure of the material, from which the number of serrations is associated to the ductility of the material. Through the observation of Fig. 19, it is possible to recognise such phenomenon through the existing differences in the number and magnitude of the serrations presented in the HF-WAAM chips, an indicator of the influence of the different physical and mechanical properties of such samples.
As can be seen in Fig. 20, regarding the generated chips from low-carbon steel samples, the primary and secondary shear zones can also be identified, being notable the flattened effect on the grains and, consequently, a parallel displacement of the shear planes, originated during the chip formation process. Such observation supports the ideal shear card model proposed by Pijspanen, based on the overlapping shear planes .
Finally, it is possible to observe the material behaviour when subjected to the shear deformation. Comparing with the conventional WAAM and HF-WAAM samples, where the effect of the refined microstructure is noted, is possible to observe larger and unequal serrations on the isotropic low-carbon steel chips. As previously stated, such characteristics are a strong indicator of uneven shear strain distribution during the cutting operation.