3.2 Characterization in the cross section of laser-treated and untreated materials

Figure 3 shows the cross-sectional analysis by OM. In this region can be observed the penetration depth of the treated region was around 250 μm, and the distance between the weld fillets was approximately 300 μm (also was shown in the first micrograph, Figure 2). Note clearly visible difference of the treated region microstructure and of the substrate.

The laser melted surface micrograph is shown at Figure 3, as can be seen it is free of microcracks and the melted regions are free of precipitates too. Fine microstructure of the melt zone is attributed to high cooling rate. Microstructure obtained in this work is similar to other laser melted aluminum alloys reported in the literature, i.e., Watkins et al. [19] reported that the microstructure of laser melted AA 2014 consists of columnar grains growing epitaxially from the substrate. Although,

Figure 3. OM micrograph in the cross-sectional area of laser-treated material.

maximum melt depth observed in this work was 250 μm (Figure 3); however the thickness of this zone depends of laser power and of the Marangoni effect, as was discussed by Pariona et al. [4, 8], these authors demonstrated when the laser beam velocity is low, therefore the molten zone depth is greater.

Figure 3 also shows zones where there is overlapping of consecutive weld fillets. This overlapping is more common in Al-2.0 wt.% Fe alloy than in Al-1.5 wt.% Fe alloy, reported by Pariona et al. [2, 3, 8]. Kalita et al. [1] also reported overlapping of consecutive weld fillets and Cordovilla et al. [20] pointed out as essential tool to understand way in which each track affects the microstructures produced by previous one.

to fine-columnar-dendrite structure. According to Pariona et al. [2], behavior of the laser treated region is homogeneous and similar to an amorphous phase; hence, it shows greater hardness, lower surface roughness, and higher corrosion resistance,

SEM micrograph in the cross-sectional sample of Al-2.0 wt.% Fe alloy LSR-treated: (a) overlapping line of consecutive weld fillets, (b) interface of treated surface and substrate, (c) substrate unaffected by laser treatment, (d) detail in the cast region, (e) interfacial region of the treated surface and substrate, and (f) detail

Effect of Microstructure on Microhardness and Electrochemical Behavior in Hypereutectic…

Vickers hardness test was accomplished in this work and by means of a microscope coupled to the tester, the "d1" and "d2" diagonals formed in area indented by pyramid were measured, and these parameters were used to calculate Vickers hardness. Figure 5 illustrates indented areas used for calculation of the hardness of

Microhardness profiles were measured along in a cross-sectional sample, for laser-treated layer and untreated. These measurements were taken along lines parallel to surface at depths of 50, 100, 200, 300, 500 and 700 μm, applying a load of HV 100 gf for 15 s. Figure 6 illustrates the 15 micro-indentations made in the cross section at each of these depths to measure the hardness. Average hardness values and standard deviation (s.d.) at each depth were calculated based on these mea-

An analysis of the data in Table 2 indicates the HV is higher for the LSR treated

The data in Table 2, also is shown in graphical form in Figure 7, it clearly show increase in hardness at treated region than untreated substrate. This difference is attributed to microstructural changes as resulting of LSR-treated. In other studies

region than the untreated region. The average hardness of the treated region is 58.8 HV, while that of the untreated region is 35.7 HV, which corresponds at 60.7% increase in hardness in the treated region compared to the untreated region.

reported by Pariona and Micene [7].

of the substrate unaffected by laser treatment.

DOI: http://dx.doi.org/10.5772/intechopen.81095

surements, and are given in Table 2.

3.3 Vickers microhardness test

Al-2.0 wt.% Fe samples.

181

Figure 4.

Figure 4 depicts a cross-sectional LSR-treated sample and analyzed by SEM, showing some regions of substrate and the as-cast microstructure. In the cast area in Figure 4, note presence of protuberances, which correspond to on the weld fillet region (also shown in Figure 3). According to Pariona et al. [4], presence of protuberances is more noticeable in Al-1.5 wt.% Fe alloy than in Al-2.0 wt.% Fe alloy. Figure 4(a) also shows an overlapping line of consecutive weld fillets. Figure 4(b) and (e) show the substrate region and the laser-treated area under higher magnification, showing a visibly different microstructure, with a dendritic-like structure. This microstructural difference between untreated substrate and LSR-treated region is attributed to temperature applied on the material surface, which exceeded its melting point but was lower than boiling point, followed by rapid cooling in laser treatment process and this leads a high thermal gradient, and so in this way produces the laser melted zone. This treatment resulted in formation of a thin recast layer with a refined microstructure practically free of precipitates, inclusions and intermetallic phases [18], as can be clearly seen at the magnified image, Figure 4 (d), with a columnar dendrite structure, Watkins et al. [19] and, Grum and Sturm [21] have also reported this characteristic in laser cast materials. Figure 4(c) shows the substrate region, which is also displayed under higher magnification in Figure 4 (f), showing presence of intermetallic phase dispersed in the matrix. A comparison in more detail of Figure 4(d) and (f) reveals that the treated region morphology is more homogeneous, without presence of the intermetallic phase that extends throughout the recast area and showing evidence of transition from coarse-grained

Effect of Microstructure on Microhardness and Electrochemical Behavior in Hypereutectic… DOI: http://dx.doi.org/10.5772/intechopen.81095

#### Figure 4.

maximum melt depth observed in this work was 250 μm (Figure 3); however the thickness of this zone depends of laser power and of the Marangoni effect, as was discussed by Pariona et al. [4, 8], these authors demonstrated when the laser beam

Figure 3 also shows zones where there is overlapping of consecutive weld fillets. This overlapping is more common in Al-2.0 wt.% Fe alloy than in Al-1.5 wt.% Fe alloy, reported by Pariona et al. [2, 3, 8]. Kalita et al. [1] also reported overlapping of consecutive weld fillets and Cordovilla et al. [20] pointed out as essential tool to understand way in which each track affects the microstructures produced by pre-

Figure 4 depicts a cross-sectional LSR-treated sample and analyzed by SEM, showing some regions of substrate and the as-cast microstructure. In the cast area in Figure 4, note presence of protuberances, which correspond to on the weld fillet region (also shown in Figure 3). According to Pariona et al. [4], presence of protuberances is more noticeable in Al-1.5 wt.% Fe alloy than in Al-2.0 wt.% Fe alloy. Figure 4(a) also shows an overlapping line of consecutive weld fillets. Figure 4(b) and (e) show the substrate region and the laser-treated area under higher magnification, showing a visibly different microstructure, with a dendritic-like structure. This microstructural difference between untreated substrate and LSR-treated region is attributed to temperature applied on the material surface, which exceeded its melting point but was lower than boiling point, followed by rapid cooling in laser treatment process and this leads a high thermal gradient, and so in this way produces the laser melted zone. This treatment resulted in formation of a thin recast layer with a refined microstructure practically free of precipitates, inclusions and intermetallic phases [18], as can be clearly seen at the magnified image, Figure 4 (d), with a columnar dendrite structure, Watkins et al. [19] and, Grum and Sturm [21] have also reported this characteristic in laser cast materials. Figure 4(c) shows the substrate region, which is also displayed under higher magnification in Figure 4 (f), showing presence of intermetallic phase dispersed in the matrix. A comparison in more detail of Figure 4(d) and (f) reveals that the treated region morphology is more homogeneous, without presence of the intermetallic phase that extends throughout the recast area and showing evidence of transition from coarse-grained

velocity is low, therefore the molten zone depth is greater.

OM micrograph in the cross-sectional area of laser-treated material.

vious one.

180

Figure 3.

Aerospace Engineering

SEM micrograph in the cross-sectional sample of Al-2.0 wt.% Fe alloy LSR-treated: (a) overlapping line of consecutive weld fillets, (b) interface of treated surface and substrate, (c) substrate unaffected by laser treatment, (d) detail in the cast region, (e) interfacial region of the treated surface and substrate, and (f) detail of the substrate unaffected by laser treatment.

to fine-columnar-dendrite structure. According to Pariona et al. [2], behavior of the laser treated region is homogeneous and similar to an amorphous phase; hence, it shows greater hardness, lower surface roughness, and higher corrosion resistance, reported by Pariona and Micene [7].

#### 3.3 Vickers microhardness test

Vickers hardness test was accomplished in this work and by means of a microscope coupled to the tester, the "d1" and "d2" diagonals formed in area indented by pyramid were measured, and these parameters were used to calculate Vickers hardness. Figure 5 illustrates indented areas used for calculation of the hardness of Al-2.0 wt.% Fe samples.

Microhardness profiles were measured along in a cross-sectional sample, for laser-treated layer and untreated. These measurements were taken along lines parallel to surface at depths of 50, 100, 200, 300, 500 and 700 μm, applying a load of HV 100 gf for 15 s. Figure 6 illustrates the 15 micro-indentations made in the cross section at each of these depths to measure the hardness. Average hardness values and standard deviation (s.d.) at each depth were calculated based on these measurements, and are given in Table 2.

An analysis of the data in Table 2 indicates the HV is higher for the LSR treated region than the untreated region. The average hardness of the treated region is 58.8 HV, while that of the untreated region is 35.7 HV, which corresponds at 60.7% increase in hardness in the treated region compared to the untreated region.

The data in Table 2, also is shown in graphical form in Figure 7, it clearly show increase in hardness at treated region than untreated substrate. This difference is attributed to microstructural changes as resulting of LSR-treated. In other studies
