Properties of 6061 aluminum alloy treated with laser alloying

In this study, a laser surface alloying method is used to alloy 1.8 mm thick 6061 aluminum alloy sheets with Ni alloy powder. The mechanical properties of the resulting alloy are investigated. The experimental results show that, following laser surface alloying, a crack-free zone can be generated but some fine porosity appears. The hardness and the wear properties of the laser alloying specimens are superior to those of the Al-matrix. A hardness of 539 HV is achieved, which is five times greater than that of the Al-matrix. As a result, the laser alloying specimens exhibit excellent wear properties and higher sliding wear distances and speeds compared to those of the Al-matrix, whereby the sliding distance is greater by up to eight times and the sliding speed is increased four-fold.


Introduction
Aluminum alloys are engineering materials of preference for many applications in fields, such as the automotive industry, electronics, and aerospace [1][2][3][4].Nowadays, with the rapid development of industry, aluminum alloys and their parts are required to maintain excellent properties under harsh environments, such as high temperature, high pressure, high corrosion, and high wear resistance.Surface modification is one of the methods for improving the surface properties of these alloys in order to extend their applications.Many methods have been attempted for enhancing the surface properties, including electrolytic plasma [5], laser treatment [6], thermal spraying [7], and nitriding [8].For certain applications, aluminum alloys do not possess adequate strength.By incorporating intermetallic compounds into aluminum alloys, their strength as well as other desired mechanical properties can be increased.Considering this, some studies have been devoted to the fabrication of intermetallic compound layers on aluminum alloys by laser treatment [9][10].The method of laser surface modification is becoming increasingly popular for improving the properties of aluminum alloys due to the various advantages of laser treatment, namely its good controllability, high accuracy, low thermal impact, low distortion, short processing time, and precise control of the treatable depth [11][12].The laser surface treatment of aluminum alloys has been reported in a number of studies involving laser surface melting, laser hardening, laser cladding, laser alloying, laser cleaning, and the fabrication of surface metal matrix composites [13][14][15][16][17][18].Laser surface alloying is a material processing method that uses the higher power density provided by a focused laser source to melt the metal coating and part of the underlying substrate.The laser alloying process causes a phase change to form a suitable hard-phase coating.The rapid heating by the laser, followed by self-quenching by cooling, results in properties that cannot be achieved by other methods.For aluminum alloys, laser alloying provides an enhanced metallurgical bonding to the substrate, wear resistance, expansion of component life, and increased strength and fatigue limits of the material [6].When performing laser alloying, the powder can be added by presetting or synchronization methods.There are two challenges in the laser alloying process of aluminum alloys.One is the poor absorption of the laser energy due to reflection from the metal surface, and the other is the low melting point of the aluminum alloy causing excessive melting of the substrate.These difficulties can be overcome by using a powder presetting method, as opposed to a synchronization method [19].The preset powder layer absorbs laser energy much better than the bulk material because of the pores of the powder present in the layer.In addition, again due to the porous structure of the powder, most of the laser energy is absorbed by the preset layer, thus ensuring that excessive melting of the substrate does not occur.Many researchers have studied the effect of Ni-based powder alloying on aluminum alloys [6,20].However, there are few studies on the effect of Ni-based powders on the laser alloying of thin aluminum alloy sheets in the literature.For example, Hussein and Kadhim [21] developed the substrate thickness to remain at 3 mm.Due to its high plasticity, the alloy is easily deformed under the influence of high temperature, which is not conducive to the development of the surface modification of the thin alloy sheet, and it is prone to issues such as penetration.Nevertheless, it is needing for the application of surface modified aluminum alloy sheets; for example, the reinforced aluminum alloy sheet can be applied to the development of all-aluminum brake discs.The existing manufacturing process for iron brake discs involves punching and cutting a 2 mm thick iron sheet, grinding both sides to a standard thickness of 1.8 mm, and then polishing the surface.In order to improve the wear resistance of all-aluminum brake discs, a laser alloying process prior to polishing would enable the feasibility of developing all-aluminum brake discs.In this study, since the laser current can be controlled below 12 A, the laser beam is concentrated, thus enabling the heat to be controlled more accurately and the laser reflection to be reduced by the preset powder, which is more advantageous for thin sheet alloying.Therefore, this study will investigate the improvement of mechanical properties by laser alloying and powder presetting to synthesize alloy layers on 6061 aluminum alloy thin sheets.

Experimental procedures
Aluminum alloy AA6061 with T6 treatment used as the substrate material.The chemical compositions of the Al-Mg-Si alloy specimens shown in Table 1.The material was machined into 40 (D) × 10 (W) × 1.8 (H) mm 3 specimen sheets.The chemical compositions of the Ni-Cr-B powder given in Table 2.The Ni-Cr-B powder consists of spherical particles with an average diameter of 50 µm and a specific density of 6.7.The powder preset onto the surface of the specimen prior to laser alloying and the preset layer reduces the reflectivity of the laser beam.Laser alloying was performed using a 50W YAG laser.The current was designed in the range of 10-12A, the pulse width was 2.0 ms, the frequency was 100 Hz, the focus length was 18 cm, the laser scanning velocity was designed in the range of 10-60 mm/s, and the diameter of the laser beam was 0.8 mm.During the laser alloying process, the specimen surface was protected with 50 SCCM of argon gas.Argon prevents rapid oxidation of the metal on the surface of the alloying pool and rapidly eliminates smoke and fine dust.The process parameters discussed in this study are shown in Table 3.To analyze the laser alloying properties, the laser alloyed specimens were cut vertically along the laser scanning direction and then the alloyed specimens were mounted, ground with sandpaper, and polished using 0.05 μ m alumina powder.Following this, the specimens were etched with 2% hydrofluoric acid for 5 s and then cleaned with pure water.Some of the laser alloyed specimens were subjected to T6 treatment, by solid solution treatment at 530 °C for 2 h and quenching in water at room temperature, followed by aging at 170 °C for 8 h and cooling in air.The resulting products were alloyed T6 specimens (referred to as "alloyed+T6" in the paper) and their properties were compared with those of the original laser alloyed specimens.Surface microstructures were observed by optical microscopy (OM: nikon AZ100) and scanning electron microscopy (SEM: Hitach-S4200、Hitach-S4100).The compositions of the surface layers were analyzed by using EDS attached to the SEM.The hardness of the alloys was measured using a Vickers hardness tester (Future-Tech Digital-FM100) with a load of 10 g for 10 s.For each specimen, the average hardness value was calculated from at least five test readings.An X-ray diffractometer(Rigaku D-max/IIB) with a Cu target was employed to analyze the alloyed layer structure and the change of precipitations at a voltage of 30 kV, a current of 10 mA, and a scanning range 2θ＝20~80°.The sliding wear behavior was examined by a pin-on-disk wear tester.Before wear testing, the laser alloyed specimens were mechanically removed from the surface by approximately 0.2 mm and then polished to an average centerline roughness (Ra) of 0.01 mm.The wear parameters were sliding wear distance and wear speed.The loading force was 3 N. Sliding speeds of 0.1 m/s, 0.2 m/s, 0.3 m/s, and 0.4/s, and sliding distances of 150 m, 500 m, 800 m, 1000 m, and 1500 m were selected.

Surface of the laser alloyed
Figure 1 shows the surface metallographic of the laser alloyed under the process conditions.Among them, (a), (b) and (c) are current 10 A; (d), (e) and (f) are current 11 A and (g), (h) and (i) are current 12 A, while their scanning speeds are 10 mm/s, 30 mm/s and 60 mm/s, in that order, respectively.Figure 1 of (a), (b) and (c) or (d), (e) and (f) or (g), (h) and (i) three groups, respectively, in the same fixed current to explore the scanning speed on the surface of the alloyed observation.As the faster the scan speed, the shorter the time to stay in the same area of alloying, and the less thermal energy required to produce a melt pool during laser alloying, thus producing a smaller melt pool area and making the laser alloying width gradually decrease as the scan speed increases.
In Figure 1, the surface of the alloyed can be seen mainly as a gray-white area material and black material at the boundary of the alloying area.The gray-white area material is analyzed , mostly aluminum trioxide, which is mainly the combination of the instantaneous high temperature of the molten pool surface aluminum and atmospheric oxygen, the black material is mostly Ni-based selffusing alloy and slag that have not been alloyed.
In Figure 1(b) and Figure 1(c), the gray-white area is not clearly visible, and the edges of the alloyed area show irregular shapes.This is due to the fact that the current is only 10 A and the scanning speed is 30 mm/s and 60 mm/s, respectively, which makes the heat generated by the laser not enough to effectively form a melt pool on the substrate, i.e., the laser alloying cannot be achieved.As a result, only the surface Ni-based self-fusion alloy melted, and due to the surface tension of the melted Nibased self-fusion alloy, it accumulated and solidified at the edges, resulting in an irregular shape of the alloyed edges.
In Figure 1(g), (h) and (i), when the current is 12 A, the irregular shape of the edge of the alloyed also appears.In addition to the part of Ni-based self-melting accumulation, the main reason is that under this parameter, the current intensity is high, providing too much heat, causing part of the substrate outside the spot irradiation area to melt and collapse causing the edge irregularity.It also causes the test specimen outside the irradiation area the substrate softening over a large area, as in figure 1(g), the overall melting width is greater than 4 mm. Figure 1(a) shows that the scan speed reduced to 10 mm/sec to produce a continuous alloying layer.In contrast, the 12 A current is too high in energy, resulting in melting of the specimen substrate due to overheating.In contrast, for the surface metallography of Figure 1(d)-(f), the current parameter was set to 11 A, and the surface pattern of the alloyed area was better regardless of the scan speed of 10 mm/sec, 30 mm/sec or 60 mm/sec.The entire alloyed area not only has continuous distinct gray-white areas, but also has relatively smooth boundaries, so the current parameter of 11 A is the best value in the range of the discussed parameters in terms of surface metallography.

Microhardness
Figure 5 shows the hardness distribution in the cross-sectional area of Figure 2(a) at different depths of the laser alloyed zone.A significant increase in the hardness value is observed in the alloyed layer.In the figure, the hardness can be discussed in three stages from the alloyed surface to the bottom of the melt pool.In the first stage, the range approximately corresponds to zone A of Figure 2(a), and the hardness of the alloyed zone is higher than that of the original substrate and decreases with increasing The average hardness at this stage is 130 HV, which is approx.1.5 times higher than that of the Al-matrix.This is mainly due to the precipitation of Al 3 N in addition to α-Al, so the hardness is higher than the Al-matrix.At this stage, when the depth increases, the hardness value decreases.This is because the cooling rate of the pool surface is faster than the pool interior.The hardness increases when the cooling rate increases.In the second stage, the range approximately corresponds to zones B and C of Figure 2(a), whereby the hardness increases as the depth increases and reaches a maximum value of 539 HV.This stage is obviously high hardness than the Al-matrix and the first stage, because it contains fine equiaxed grains and various Al 3 Ni 2 /AlNi precipitations.In particular, the equiaxed grains also increase the alloyed zone hardness values during the rapid solidification.In the third stage, corresponding to zone D of Figure 2(a), the hardness values drop sharply and approach the hardness values similar to those of the Al-matrix.The average hardness of the alloyed zone is higher than that of the Al-matrix.Precipitations of Al 3 Ni 2 and AlNi are not observed in this zone and the hardness value decreases rapidly.It is clear that the alloying zone has higher hardness than the Al matrix due to the precipitation of Al 3 Ni, Al 3 Ni 2 , and AlNi compounds.In Figure 5, the hardness of the alloyed T6 specimens is lower than the alloyed specimens because of the grain growth by the T6 treatment.

Sliding wear distance
Experiment number LA1130 was used to make the specimens required for the abrasion.The relationship between the wear mass and the sliding distance was tested under fixed wear conditions at a sliding speed of 0.1 m/s and a load of 3 N. Figure 6 gives the curve of the wear mass as a function of sliding distance.By the wear mass of the original substrate increases steeply when a sliding distance of 100 m is reached, indicating a transition from mild wear to severe wear.For the alloyed specimen, this transition occurs at 800 m, whereas for the alloyed layer that underwent T6 heat treatment, it occurs sooner, at 500 m. Figure 7 shows SEM images of the surface of the alloyed layer at different distances of wear, between 150 m and 1000 m respectively.In Figure 6, no serious peeling traces observed at 150 m, slight plastic deformation spalling traces began to appear at 500 m, whereas large flaky spalling traces emerged at 800 m and furrows were obvious at 1000 m. Figure 8 shows the shape of the wear debris of the alloyed layer at different sliding distances.Before 150 m, small particles of wear debris are present, at 500 m, small pieces of wear debris are produced, and after 800 m, large pieces of wear debris are produced.This means that the alloyed layer changes from mild wear to severe wear at a sliding distance of 800 m. Figure 9 shows the surface of the wear scar at different distances after heat treatment of the alloyed alloy, and Figure 10 shows the corresponding wear debris.In Figure 9, plastic deformation traces can be observed at 150 m, at 500 m there are large areas of spalling traces, and at 800 m, the surface has been marked by furrows.A large piece of wear debris is featured at 800 m in Figure 10 that, after T6 heat treatment, the alloyed layer will start to fail when its sliding distance reaches 500 m.The stress relief during the T6 treatment will slightly reduce the hardness and the wear resistance of the alloyed specimen.
In terms of the sliding distance, the wear properties of the alloyed layer increase by eight-fold compared to the original substrate material.However, the wear resistance of the alloyed layer that undergoes T6 heat treatment is five times greater than that of the original substrate due to the release of internal stress caused by rapid solidification and the growth of fine grains due to heat treatment.

Sliding wear speed
The wear conditions were fixed at a sliding wear distance of 800 m with a load of 3 N, and the effect of the sliding speed on the wear mass was tested, as given in Figure 11.The wear mass of both the alloyed specimen and the alloyed T6-treated specimen tends to increase significantly after a sliding speed of 0.2 m/s.In other words, the safety range is within 0.2 m/s.The recommended sliding speed of the original substrate should be within 0.05 m/s, so, the alloyed specimen its wear speed is four times greater than that of the original substrate.When comparing the alloyed T6 treated layer with the alloyed layer, the wear mass value did not increase significantly.This is most likely due to the fact that the stress relief during the T6 treatment will slightly reduce the hardness of the alloyed layer.However, it can be seen from Figure 5 that the hardness value of the surface in the alloyed area does not change much after the T6 treatment.Therefore, there is no significant increase in the wear mass value.(2) The laser alloying specimens are much harder than the Al matrix.The hardness of the laser alloyed zone reaches HV 539, which is approximately five times that of the Al matrix.
(3) The laser alloying specimens have excellent wear performance, which is approximately eight times greater than that of the Al-matrix in terms of the sliding distance, and approximately four times greater in terms of the sliding speed.

Figure 1 .
Figure 1.Metallographic of laser surface alloying area of each process.The metallographic diagrams from (a) to (i) in the figure are experiment numbers LA1010 to LA1260 in sequence.

Figure 2 .
Figure 2. (a) SEM metallographic cross-section and (b) XRD patterns of the alloyed area of the 6061 aluminum alloy.

Figure 3 .
Figure 3. SEM image of metallographic zone B of the alloyed area of 6061 aluminum alloy.

Figure 4 .
Figure 4. SEM image of metallographic zone C of the alloyed area of 6061 aluminum alloy.

Figure 5 .
Figure 5. Microhardness as a function of distance from the surface to the Al-matrix for the Al-matrix, alloyed, and alloyed+T6 specimens.

Figure 6 .
Figure 6.Wear mass as a function of sliding wear distance for the Al-matrix, alloyed, and alloyed+T6 specimens.

Figure 7 .
Figure 7. SEM micrographs of the wear surfaces of alloyed specimens at sliding wear distances of (a) 150 m, (b) 500 m, (c) 800 m, and (d) 1000 m.

Figure 8 .
Figure 8. SEM micrographs of the wear debris of alloyed specimens at sliding wear distances of (a) 150 m, (b) 500 m, (c) 800 m, and (d) 1000 m.

Figure 11 .
Figure 11.Wear mass as a function of sliding wear speed for the alloyed specimen and the alloyed+T6 specimen.

Table 1 .
Chemical composition of the 6061 aluminum alloy.

Table 2 .
Chemical composition of the Ni-based self-melting alloy.

Table 3 .
Experiment number and process parameters number.