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AlN samples
Journal of Manufacturing Processes 46 (2019) 271–278
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Effect of the process parameters in the additive manufacturing of in situ Al/ AlN samples
T
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A. Riquelme , P. Rodrigo, M.D. Escalera-Rodriguez, J. Rams Universidad Rey Juan Carlos, C/Tulipán S/N Móstoles, Spain
A R T I C LE I N FO
A B S T R A C T
Keywords: Direct laser deposition Laser parameters Metal matrix composites Al/AlN Focal position
Additive manufacturing has revolutionized the manufacturing industry. Researchers have investigated various techniques for increasing the reliability of the metal Additive Manufacturing processes. However, there are few studies about Additive Manufacturing of metal matrix composite components, so, it is necessary increase the research about this topic. In situ Al/AlN composite was synthesized using Direct Laser Deposition equipment in which aluminum powder was deposited through a laser beam using N2 as carrying and reactive gas. This composite powder has been melted and directly deposited layer by layer to form an additive structure. The effect of the laser parameters on the geometry, microstructure and properties of the fabricated structures have been analysed. This allowed the development of a process map for Al and Al/AlN pieces fabricated by Direct Laser Deposition. In situ Al/AlN samples have higher mechanical properties than Al samples.
1. Introduction Additive Manufacturing (AM) comprises a set of new fabrication techniques in which pieces are fabricated layer by layer from powders or wires. It is receiving attention because it helps to reduce the amount of material and energy used. It also is a solution to fabricate difficult geometry components and to personalize products [1–4]. Different metal structures have been successfully manufactured by Selective Laser Melting (SLM) and pieces valid for many applications have been developed with adequate properties [5–7]. However, SLM process results in high porosity and components fabricated by this technique usually require post processing treatments. Some researchers found that Direct Laser Deposition (DLD) results in lower porosity than with other AM techniques [8]. Considerable efforts have been made to analyze these process to fabricate metal components [9,10], however, few researches focus in the fabrication of metal matrix composites (MMC). These materials have become significant, especially in industries like aeronautic or automotive, and are promising candidates to improve the properties of additively manufactured pieces. Transport industry has been forced to reduce the components weight and to use lighter materials. One possible strategy is to use aluminium matrix composites (Al-MMC) [11–13]. Aluminum matrix composites have received considerable attention due to the good combination of their tribological properties and density. Typical applications include aerospace components, electronic packaging, high
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precision instrumentation and automobile engine components [14]. AlN in the shape of particles is an excellent reinforcement material for Al–MMC, because AlN offers high thermodynamic stability and good compatibility and wettability by molten aluminum. AlN Young’s modulus (343 GPa) is similar to other typical reinforcements like SiC, BeO or Al2O3 (460, 378 and 378 GPa, respectively), but AlN flexural strength (400–500 MPa) is higher than these others (SiC = 450 MPa; BeO = 250 MPa and Al2O3 = 300 MPa)[15]. Moreover, it has high thermal conductivity (170–220 W/m K at 25 °C) and a coefficient of thermal expansion (4.3–4.6·10−6 ºC-1) that is similar to that of silicon [16–19]. These properties makes it a good candidate to be used in thermal management applications and to be deposited as coatings [9–11]. However, AlN powder is expensive and induces high fabrication costs. Thus, in situ fabrication of Al-AlN composite is an attractive manufacturing route to form the reinforcement and its integration in the matrix [12,13]. Composites have been fabricated by conventional processes like casting [5], but its cost and complexity, as well as the high cost of mechanizing these materials make necessary to increase the research about the fabrication of MMC components by AM to combine the benefits of this technique with the properties of Al-MMC [5,23–26]. In situ metal matrix composites are novel composites in which the reinforcements are formed within the matrix material by controlled chemical reactions during the composite fabrication. In recent years,
Corresponding author. E-mail address: [email protected] (A. Riquelme).
https://doi.org/10.1016/j.jmapro.2019.09.011 Received 3 October 2018; Received in revised form 30 July 2019; Accepted 12 September 2019 1526-6125/ © 2019 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
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there have been attempts to produce AlN composites using the reaction between molten Al and nitrogen or NH3 [20,21]. It is the case of the Direct Melt Nitridation (PRIMEX) method [22,27]. These processing methods are sub-optimal and they result in porosity, matrix-reinforcement interface deterioration, and high processing costs [17,23,24]. The aim of this research is to develop a methodology to manufacture high-quality in situ Al/AlN composite structures in a cost effective way by Direct Laser Deposition. In situ Al/AlN composite powders have been manufactured during the additive manufacturing process by spraying aluminium powder through a laser beam using N2 as carrying gas. The laser heats the particles and the nitridation reaction takes place by its reaction with the surrounding nitrogen. Aluminum without reinforcement samples were also fabricated as control samples, in order to evaluate the effect of the reinforcement. The effect of the laser parameters on the geometry of the additively fabricated samples and the properties of the fabricated structures have been evaluated.
Table 2 Set of experiments.
2. Experimental procedure Thin wall samples (length =60 mm and height =15 mm) of Al and Al/in-situ AlN were made by DLD using AA2024 sheets (100 × 100 × 30 in mm) as support. Commercial powder of eutectic aluminum silicon has been used (Table 1). The laser used was a high-power diode laser (1300 W) (ROFIN DL013S) with a wavelength between 808 and 940 nm. The metallic powder was sprayed coaxially with the laser beam trough a coaxial nozzle Fraunhofer IWS COAX 8. The powder and the laser focus were 12.5 mm below the nozzle tip. The powder feed ratio was 5 g/min and the laser focus overlapped with the powder focus. In addition, this system is placed in an ABB IRB2400 robot. A hot-plate connected to a temperature control system was used as substrate support. Aluminum samples have been fabricated by spraying the aluminum powder through the laser beam using argon as carrying gas (4.5 atm pressure and 0.05 L s−1 flow rate). The in situ aluminum matrix composite samples were fabricated by spraying aluminum powder through the laser beam using N2 as carrying and reactive gas (4.5 atm pressure and 0.05 L s−1 flow rate) so that nitrogen reacts with the aluminum and forms Al/AlN. The processing parameters studied were the laser power (600–1200 W), the laser scanning speed (5–20 mm/s) and the height increase per layer (1–0.5 mm). In all cases, to avoid heat build-up, the laser power gradually decreases 100 W during the deposition process. For each fabricated samples, the first deposited layer was manufactured using the initial power and, the last deposited layer was manufactured using a final power that was 100 W lower than the initial one. Table 2 shows the set of samples fabricated with its corresponding fabrication conditions. The heights and widths of fabricated samples were measured using a caliper with precision ± 0.01 mm. Samples have been metallography prepared. Samples cross-sections were cut, mounted in an electrically conductive resin, wet grounded using a sequence of silicon carbide abrasive (400–4000 grit) and 1 μm diamond polished using ethylene glycol as lubricant in order to analyze its microstructure and its microhardness profile. Microstructures and porosity were evaluated by image analysis software (Leica Application Suite) on the captured images obtained by a light microscope (OM; Leica DMR) and by scanning electron microscope (SEM; Hitachi S-3400 N) equipped with an energy dispersive X-ray
Supplier
D50 (μm)
ρ (g·cm−3)
Composition (wt.%)
Al 12 wt. % Si
Metco 52C-NS
71
2.7
12 Si; balance Al.
Laser power (W)
Scanning speed (mm/s)
Height increase per layer (mm)
C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 C15 C16 C17 C18 C19 C20 C21 C22 C23 C24 C25
700–600 700–600 700–600 700–600 1000–900 1000–900 1000–900 1000–900 1050–950 1050–950 1050–950 1050–950 1100–1000 1100–1000 1100–1000 1100–1000 1150–1050 1150–1050 1150–1050 1150–1050 1200–1100 1200–1100 1200–1100 1200–1100 1150–1050
5 10 15 20 5 10 15 20 5 10 15 20 5 10 15 20 5 10 15 20 5 10 15 20 10
0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 1
spectrometer (EDX). A Shimadzu microhardness tester was used to obtain Vickers microhardness (HV0.1) (0.9807 N and 15 s) profiles on the cross-section of the specimens. The microhardness measurements were made from the substrate to the top of the sample with distances of 1500 μm between each measurement. 3. Results Fig. 1a shows the processing map obtained for the additive manufacturing of Al walls by Direct Laser Deposition. The process map associates the fabricated samples characteristics (dimensions and porosity) with the laser power and scan speed. The different points inside the process map correspond to the different experimental conditions shown in Table 2. The objective was to fabricate a sample with 30 layers and with a height of 15 mm (height increase per layer was 0.5 mm). In all cases, to avoid heat build-up, the laser power gradually decreased 3.33 W per layer. The effect of the laser power and the scan speed have been analyzed. Laser power and scanning speed have a direct effect in the dimensions of the fabricated sample. The key parameter for the energy supply is the energy density in the material during the deposition process, which is directly related to these parameters and to the laser spot size (Eq. 1).
Energy Density (J/mm2) =
laser power (W ) scanning speed
( )·laser spot size (mm ) mm
s
2
(1) For high laser power (higher than 1150 W), the second layer deposited on the first one produces the re-melting of this first one. This occurs in each layer deposited and the melted material flows by gravity and the sample height increase per layer is smaller than the programmed one (0.5 mm). During the deposition of the first layer, the laser is in focus on the built plate but as more layers are deposited, the focus is no longer on the top of the of the last deposited layer; i.e., the laser is not focused in the growing sample. The possible laser focal height is schematically showed in Fig. 1b. On positive defocus mode the laser and powder focus are above the last deposited surface. On focus plane, the laser and powder focus are exactly on the surface of the last deposited layer and, finally on negative
Table 1 Powder properties. Product
Condition
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Fig. 1. (a) Process map; (b) sample fabricated using 1 mm as height increase per layer; (c) sample fabricated using 0.5 mm as height increase per layer; (d) Scheme of focus laser.
decreases and the focus is in slightly negative defocus mode, the spot size increases and the resulting Energy Density decreases. It causes that there was not re-melting of the samples, at a difference from what happened in the case of laser power higher than 1150 W. As the scanning speed increases, the effect is the opposite and the focus is in slightly positive defocus mode. In the second zone, the amount of material per unit time and the energy provided is lower than in the first zone because the scanning speed is higher, so the real height increase per layer is lower than the programmed one, resulting in samples with lower height than programmed. In addition, in the third zone, the scanning speed is lower and the amount of deposited material is higher. That results in higher growing of the sample regards to the programed height increase per layer. The gas used affects lightly to the dimensions and porosity of the sample. In general, the fabrication zones are the same independently of the carrier/reactive gas used, so the scheme shown in Fig. 1a can be used to fabricate Al samples using argon as carrier gas or to fabricate Al/AlN ones using nitrogen as reactive and carrier gas. However, there are some differences between the use of both gases, which will be extensively discussed later. Fig. 1c–d shows the samples fabricated in which the differences in the height can be clearly observed. Fig. 1c shows the effect of the height increase per layer on the relative laser focus position. Aluminum samples fabricated with power in the range 1150–1050 W and 10 mm/s with height increments of 0.5 mm (Fig. 1c) are smaller than equivalent samples fabricated with height increments of 1 mm (Fig. 1d). In the first one, the layers do not grow at the same rate as programmed, so the laser focus remains in slightly negative defocus condition. In the second sample, the pieces also do not grow at the same rate as the height increase per layer, but in this case, the laser focus remains in positive defocus condition as the laser focus is focused below the surface. Fig. 2 shows the effect of the laser speed on the height and the width of the thin wall in the fabrication zone as a function of the number of deposited layers using argon and nitrogen. In all cases, the laser power was kept nearly constant, with only a gradual decrease of 100 W from the first to the last layer (deposition started with 1100 W and finished with 1000 W). Scanning speeds of 5, 10 and 20 mm/s were evaluated. The samples were fabricated using argon as carrier gas (red lines) to obtain aluminum additive manufacturing samples and nitrogen as carrier/reactive gas (black lines) in order to obtain Al/AlN additive manufacturing samples. In all cases, the sample height per layer decreases when the laser speed increases. At very low laser speeds (5 mm/s), the pieces grow more than the
defocus mode, the laser and powder focus are under the surface of the last deposited layer. With laser power above 1150 W, the slow growth of the material makes that the laser gets defocused in the positive mode because the actual growth of the sample is smaller than the programmed height increase of the laser after each layer. The defocus causes that the spot size at the sample is bigger than when it is in focus, so the energy density is lower for each deposited layer. Also, the distance from the last deposited layer to the coaxial nozzle increases in each deposited layer, so, the temperature with which the particles impact on the samples is lower and they get partially solidified. This combined with a lower temperature of the samples makes that many of the sprayed particles rebound at the surface. With lower power (< 1050 W), there was insufficient melting and very porous samples were obtained independently from the scanning speed (average porosity of more than 15%). The energy density was very low and the powders were only partially melted so, the porosity of the samples was high and in most cases, the pores observed had sizes that were similar to the initial powder used. With scanning speed higher than 20 mm/s and laser power between 1050 and 1250 W, there was also insufficient melting and the fabricated samples showed 1.5 ± 0.5% porosity. Despite the laser power being higher than in the last case, the scanning speed causes that the energy input decreases and it results in porosity. In addition, with scanning speed below 5 mm/s and, laser power between 1050 and 1250 W, the real height increase per layer samples grows more than the programmed height increase per layer (0.5 mm). It is because the amount of deposited material per unit length is high and the heat density provided by laser is greater. It produces that the laser focus is in negative defocus mode with regards to the last deposited layer. The combination of power, scanning speed and spot size causes an increase in the particles deposition ratio. Therefore, the valid fabrication zone was delimited to three different areas all of them between 1050 and 1150 W of laser power. The first one is the optimal fabrication zone, which takes place between 10–15 mm/s of scanning speed. The second fabrication zone appears with slow growing using scanning speed up 15 mm/s. The third fabrication zone appears with slow growing operating under 5 mm/s of scanning speed. In the first zone, the fabricated samples dimensions coincide with the programmed ones with an error equivalent to one layer height ( ± 0.5 mm). Also, porosity was low (0.2 ± 0.1%). As the scanning speed decreases the amount of material per unit length increases and, it causes that the real height increase per layer is higher than the programmed one. Also, the distance from the nozzle to the last layer 273
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Fig. 2. Effect of laser speed on (a) height of the thin wall; (v) width of the thin wall.
programmed height because the amount of deposited material and the laser input heat per unit of time is high. The distance from the last layer to the focus decreases with each deposited layer (negative defocus mode). This causes a constant diminution in the sample to laser distance that obliges to stop the wall growth at the 15th layer to avoid the contact between sample and DLD system. In the other cases, 10 and 20 mm/s of scanning speed, the number of layers grown was 30. At high speeds (20 mm/s) the pieces grew less than programmed because of an insufficient deposited material. In relation with the samples width, the same tendency was observed: the sample width decreases when the scan speed increases. At higher scan speed, less deposited material per unit of time, and lower sample width. Therefore, overall, the higher the laser scan speed the lower the height and the width of the samples. In all cases, Al/AlN samples are higher and thinner than Al ones. In addition, the width decrease is lower in the AlN samples than in the Al ones. These differences can be justified by the different carrying gas used. In the case of Al samples, the carrying gas was argon and in the case of the Al/AlN ones, the carrying and reactive gas was nitrogen. Nitrogen has lower ionization energy (1.402 kJ/mol) and higher thermal conductivity (256.6·10−4 W/mK) than argon (1.521 kJ/mol and 172.8·10−4 W/mK, respectively). The lower ionization energy produces that when the dissociated nitrogen atoms contact with the deposited Al/AlN layer transfers less heat that in the case of argon. In addition, the higher thermal conductivity of nitrogen causes that the heat was removed faster from the aluminum. Both properties cause lower re-melting because the first deposited layer was fully solidified before the second layer was deposited, and so on. In addition, argon has higher ionization energy than nitrogen, so, nitrogen plasma formation being at lowers energy density than argon. This plasma would attenuate the laser beam, having an important effect on the power intensity at the deposition point [31]. For these reasons, Al/AlN layer grow up higher and thinner with nitrogen as carrying and reactive gas than Al layer with argon as carrying gas. The height increase per layer of the laser was kept constant during the fabrication process, so, the effect of the laser speed in the growth of the wall also influences the laser focal height related to the last deposited layer. Fig. 3 shows the effect of laser speed on the focal height. 5, 10 and 20 mm/s of laser speed have been analyzed; the laser power used was the same in all cases and the two studied gases have been also used. In all cases, the focus mode switches gradually from negative defocus mode at low scanning speed to positive defocus mode at high
Fig. 3. Effect of laser speed on the focal height.
scanning speed. It can be observed that at low speeds, the sample surface remains in negative focus because the pieces grow faster than the programmed height. With high speeds, the piece growth do not follow the programmed height increases, so the surface of the piece remains in positive focus. In addition, differences between using argon and nitrogen were observed. With 5 mm/s of scanning speed, Al and in situ Al/AlN samples remain in negative defocus from the 5th layer. At 10 mm/s can be observed a difference between samples with and with-out reinforcement. Al samples remain in positive focus from the 15th layer and Al/ AlN samples remain in negative defocus from the 15th layer. At 20 mm/ s of scanning speed, Al and in situ Al/AlN samples remain in positive defocus from the 13th and 20th layers, respectively. This evidences that the rate of material deposited is high and the net energy provided by the laser is also high, causing that the laser focus is in positive defocus mode with regard to the last deposited layer. Also, there are some differences at different laser speeds that can justified by the different carrying gas used, i.e., argon or nitrogen. Using argon as carrier gas, the change in focus mode was produced at a lower scanning speed than when using nitrogen. Nitrogen has higher thermal conductivity than argon, so, in Al samples there were higher re-melting in consecutive deposited layers, and therefore, Al pieces grow slightly more than the height increase per layer. The temperature reached by the powder along the laser beam depends on the position with respect to the focal plane [32]. When the 274
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Fig. 4. Scheme of fabrication process (a) negative defocus; (b) positive defocus (c) accumulation of partially melted particles around the fabricated walls.
be easily removed after the fabrication process. Under the negative defocus condition (Fig. 4a), the powder is only heated during its flight across the laser beam, never passing through the highest temperature zone and it is not completely molten at the moment of its impact on the surface [33]; so it tends to rebound away from the surface, and the external mold is formed. Vortices of powder on the sides of the manufactured walls were observed. Under the focus condition most of the powder reaches the substrate surface in the middle of the laser beam. In the moment of the powder impact with the surface, the laser quickly heats the powder and the substrate surface simultaneously. Then, the powder is melted and deposited without rebound away. The vortices of powder on the sides of the manufactured walls were less relevant, and the porous mold stops growing. Under the positive condition (Fig. 4b), the powder passes through the highest temperature zone at the edge of the laser beam and through the lower temperature zone in the center [33]. The wall formation predominates over the porous mold formation. The microstructure of the different samples was studied. Fig. 5 shows the differences observed in the microstructure of the walls. Fig. 5a shows the cross-section scheme in which the building direction
laser is in negative defocus mode, the particles temperature is lower and the spot size is higher than at focus mode. This, combined with the low scan speed produces that the adequate energy density value that allows the particles deposition ratio to increases. When the laser is in positive defocus mode, the temperature distribution in the perpendicular plane to the laser beam does not follow a Gaussian distribution and there is a higher temperature at the edges of the stream below the focal point. This produces re-remelting of consecutives layers and the samples grow less than the programmed height increase per layer. There is also another effect of the relative position between the focus and the material surface. Fig. 4 shows schematically the powder movement at the sides of the manufactured walls. The effect of the negative defocus mode and positive defocus mode are shown in Fig. 4a and b, respectively. In both cases, there was an accumulation of partially melted particles around the fabricated wall. This agglomerated powder forms an “external porous mold" that acts as a casting mold and promotes the growth of the piece. This effect is increased when the system is in negative defocus. Fig. 4c shows different samples after the fabrication process where these “porous mold” can be observed. This material can 275
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Fig. 5. Effect of laser scan speed in the microstructure; (a) Cross-section scheme that show the build direction; (b) Al and Al/AlN microstructure along the crosssection samples fabricated using argon or nitrogen and different scanning speeds.
reinforcement (lines in the figure show the data tendency). In all cases, the hardness increased by increasing the distance from the substrate due to the microstructure refinement. Also, measurements near the substrate show dilution of the substrate, so, the microhardness was influenced by the composition change. For this reason, the microhardness is significantly higher from the middle height of the sample. At 20 mm/s of scanning speed the microhardness increases from 2.5 mm. For scanning speeds down to 10 mm/s from 8 mm. In addition, reinforced samples were harder than un-reinforced ones. Fig. 8 shows the average microhardness obtained for each sample and the hardness increase as a function of the effect of the reinforcement. The average microhardness was obtained with no dilution-influenced data. All pieces manufactured with N2 have higher hardness. Hardness increases of 27% were achieved for the lowest speeds. For the fastest speed, the average hardness was higher but the effect of the reinforcement increase was lower. At the highest scanning speed, the laser remains less time over each zone, so the solidification speed is faster and, the microstructure of the eutectic silicon is refined. This causes that the average hardness of the samples fabricated with 20 mm/ s of scan speed was higher than that of the samples fabricated with 5 mm/s of scanning speed. Furthermore, at lower scanning speed the amount of material and heat provided per unit length was grater, so the solidification speed is slower. It is possible that the nitrogen and the aluminum continue reacting during the melt pool formation and not only during the deposition process. So at low scan speed the melt pool solidification is slower, the ratio of reaction between the aluminum and the nitrogen is
and the location of different zones analyzed are indicated. In addition, the effect of laser scan speed in the Al and Al/AlN microstructure is shown in Fig. 5 b. In both cases (Al and Al/AlN) the microstructure is eutectic aluminum silicon. In the case of Al/AlN samples there was AlN nanometric reinforcement. The eutectic silicon morphology varies as a function of the scanning speed. The faster the laser speed, the thinner the microstructure because there was less amount of material per layer. In addition, there was less re-melting because the first deposited layer solidified before the second layer being deposited, and so on. The energy density is lower and, therefore, the solidification is faster. In all cases, zones near the substrate have eutectic silicon phase is refiner in respect with zones further from the substrate because the heat loss due to thermal conductivity through the substrate is higher than the heat loss due to convection and thermal conductivity through the deposited layers. The distribution of the different phases was also analyzed by SEM. Fig. 6 shows a detail of the Al/AlN sample microstructure fabricated at 10 mm/s of laser speed, and its corresponding EDX element map and EDX analysis. It can be observed the formation of AlN particles with diameters less than 1 μm. (Fig. 6a). AlN is detected in the interface between the eutectic silicon and aluminum (Fig. 6b). The EDX analysis confirms the AlN formation (Fig. 6c). Mechanical properties were also measured. Microhardness data obtained on the wall cross-section were measured from the bottom of the substrate to the top of the wall as it is schematically shown in Fig. 7a. Fig. 7b shows the effect of the distance from the substrate in the microhardness of the different samples fabricated. In addition, it can be observed the effect of the scanning speed and the effect of the AlN
Fig. 6. SEM micrograph of (a) Al/AlN thin wall fabricated at 10 mm/s of laser scan speed; (b) EDX elements map of a) aluminum is color code in blue, silicon is in green and nitrogen in red; (c) EDX analysis on an AlN particle. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article). 276
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Fig. 7. Evolution of the cross-section microhardness (a) microhardness test scheme; (b) microhardness results.
- The fabrication zone is restricted to laser powers between 1050 and 1150 W and scanning speeds between 5 and 15 mm/s. - Powers higher than 1150 W result in re-melting of the samples and defocusing of the laser in positive defocus mode. - Powers below 950 W result in insufficient melting and very porous samples were obtained. - At low scanning speeds, the layer height is higher than the programmed height increase per layer (0.5 mm), so the sample grows faster than designed, the surface gets into negative focus and the final height of the sample is higher than designed.
higher and it is possible that more AlN particles are formed in these samples than in the samples fabricated with 20 mm/s of scanning speed.
4. Conclusions A process map for Al and in situ Al/AlN samples fabricated by Direct Laser Deposition that relates the laser power and the scan speed has been developed. The main conclusions are the following:
Fig. 8. Average microhardness (HV0.1). (a) microhardness test scheme. (b) Average microhardness (HV0.1). Data obtained from no dilution-influence zone. 277
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- With high speeds, the piece growth does not follow the programmed height increase per layer and the surface of the sample remains in positive focus. The total height of the wall is lower that designed. - The focus mode switches gradually from negative defocus mode at low scan speed to positive defocus mode at high scan speed because the amount of deposited material per unit length and the heat provided increase. - The type of carrier gas used, i.e., argon of nitrogen has not a relevant effect in the dimensions of porosity of the sample and the regions previously indicated are mainly valid. Using argon as carrier gas, the change in focus mode was produced at lower scan speed than using nitrogen. Fabricated samples using nitrogen as carrier/reactive gas result in lower remelting of consecutive deposited layers because of the lower ionization energy and higher thermal conductivity of nitrogen versus argon. - Accumulation of partially melted particles around the fabricated wall forms an “external porous mold” that acts as a casting mold and promotes the growth of the piece. - No dilution-influenced microhardness increases from the bottom to the top. - The faster laser speed, the thinner microstructure and the higher microhardness. All this because the solidification is faster. - All samples manufactured with N2 have higher microhardness than the equivalent ones with argon due to the reaction between the nitrogen and aluminum, which forms AlN particles. This results in the fabrication of an aluminum matrix composite reinforced with AlN particles with diameters below 1 μm. AlN samples show hardness values that are up to 27% higher than those of Al samples. This value was achieved for the lowest scanning speeds because the lower solidification speeds results in a higher reactivity between Al and nitrogen.
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Acknowledgments The authors wish to express their gratitude to the Agencia Estatal de Investigación (Project RTI2018-096391-B-C31) and Comunidad de Madrid (Spain) (Project ADITIMAT-CM S2018/NMT-4411). References [1] Bandyopadhyay A, Traxel KD. Invited Review Article: metal-additive manufacturing — modeling strategies for application-optimized designs. Addit Manuf 2018;22:758–74. https://doi.org/10.1016/J.ADDMA.2018.06.024. [2] Pinkerton AJ. Lasers in additive manufacturing. Opt Laser Technol 2015;78:25–32. https://doi.org/10.1016/j.optlastec.2015.09.025. [3] Klahn C, Leutenecker B, Meboldt M. Design strategies for the process of additive manufacturing. Procedia CIRP 2015;36:230–5. https://doi.org/10.1016/j.procir. 2015.01.082. [4] Lawrence JR, Pou J, Low DKY, Toyserkani E. Advances in laser materials processing. 2010. https://doi.org/10.1533/9781845699819.1.20. [5] Kang N, Ma W, Heraud L, El Mansori M, Li F, Liu M, et al. Selective laser melting of tungsten carbide reinforced maraging steel composite. Addit Manuf 2018;22:104–10. https://doi.org/10.1016/j.addma.2018.04.031. [6] Onuike B, Bandyopadhyay A. Additive manufacturing of Inconel 718 – Ti6Al4V bimetallic structures. Addit Manuf 2018. https://doi.org/10.1016/J.ADDMA.2018. 06.025.
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Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
Effect of the process parameters in the additive manufacturing of in situ Al/ AlN samples
T
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A. Riquelme , P. Rodrigo, M.D. Escalera-Rodriguez, J. Rams Universidad Rey Juan Carlos, C/Tulipán S/N Móstoles, Spain
A R T I C LE I N FO
A B S T R A C T
Keywords: Direct laser deposition Laser parameters Metal matrix composites Al/AlN Focal position
Additive manufacturing has revolutionized the manufacturing industry. Researchers have investigated various techniques for increasing the reliability of the metal Additive Manufacturing processes. However, there are few studies about Additive Manufacturing of metal matrix composite components, so, it is necessary increase the research about this topic. In situ Al/AlN composite was synthesized using Direct Laser Deposition equipment in which aluminum powder was deposited through a laser beam using N2 as carrying and reactive gas. This composite powder has been melted and directly deposited layer by layer to form an additive structure. The effect of the laser parameters on the geometry, microstructure and properties of the fabricated structures have been analysed. This allowed the development of a process map for Al and Al/AlN pieces fabricated by Direct Laser Deposition. In situ Al/AlN samples have higher mechanical properties than Al samples.
1. Introduction Additive Manufacturing (AM) comprises a set of new fabrication techniques in which pieces are fabricated layer by layer from powders or wires. It is receiving attention because it helps to reduce the amount of material and energy used. It also is a solution to fabricate difficult geometry components and to personalize products [1–4]. Different metal structures have been successfully manufactured by Selective Laser Melting (SLM) and pieces valid for many applications have been developed with adequate properties [5–7]. However, SLM process results in high porosity and components fabricated by this technique usually require post processing treatments. Some researchers found that Direct Laser Deposition (DLD) results in lower porosity than with other AM techniques [8]. Considerable efforts have been made to analyze these process to fabricate metal components [9,10], however, few researches focus in the fabrication of metal matrix composites (MMC). These materials have become significant, especially in industries like aeronautic or automotive, and are promising candidates to improve the properties of additively manufactured pieces. Transport industry has been forced to reduce the components weight and to use lighter materials. One possible strategy is to use aluminium matrix composites (Al-MMC) [11–13]. Aluminum matrix composites have received considerable attention due to the good combination of their tribological properties and density. Typical applications include aerospace components, electronic packaging, high
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precision instrumentation and automobile engine components [14]. AlN in the shape of particles is an excellent reinforcement material for Al–MMC, because AlN offers high thermodynamic stability and good compatibility and wettability by molten aluminum. AlN Young’s modulus (343 GPa) is similar to other typical reinforcements like SiC, BeO or Al2O3 (460, 378 and 378 GPa, respectively), but AlN flexural strength (400–500 MPa) is higher than these others (SiC = 450 MPa; BeO = 250 MPa and Al2O3 = 300 MPa)[15]. Moreover, it has high thermal conductivity (170–220 W/m K at 25 °C) and a coefficient of thermal expansion (4.3–4.6·10−6 ºC-1) that is similar to that of silicon [16–19]. These properties makes it a good candidate to be used in thermal management applications and to be deposited as coatings [9–11]. However, AlN powder is expensive and induces high fabrication costs. Thus, in situ fabrication of Al-AlN composite is an attractive manufacturing route to form the reinforcement and its integration in the matrix [12,13]. Composites have been fabricated by conventional processes like casting [5], but its cost and complexity, as well as the high cost of mechanizing these materials make necessary to increase the research about the fabrication of MMC components by AM to combine the benefits of this technique with the properties of Al-MMC [5,23–26]. In situ metal matrix composites are novel composites in which the reinforcements are formed within the matrix material by controlled chemical reactions during the composite fabrication. In recent years,
Corresponding author. E-mail address: [email protected] (A. Riquelme).
https://doi.org/10.1016/j.jmapro.2019.09.011 Received 3 October 2018; Received in revised form 30 July 2019; Accepted 12 September 2019 1526-6125/ © 2019 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
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there have been attempts to produce AlN composites using the reaction between molten Al and nitrogen or NH3 [20,21]. It is the case of the Direct Melt Nitridation (PRIMEX) method [22,27]. These processing methods are sub-optimal and they result in porosity, matrix-reinforcement interface deterioration, and high processing costs [17,23,24]. The aim of this research is to develop a methodology to manufacture high-quality in situ Al/AlN composite structures in a cost effective way by Direct Laser Deposition. In situ Al/AlN composite powders have been manufactured during the additive manufacturing process by spraying aluminium powder through a laser beam using N2 as carrying gas. The laser heats the particles and the nitridation reaction takes place by its reaction with the surrounding nitrogen. Aluminum without reinforcement samples were also fabricated as control samples, in order to evaluate the effect of the reinforcement. The effect of the laser parameters on the geometry of the additively fabricated samples and the properties of the fabricated structures have been evaluated.
Table 2 Set of experiments.
2. Experimental procedure Thin wall samples (length =60 mm and height =15 mm) of Al and Al/in-situ AlN were made by DLD using AA2024 sheets (100 × 100 × 30 in mm) as support. Commercial powder of eutectic aluminum silicon has been used (Table 1). The laser used was a high-power diode laser (1300 W) (ROFIN DL013S) with a wavelength between 808 and 940 nm. The metallic powder was sprayed coaxially with the laser beam trough a coaxial nozzle Fraunhofer IWS COAX 8. The powder and the laser focus were 12.5 mm below the nozzle tip. The powder feed ratio was 5 g/min and the laser focus overlapped with the powder focus. In addition, this system is placed in an ABB IRB2400 robot. A hot-plate connected to a temperature control system was used as substrate support. Aluminum samples have been fabricated by spraying the aluminum powder through the laser beam using argon as carrying gas (4.5 atm pressure and 0.05 L s−1 flow rate). The in situ aluminum matrix composite samples were fabricated by spraying aluminum powder through the laser beam using N2 as carrying and reactive gas (4.5 atm pressure and 0.05 L s−1 flow rate) so that nitrogen reacts with the aluminum and forms Al/AlN. The processing parameters studied were the laser power (600–1200 W), the laser scanning speed (5–20 mm/s) and the height increase per layer (1–0.5 mm). In all cases, to avoid heat build-up, the laser power gradually decreases 100 W during the deposition process. For each fabricated samples, the first deposited layer was manufactured using the initial power and, the last deposited layer was manufactured using a final power that was 100 W lower than the initial one. Table 2 shows the set of samples fabricated with its corresponding fabrication conditions. The heights and widths of fabricated samples were measured using a caliper with precision ± 0.01 mm. Samples have been metallography prepared. Samples cross-sections were cut, mounted in an electrically conductive resin, wet grounded using a sequence of silicon carbide abrasive (400–4000 grit) and 1 μm diamond polished using ethylene glycol as lubricant in order to analyze its microstructure and its microhardness profile. Microstructures and porosity were evaluated by image analysis software (Leica Application Suite) on the captured images obtained by a light microscope (OM; Leica DMR) and by scanning electron microscope (SEM; Hitachi S-3400 N) equipped with an energy dispersive X-ray
Supplier
D50 (μm)
ρ (g·cm−3)
Composition (wt.%)
Al 12 wt. % Si
Metco 52C-NS
71
2.7
12 Si; balance Al.
Laser power (W)
Scanning speed (mm/s)
Height increase per layer (mm)
C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 C15 C16 C17 C18 C19 C20 C21 C22 C23 C24 C25
700–600 700–600 700–600 700–600 1000–900 1000–900 1000–900 1000–900 1050–950 1050–950 1050–950 1050–950 1100–1000 1100–1000 1100–1000 1100–1000 1150–1050 1150–1050 1150–1050 1150–1050 1200–1100 1200–1100 1200–1100 1200–1100 1150–1050
5 10 15 20 5 10 15 20 5 10 15 20 5 10 15 20 5 10 15 20 5 10 15 20 10
0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 1
spectrometer (EDX). A Shimadzu microhardness tester was used to obtain Vickers microhardness (HV0.1) (0.9807 N and 15 s) profiles on the cross-section of the specimens. The microhardness measurements were made from the substrate to the top of the sample with distances of 1500 μm between each measurement. 3. Results Fig. 1a shows the processing map obtained for the additive manufacturing of Al walls by Direct Laser Deposition. The process map associates the fabricated samples characteristics (dimensions and porosity) with the laser power and scan speed. The different points inside the process map correspond to the different experimental conditions shown in Table 2. The objective was to fabricate a sample with 30 layers and with a height of 15 mm (height increase per layer was 0.5 mm). In all cases, to avoid heat build-up, the laser power gradually decreased 3.33 W per layer. The effect of the laser power and the scan speed have been analyzed. Laser power and scanning speed have a direct effect in the dimensions of the fabricated sample. The key parameter for the energy supply is the energy density in the material during the deposition process, which is directly related to these parameters and to the laser spot size (Eq. 1).
Energy Density (J/mm2) =
laser power (W ) scanning speed
( )·laser spot size (mm ) mm
s
2
(1) For high laser power (higher than 1150 W), the second layer deposited on the first one produces the re-melting of this first one. This occurs in each layer deposited and the melted material flows by gravity and the sample height increase per layer is smaller than the programmed one (0.5 mm). During the deposition of the first layer, the laser is in focus on the built plate but as more layers are deposited, the focus is no longer on the top of the of the last deposited layer; i.e., the laser is not focused in the growing sample. The possible laser focal height is schematically showed in Fig. 1b. On positive defocus mode the laser and powder focus are above the last deposited surface. On focus plane, the laser and powder focus are exactly on the surface of the last deposited layer and, finally on negative
Table 1 Powder properties. Product
Condition
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Fig. 1. (a) Process map; (b) sample fabricated using 1 mm as height increase per layer; (c) sample fabricated using 0.5 mm as height increase per layer; (d) Scheme of focus laser.
decreases and the focus is in slightly negative defocus mode, the spot size increases and the resulting Energy Density decreases. It causes that there was not re-melting of the samples, at a difference from what happened in the case of laser power higher than 1150 W. As the scanning speed increases, the effect is the opposite and the focus is in slightly positive defocus mode. In the second zone, the amount of material per unit time and the energy provided is lower than in the first zone because the scanning speed is higher, so the real height increase per layer is lower than the programmed one, resulting in samples with lower height than programmed. In addition, in the third zone, the scanning speed is lower and the amount of deposited material is higher. That results in higher growing of the sample regards to the programed height increase per layer. The gas used affects lightly to the dimensions and porosity of the sample. In general, the fabrication zones are the same independently of the carrier/reactive gas used, so the scheme shown in Fig. 1a can be used to fabricate Al samples using argon as carrier gas or to fabricate Al/AlN ones using nitrogen as reactive and carrier gas. However, there are some differences between the use of both gases, which will be extensively discussed later. Fig. 1c–d shows the samples fabricated in which the differences in the height can be clearly observed. Fig. 1c shows the effect of the height increase per layer on the relative laser focus position. Aluminum samples fabricated with power in the range 1150–1050 W and 10 mm/s with height increments of 0.5 mm (Fig. 1c) are smaller than equivalent samples fabricated with height increments of 1 mm (Fig. 1d). In the first one, the layers do not grow at the same rate as programmed, so the laser focus remains in slightly negative defocus condition. In the second sample, the pieces also do not grow at the same rate as the height increase per layer, but in this case, the laser focus remains in positive defocus condition as the laser focus is focused below the surface. Fig. 2 shows the effect of the laser speed on the height and the width of the thin wall in the fabrication zone as a function of the number of deposited layers using argon and nitrogen. In all cases, the laser power was kept nearly constant, with only a gradual decrease of 100 W from the first to the last layer (deposition started with 1100 W and finished with 1000 W). Scanning speeds of 5, 10 and 20 mm/s were evaluated. The samples were fabricated using argon as carrier gas (red lines) to obtain aluminum additive manufacturing samples and nitrogen as carrier/reactive gas (black lines) in order to obtain Al/AlN additive manufacturing samples. In all cases, the sample height per layer decreases when the laser speed increases. At very low laser speeds (5 mm/s), the pieces grow more than the
defocus mode, the laser and powder focus are under the surface of the last deposited layer. With laser power above 1150 W, the slow growth of the material makes that the laser gets defocused in the positive mode because the actual growth of the sample is smaller than the programmed height increase of the laser after each layer. The defocus causes that the spot size at the sample is bigger than when it is in focus, so the energy density is lower for each deposited layer. Also, the distance from the last deposited layer to the coaxial nozzle increases in each deposited layer, so, the temperature with which the particles impact on the samples is lower and they get partially solidified. This combined with a lower temperature of the samples makes that many of the sprayed particles rebound at the surface. With lower power (< 1050 W), there was insufficient melting and very porous samples were obtained independently from the scanning speed (average porosity of more than 15%). The energy density was very low and the powders were only partially melted so, the porosity of the samples was high and in most cases, the pores observed had sizes that were similar to the initial powder used. With scanning speed higher than 20 mm/s and laser power between 1050 and 1250 W, there was also insufficient melting and the fabricated samples showed 1.5 ± 0.5% porosity. Despite the laser power being higher than in the last case, the scanning speed causes that the energy input decreases and it results in porosity. In addition, with scanning speed below 5 mm/s and, laser power between 1050 and 1250 W, the real height increase per layer samples grows more than the programmed height increase per layer (0.5 mm). It is because the amount of deposited material per unit length is high and the heat density provided by laser is greater. It produces that the laser focus is in negative defocus mode with regards to the last deposited layer. The combination of power, scanning speed and spot size causes an increase in the particles deposition ratio. Therefore, the valid fabrication zone was delimited to three different areas all of them between 1050 and 1150 W of laser power. The first one is the optimal fabrication zone, which takes place between 10–15 mm/s of scanning speed. The second fabrication zone appears with slow growing using scanning speed up 15 mm/s. The third fabrication zone appears with slow growing operating under 5 mm/s of scanning speed. In the first zone, the fabricated samples dimensions coincide with the programmed ones with an error equivalent to one layer height ( ± 0.5 mm). Also, porosity was low (0.2 ± 0.1%). As the scanning speed decreases the amount of material per unit length increases and, it causes that the real height increase per layer is higher than the programmed one. Also, the distance from the nozzle to the last layer 273
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Fig. 2. Effect of laser speed on (a) height of the thin wall; (v) width of the thin wall.
programmed height because the amount of deposited material and the laser input heat per unit of time is high. The distance from the last layer to the focus decreases with each deposited layer (negative defocus mode). This causes a constant diminution in the sample to laser distance that obliges to stop the wall growth at the 15th layer to avoid the contact between sample and DLD system. In the other cases, 10 and 20 mm/s of scanning speed, the number of layers grown was 30. At high speeds (20 mm/s) the pieces grew less than programmed because of an insufficient deposited material. In relation with the samples width, the same tendency was observed: the sample width decreases when the scan speed increases. At higher scan speed, less deposited material per unit of time, and lower sample width. Therefore, overall, the higher the laser scan speed the lower the height and the width of the samples. In all cases, Al/AlN samples are higher and thinner than Al ones. In addition, the width decrease is lower in the AlN samples than in the Al ones. These differences can be justified by the different carrying gas used. In the case of Al samples, the carrying gas was argon and in the case of the Al/AlN ones, the carrying and reactive gas was nitrogen. Nitrogen has lower ionization energy (1.402 kJ/mol) and higher thermal conductivity (256.6·10−4 W/mK) than argon (1.521 kJ/mol and 172.8·10−4 W/mK, respectively). The lower ionization energy produces that when the dissociated nitrogen atoms contact with the deposited Al/AlN layer transfers less heat that in the case of argon. In addition, the higher thermal conductivity of nitrogen causes that the heat was removed faster from the aluminum. Both properties cause lower re-melting because the first deposited layer was fully solidified before the second layer was deposited, and so on. In addition, argon has higher ionization energy than nitrogen, so, nitrogen plasma formation being at lowers energy density than argon. This plasma would attenuate the laser beam, having an important effect on the power intensity at the deposition point [31]. For these reasons, Al/AlN layer grow up higher and thinner with nitrogen as carrying and reactive gas than Al layer with argon as carrying gas. The height increase per layer of the laser was kept constant during the fabrication process, so, the effect of the laser speed in the growth of the wall also influences the laser focal height related to the last deposited layer. Fig. 3 shows the effect of laser speed on the focal height. 5, 10 and 20 mm/s of laser speed have been analyzed; the laser power used was the same in all cases and the two studied gases have been also used. In all cases, the focus mode switches gradually from negative defocus mode at low scanning speed to positive defocus mode at high
Fig. 3. Effect of laser speed on the focal height.
scanning speed. It can be observed that at low speeds, the sample surface remains in negative focus because the pieces grow faster than the programmed height. With high speeds, the piece growth do not follow the programmed height increases, so the surface of the piece remains in positive focus. In addition, differences between using argon and nitrogen were observed. With 5 mm/s of scanning speed, Al and in situ Al/AlN samples remain in negative defocus from the 5th layer. At 10 mm/s can be observed a difference between samples with and with-out reinforcement. Al samples remain in positive focus from the 15th layer and Al/ AlN samples remain in negative defocus from the 15th layer. At 20 mm/ s of scanning speed, Al and in situ Al/AlN samples remain in positive defocus from the 13th and 20th layers, respectively. This evidences that the rate of material deposited is high and the net energy provided by the laser is also high, causing that the laser focus is in positive defocus mode with regard to the last deposited layer. Also, there are some differences at different laser speeds that can justified by the different carrying gas used, i.e., argon or nitrogen. Using argon as carrier gas, the change in focus mode was produced at a lower scanning speed than when using nitrogen. Nitrogen has higher thermal conductivity than argon, so, in Al samples there were higher re-melting in consecutive deposited layers, and therefore, Al pieces grow slightly more than the height increase per layer. The temperature reached by the powder along the laser beam depends on the position with respect to the focal plane [32]. When the 274
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Fig. 4. Scheme of fabrication process (a) negative defocus; (b) positive defocus (c) accumulation of partially melted particles around the fabricated walls.
be easily removed after the fabrication process. Under the negative defocus condition (Fig. 4a), the powder is only heated during its flight across the laser beam, never passing through the highest temperature zone and it is not completely molten at the moment of its impact on the surface [33]; so it tends to rebound away from the surface, and the external mold is formed. Vortices of powder on the sides of the manufactured walls were observed. Under the focus condition most of the powder reaches the substrate surface in the middle of the laser beam. In the moment of the powder impact with the surface, the laser quickly heats the powder and the substrate surface simultaneously. Then, the powder is melted and deposited without rebound away. The vortices of powder on the sides of the manufactured walls were less relevant, and the porous mold stops growing. Under the positive condition (Fig. 4b), the powder passes through the highest temperature zone at the edge of the laser beam and through the lower temperature zone in the center [33]. The wall formation predominates over the porous mold formation. The microstructure of the different samples was studied. Fig. 5 shows the differences observed in the microstructure of the walls. Fig. 5a shows the cross-section scheme in which the building direction
laser is in negative defocus mode, the particles temperature is lower and the spot size is higher than at focus mode. This, combined with the low scan speed produces that the adequate energy density value that allows the particles deposition ratio to increases. When the laser is in positive defocus mode, the temperature distribution in the perpendicular plane to the laser beam does not follow a Gaussian distribution and there is a higher temperature at the edges of the stream below the focal point. This produces re-remelting of consecutives layers and the samples grow less than the programmed height increase per layer. There is also another effect of the relative position between the focus and the material surface. Fig. 4 shows schematically the powder movement at the sides of the manufactured walls. The effect of the negative defocus mode and positive defocus mode are shown in Fig. 4a and b, respectively. In both cases, there was an accumulation of partially melted particles around the fabricated wall. This agglomerated powder forms an “external porous mold" that acts as a casting mold and promotes the growth of the piece. This effect is increased when the system is in negative defocus. Fig. 4c shows different samples after the fabrication process where these “porous mold” can be observed. This material can 275
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Fig. 5. Effect of laser scan speed in the microstructure; (a) Cross-section scheme that show the build direction; (b) Al and Al/AlN microstructure along the crosssection samples fabricated using argon or nitrogen and different scanning speeds.
reinforcement (lines in the figure show the data tendency). In all cases, the hardness increased by increasing the distance from the substrate due to the microstructure refinement. Also, measurements near the substrate show dilution of the substrate, so, the microhardness was influenced by the composition change. For this reason, the microhardness is significantly higher from the middle height of the sample. At 20 mm/s of scanning speed the microhardness increases from 2.5 mm. For scanning speeds down to 10 mm/s from 8 mm. In addition, reinforced samples were harder than un-reinforced ones. Fig. 8 shows the average microhardness obtained for each sample and the hardness increase as a function of the effect of the reinforcement. The average microhardness was obtained with no dilution-influenced data. All pieces manufactured with N2 have higher hardness. Hardness increases of 27% were achieved for the lowest speeds. For the fastest speed, the average hardness was higher but the effect of the reinforcement increase was lower. At the highest scanning speed, the laser remains less time over each zone, so the solidification speed is faster and, the microstructure of the eutectic silicon is refined. This causes that the average hardness of the samples fabricated with 20 mm/ s of scan speed was higher than that of the samples fabricated with 5 mm/s of scanning speed. Furthermore, at lower scanning speed the amount of material and heat provided per unit length was grater, so the solidification speed is slower. It is possible that the nitrogen and the aluminum continue reacting during the melt pool formation and not only during the deposition process. So at low scan speed the melt pool solidification is slower, the ratio of reaction between the aluminum and the nitrogen is
and the location of different zones analyzed are indicated. In addition, the effect of laser scan speed in the Al and Al/AlN microstructure is shown in Fig. 5 b. In both cases (Al and Al/AlN) the microstructure is eutectic aluminum silicon. In the case of Al/AlN samples there was AlN nanometric reinforcement. The eutectic silicon morphology varies as a function of the scanning speed. The faster the laser speed, the thinner the microstructure because there was less amount of material per layer. In addition, there was less re-melting because the first deposited layer solidified before the second layer being deposited, and so on. The energy density is lower and, therefore, the solidification is faster. In all cases, zones near the substrate have eutectic silicon phase is refiner in respect with zones further from the substrate because the heat loss due to thermal conductivity through the substrate is higher than the heat loss due to convection and thermal conductivity through the deposited layers. The distribution of the different phases was also analyzed by SEM. Fig. 6 shows a detail of the Al/AlN sample microstructure fabricated at 10 mm/s of laser speed, and its corresponding EDX element map and EDX analysis. It can be observed the formation of AlN particles with diameters less than 1 μm. (Fig. 6a). AlN is detected in the interface between the eutectic silicon and aluminum (Fig. 6b). The EDX analysis confirms the AlN formation (Fig. 6c). Mechanical properties were also measured. Microhardness data obtained on the wall cross-section were measured from the bottom of the substrate to the top of the wall as it is schematically shown in Fig. 7a. Fig. 7b shows the effect of the distance from the substrate in the microhardness of the different samples fabricated. In addition, it can be observed the effect of the scanning speed and the effect of the AlN
Fig. 6. SEM micrograph of (a) Al/AlN thin wall fabricated at 10 mm/s of laser scan speed; (b) EDX elements map of a) aluminum is color code in blue, silicon is in green and nitrogen in red; (c) EDX analysis on an AlN particle. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article). 276
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Fig. 7. Evolution of the cross-section microhardness (a) microhardness test scheme; (b) microhardness results.
- The fabrication zone is restricted to laser powers between 1050 and 1150 W and scanning speeds between 5 and 15 mm/s. - Powers higher than 1150 W result in re-melting of the samples and defocusing of the laser in positive defocus mode. - Powers below 950 W result in insufficient melting and very porous samples were obtained. - At low scanning speeds, the layer height is higher than the programmed height increase per layer (0.5 mm), so the sample grows faster than designed, the surface gets into negative focus and the final height of the sample is higher than designed.
higher and it is possible that more AlN particles are formed in these samples than in the samples fabricated with 20 mm/s of scanning speed.
4. Conclusions A process map for Al and in situ Al/AlN samples fabricated by Direct Laser Deposition that relates the laser power and the scan speed has been developed. The main conclusions are the following:
Fig. 8. Average microhardness (HV0.1). (a) microhardness test scheme. (b) Average microhardness (HV0.1). Data obtained from no dilution-influence zone. 277
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- With high speeds, the piece growth does not follow the programmed height increase per layer and the surface of the sample remains in positive focus. The total height of the wall is lower that designed. - The focus mode switches gradually from negative defocus mode at low scan speed to positive defocus mode at high scan speed because the amount of deposited material per unit length and the heat provided increase. - The type of carrier gas used, i.e., argon of nitrogen has not a relevant effect in the dimensions of porosity of the sample and the regions previously indicated are mainly valid. Using argon as carrier gas, the change in focus mode was produced at lower scan speed than using nitrogen. Fabricated samples using nitrogen as carrier/reactive gas result in lower remelting of consecutive deposited layers because of the lower ionization energy and higher thermal conductivity of nitrogen versus argon. - Accumulation of partially melted particles around the fabricated wall forms an “external porous mold” that acts as a casting mold and promotes the growth of the piece. - No dilution-influenced microhardness increases from the bottom to the top. - The faster laser speed, the thinner microstructure and the higher microhardness. All this because the solidification is faster. - All samples manufactured with N2 have higher microhardness than the equivalent ones with argon due to the reaction between the nitrogen and aluminum, which forms AlN particles. This results in the fabrication of an aluminum matrix composite reinforced with AlN particles with diameters below 1 μm. AlN samples show hardness values that are up to 27% higher than those of Al samples. This value was achieved for the lowest scanning speeds because the lower solidification speeds results in a higher reactivity between Al and nitrogen.
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