AlSi7Mg specimens made by Directed Energy
Deposition
4.1 Résumé
Malgré le fait que cette famille de matériau présente des propriétés physiques et mécaniques intéressantes pour l’industrie du transport, le nombre d’études portant sur les alliages d’aluminium (AA) mise en forme par Fabrication Additive (FA) est limité comparativement aux autres matériaux tels que les alliages de titane et les alliages d’acier inoxydable. La technologie de Déposition sous Énergie Dirigée est une sous branche de la FA offrant des possibilités de mise en forme uniques grâce à la conception de son procédé de déposition de matière. En effet, la DED est particulièrement bien conçue pour l’ajout de sections complexes à des pièces déjà existantes, pour varier localement la composition chimique en cours de construction et pour la réparation de pièces. Cette dernière application est spécialement intéressante pour l’industrie manufacturière puisqu’elle permet de sauver des coûts de remplacement de pièces défectueuses ou brisées. Deux applications ont été étudié au cours de cet ouvrage; 1) la construction complète d’échantillons d’alliage d’aluminium AlSi7Mg et 2) la réparation de support fait d’alliage d’aluminium AlSi7Mg coulés par fonderie conventionnelle. Tous les échantillons ont été caractérisés en termes de microstructure et de propriétés mécaniques à l’aide de microscopes optiques et électroniques (MEB) et à l’aide d’un appareil de diffraction d’électrons rétrodiffusé (EBSD). Les réparations ainsi conçues par DED présentent un allongement inférieur ainsi qu’une limite élastique et une résistance à la traction supérieures aux éprouvettes de traction mise en forme par fonderie. Les éprouvettes de traction entièrement construites le long de l’axe X présentent une limite élastique et une résistance à la traction significativement plus élevées que celles des éprouvettes de traction construites le long de l’axe Z. Ces dernières présentent d’ailleurs un allongement beaucoup plus élevé soulignant davantage une forte anisotropie des propriétés mécaniques.
4.2 Abstract
Although this family of materials presents interesting mechanical and physical characteristics that are key to the transportation industry, the number of studies on aluminum and its alloys built with Additive Manufacturing (AM) is still minor compared to other materials such as Ti-6Al-4V and stainless steels. Directed Energy Deposition (DED) is a sub-category within AM processes that offers unique possibilities thanks to the clever design of its deposition process. Indeed, DED is particularly well suited to add new complex features to existing parts, to vary the chemical composition in function of the XYZ coordinates within a build and to repair surface defects or worn
parts. The latter is quite interesting for the manufacturing industry since it enables significant cost savings and is the focus of this study. In this work two different applications were studied: 1) the complete construction of samples made of AlSi7Mg and 2) the repair of simulated broken AlSi7Mg brackets with DED. Samples were characterized in terms of mechanical properties, microstructure, crystallographic textures as well as grain size using SEM and EBSD. Repairs display lower El, but higher YS and UTS than fully cast samples. Completely DED constructed tensile samples built along the X axis exhibit significant higher YS and UTS than samples build along the Z axis highlighting a strong anisotropy of mechanical properties.
4.3 Introduction
With their significant specific strength and interesting physical properties such as corrosion resistance and thermal conductivity, Aluminum Alloys (AA) are notably used in the design of parts for the transportation industry [1]. Representing about 90% of the total production of aluminum castings, the 3xx.xx cast AA series is predominant in the market [1]. Additive Manufacturing (AM) is a rather novel fabrication process that permits the creation of complex 3D parts layer by layer as opposed to traditional subtractive processes [2]. Paired with AM, AA have a great potential to further reduce the weight of crucial parts in transportation applications thanks to topology optimization and design for AM [3]. Directed Energy Deposition (DED) is an AM process that uses a highly concentrated energy source such as a laser to melt particles or wire feedstock onto a base plate to create complex parts [4]. Thanks to its unique deposition process, DED offers unique capabilities that other AM technologies can hardly meet. One of these is the possibility to repair defective or worn parts for a fraction of their replacement cost [5].
Despite their high potential with the AM route, AA didn’t grab the interest of researchers at the beginning of the craze for AM. The latter was rather focused on Ti-6Al-4V, Inconel and Stainless Steel alloys, putting AA aside. It’s only recently that AA managed to catch up with more and more papers published every year. Foundry alloys are certainly the most studied, AlSi10Mg being the main one with its near eutectic composition. Indeed, AA are prone to hot cracking caused by the lack of melt feeding the shrinkage-induced voids upon solidification [6]. The closer the composition is to the eutectic, the smaller is the liquidus to solidus temperature range and the better is the fluidity of the melt to compensate solidification shrinkage. Their high sensitivity to hot cracking is the main reason why high strength wrought AA are non-compatible with AM. However, some interesting recent findings suggest a way to overcome this problematic using nanoparticles nucleants [7].
Few researchers studied the response and performance of AA created with AM. Brandl et al. studied the fatigue performance, the microstructure and the fracture behaviour of AlSi10Mg samples made by Selective Laser Melting (SLM) [8]. They mainly found that a combination of a 300 °C pre-heated build platform with a peak- hardening (T6) treatment allowed to significantly increase the fatigue resistance above standard values and limit samples’ anisotropy. Kimura et al. investigated the microstructure and mechanical properties of A356 samples made by SLM [9]. Ultimate tensile strength (UTS), yield strength (YS) and elongation (El) properties of their samples were all above typical values for conventionally cast A356 samples. Their samples displayed a fine dendritic cellular microstructure and a high relative density of about 99.8 % with optimum process parameters combination.
Few more recent studies have been made on A357 alloy. Rao et al. studied the effect of processing parameters on the microstructure and mechanical properties of A357 samples made by SLM [10]. Their samples showed
mechanical properties equal or higher than the cast standard and a high average relative density with a maximum of 99.8 %. Significant anisotropy was observed, vertical specimens showing the lowest tensile properties especially elongation which was 2 times lower than horizontal specimens. Vertical specimens didn’t reached necking because of their high brittleness. This phenomenon is explained by the fact that the interdendritic Si phase is more brittle than Al phase and that its effect on the properties is higher for the Z planes than the XY ones [11]. Aversa et al. investigated the mechanical properties of A357 samples made by SLM upon different process and post-process conditions [12]. They mainly found that a pre-heated platform within the 140 °C to 170 °C range leads to better as-built mechanical properties because of its ageing effect that induces precipitation hardening.
Regarding DED’s unique application that is repair, not many researchers studied its full potential yet regardless of material. Pinkerton et al. studied the quality of DED repaired grooves on H13 substrates. Tensile properties of their samples were lower than typical soft annealed H13 and little to no plastic deformation was observed. This observation has been linked to insufficient density and as-built residual stresses that led to premature failure. Liu et al. explored the possibility to repair 4140 steel using nickel and cobalt based superalloys [13]. To do so, they repaired half final length 4140 steel cylindrical samples that were latter machined down to tensile specimen’s needed geometry. Inconel 718-4140 steel hybrid samples displayed increased tensile properties compared to 4140 steel repaired ones. UTS of repaired hybrid samples consisted of a compromise between the deposited and substrate material, i.e, UTS lower than Inconel 718 but higher than 4140 steel.
The lack of studies on DED processed and especially repaired AlSi7Mg was the motivation behind this paper. This work explores the possibility to repair and build AlSi7Mg specimens using AlSi7Mg powder feedstock and a DED system. A LENS 450 equipped with a 1000 W YAG laser, 4 powder hoppers and a build envelope of 100 x 250 x 100 mm was used throughout the study. Repairs and entirely built specimens were characterized in terms of mechanical properties and microstructure.
4.4 Experimental procedures
Samples fabrication
Two series of samples were built in this study; 1) entirely built tensile specimens and 2) repairs. Samples of the former type were built along the X and Z axis of the DED system to investigate possible anisotropy inherent to the process. Three samples were built for each orientation. Repairs were done on half cast brackets to simulate the repair process of a broken bracket. A simple bracket design was adapted to the LENS 450 system used. The final design was 118 mm in height, 85 mm in width and 56 mm in depth. A foundry sand mold was designed and printed with an ExOne binder jetting AM system to conventionally cast a total of 4 half and 2 full brackets. The latter were cast for comparison purposes. Three tensile specimens were sampled along the vertical portion of each studied bracket. Fig. 1 a) shows the repair set-up used within the LENS building chamber while Fig. 1 b) displays an example of a half size bracket, a repaired bracket and a full-size cast bracket. Process parameters used for the construction of both types of samples are shown at Table 1. The hatch pattern used to fill each new layer was rotated by 45° to limit possible anisotropy caused by the deposition of the material along the same direction throughout the construction of the sample. Spherical AlSi7Mg powder with a particle size distribution between 45 µm and 150 µm was used for the construction of both type of specimen. This specific particle size distribution is recommended by the manufacturer of the LENS system for optimum flow characteristics. Its chemical composition is displayed at Table 2. Oxygen concentration in the building chamber was purged and kept below 100 ppm using Argon during the construction of every samples.
Figure 4-1 : a) repair set-up inside the LENS 450 building chamber and b) from left to right; half simulated broken bracket, DED repaired bracket and completely cast bracket
Table 4-1: Process parameters used to repair and build tensile specimens and repairs throughout the study
Parameters
Value
Fiber size (µm)
200
Laser power (W)
750
Feed Rate (g/min)
2.04
Travel speed (mm/s)
21.17
Layer thickness (µm)
305
Hatch distance (µm)
381
Table -2: Chemical composition of the AlSi7Mg powder used
Si
Mg
Fe
Ti
Al
Wt%
7.07 0.64 0.19 0.024
Balance
Samples construction
Once built or repaired, all specimens were machined to respect subsize tensile specimen dimensions according to ASTM standard E8 [14]. All samples tested in this study were in either as-DEDed or as-cast condition, no heat treatments were studied.
a)
)
Mechanical Testing
Tensile properties characterization was done using an Exceed model E43 MTS standard apparatus. Vickers and Brinell hardness measurements were done using a Mitutoyo MVK-HV and a QNDE DTLC-3000 with 200gf and 500gf respectively.
Microstructural characterization
Typical preparation methods were applied to samples destined for microstructural characterization. Whenever needed, samples were etched using Weck’s reagent. A Nikon Eclipse LV150 optical microscope and a Hitachi SU3500 SEM equipped a Nordlys Max2 EBSD from Oxford were used to characterize the microstructure and the crystal orientation of the samples. Samples destined for EBSD analysis were polished with a Hitachi IM4000 plus Argon beam ion milling system. EBSD images were taken with a tension of 20 kV, a spot intensity of 60, a step size of 2 µm, a binning of 2x2 and a gain of 4. Every image was taken at 100X displaying an area of 1200 µm by 900 µm. A transition angle of 15° was used to distinguish grains.
Grain characterization
Oxford’s Tango software was used to determine the average diameter of grains. It is important to note that this diameter corresponds to the equivalent diameter of a perfect circle with the same surface area of the grains corresponding to the following equation:
𝑑𝑒𝑞= √(4 ∗ 𝑆)/𝜋
Where S is the surface area of the analysed grain and deq,its equivalent diameter. Grains located at the outskirt
of the analysed region were not taken into account for this analysis.
Powder characterization
A typical sequence of sieves ranging from 45 µm to 150 µm with a mechanical sieve shaker were used to characterize the size distribution of the powder used throughout the study.
4.5 Results and Discussion
Powder characterization
Fig. 2 displays a SEM image taken in SE mode of a sample of the AlSi7Mg powder used throughout the study. It is possible to notice the high sphericity and high quality of the AlSi7Mg powder. Indeed, only small satellites of a couple microns in size are present on the surface of the particles. Results from the sieve analysis made on the same powder is presented at Fig. 3. It is possible to confirm a tight particles size distribution between 53 µm and 90 µm which helps to maintain a stable mass flow during the construction process. D10, D50 and D90 of the powder used are 54.8 µm, 55.3 µm and 80.1 µm respectively
Figure 4-2 : SEM SE image of the AlSi7Mg 7 powder used
Figure 4-3 : Particle size distribution of the AlSi7Mg powder usedMetallurgical characterization
Porosity
Entirely built tensile coupons created along the X axis exhibit an average of 0.8 % porosity content. Minimum porosity content achieved is 0.27 % and is close to others achieved with SLM processed AlSi7Mg in other works [10] [12]. Fig. 4 shows typical defects found in tensile specimens taken with SEM in SE mode. The most prominent type of porosity is unmelted particles also known as Lack of Fusion (LOF) type defects. This hints to the fact that the total energy density transferred to the powder during the deposition process was not high enough to melt all particles and create a perfectly dense deposit. Small gas-type porosity can also be distinguished in
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the Al matrix. These are the result of either entrapped process Argon gas or adsorbed Hydrogen on the original powder. However, the influence of these on the overall density of the specimens is significantly lower than LOF- type porosities.
Figure 4-4 : SEM SE micrograph displaying lack of fusion (LOF) type and gas type porosities inside one of the tensile
Microstructure
Fig. 5 displays a cubic montage showing the different microstructure patterns in relation to building coordinates, the Z axis being the building direction. First, AM’s characteristic "fish scale" like macrostructure pattern caused
the repetitive solidification of deposited tracks is easily identifiable. Furthermore, it’s possible to discern a big difference in microstructure between the center of deposited beads and their surroundings. Indeed, the now solidified melt pools display a small cellular-type structure at their center while their periphery are characterized by bands made of a larger dendritic structure. The resulting cellular structure can be associated with the high cooling rate of DED [15] while the larger dendritic structure delimiting the deposited tracks can be attributed to the remelting and reheating of previously deposited beads.
Figure 4-5 : Cubic montage of typical microstructure at low magnification (left) and high magnification (right) of DED processed AlSi7Mg samples (etched)
Fig. 6 displays a high magnification micrograph taken in the band region right between two deposited tracks of the XZ plane. The left track was deposited right after the right one. Such regions are characterized by four distinct microstructures that have already been discussed in other works on the processing of AA with DED [16], SLM [17-22] and Direct Metal Laser Sintering (DMLS) [23] technologies. Region A, which is in the first deposited track, is composed of several small α cells of couple microns resulting from the high cooling rate with a clear intercellular eutectic phase. The next two regions, B and C make the band region described earlier. Region B exhibits a unique structure with a broken down and a modified-like eutectic phase. This structure is the result of the coarsening of the Si phase caused by an increased diffusion rate at high temperature. This zone can be interpreted as the Heat-Affected Zone (HAZ) of the melt pool of the second deposited track [17] [20]. Next, region C is characterized by relatively coarse α cells with a sound interdendritic eutectic network. The solidification rate at this exact location was lower because of the residual heat left by the first deposited track, hence the coarser structure [20]. Finally, zone D delimits the beginning of the new deposited track. This zone is characterized by bigger dendrites with a clear radial preferential orientation. The latter can be explained by the radial solidification front of the melt pool upon solidification. Although not visible on this micrograph, these elongated dendrites make place to a similar cellular structure as the one described in region A as they get away from the periphery of the track. It is interesting to note that such radially oriented dendrites were not described in other SLM processed AA works. This can be explained by the fact the SLM systems normally use a faster scanning velocity with a smaller layer thickness and a smaller beam size than DED systems resulting in a significantly higher cooling rate and a finer structure [24]. AA processed by SLM exhibit a microcellular structure free of large elongated dendrites and display the same A, B and C regions as the ones described here. Interestingly enough, elongated dendrites were not described in Dinda’s work on DED processed AA either [16]. This can be explained by the difference in sample geometry. Indeed, samples built in this present study are significantly bulkier than thin wall samples built in the other one. The thinner the sample, the higher is the cooling rate since heat is more easily extracted by the base plate. Hence, it is less likely to see relatively large dendrites in thin samples.
Figure 4-6 : High magnification micrograph taken in the band region between two deposited tracks (etched)
A
B
C
D
Grain size and EBSD characterization
Fig. 7 shows a suite of EBSD images taken along the building direction of a complete tensile sample built along the Z axis. From a) to f), images were taken at 8, 24, 40, 56, 72 and 88 mm from the base plate respectively. First, it is possible to see that no color is predominant regardless of the distance from the base plate. This indicates that grains don’t have a preferential orientation and that no specific texture is formed over the whole length of the sample upon its construction with the DED apparatus. Next, it is possible to note elongated grains and a shift in their growth direction between each layer. This observation is easily discernible and outlined in Fig. 7 f). This phenomenon can be explained by the back and forth scanning nature of the process which control the direction of the temperature gradient of the melt pool. It is also possible to notice that grains tend to align along the building direction and have a relatively high aspect ratio. This is caused by the fact that heat is extracted downwards by the sample itself and by the base plate.
When comparing these images with classic micrographs shown previously, it is possible to confirm that several cells and dendrites make up larger grains. Elongated grains are more than likely made up of several relatively large preferentially oriented dendrites and smaller grains, of the other finer structures. Compared to EBSD images of SLM processed AA found in the literature [17], AM’s characteristic circular patterns are difficult to discern here. The structure is also significantly coarser. This can be linked to the difference in local cooling rate