A review on selective laser sintering/melting (SLS/SLM) of aluminium alloy powders: Processing, microstructure, and properties

TL;DR: In this article, the state of the art in selective laser sintering/melting (SLS/SLM) processing of aluminium powders is reviewed from different perspectives, including powder metallurgy (P/M), pulsed electric current (PECS), and laser welding of aluminium alloys.

About: This article is published in Progress in Materials Science.The article was published on 2015-10-01 and is currently open access. It has received 1172 citations till now. The article focuses on the topics: Aluminium alloy & Selective laser sintering.

Figures

Fig. 36. (A) Microstructure of SLM processed Ti-6Al-4V exhibiting porosity and insufficient substrate remelts and (B) Etched microstructure of fully dense Ti-6Al-4V specimen [46].

Fig. 62. Typical fracture surfaces of SLM processed AlSi10Mg parts (powder bed temperature of 300 oC / build direction of 0o / peak hardened): (A) crack initiation site and (B) area of forced fracture [30].

Fig. 43. Typical microstructure of SLM processed AlSi10Mg parts at powder bed temperature of 300 oC, and build orientation of 0o: (A) As built (B) Peak-hardened [30].

Fig. 5. LPS of WC-Co powder mixture; (A) before infiltration (grey portion: non-molten WC particle, white portion: molten Co, dark portion: porosity); (B) after infiltration with low melting point material (copper) [55].

Fig. 61. Typical fracture surfaces of SLM processed AlSi10Mg parts (powder bed temperature of 300 oC / build direction of 0o / as-built): (A) crack initiation site and (B) area of forced fracture. [30].

Fig. 40. Characteristic microstructures of SLS processed AlSi12 powder with varying processing conditions: (A, B) 100 J/mm3; (C, D) 67 J/mm3; (E, F) 50 J/mm3 [28].

Fig. 32. Density and relative density of the SLM parts. The density was measured by dimensional method while relative density was achieved according to CT scanning experiments [31].

Fig. 58. Ultimate tensile strength Rm and yield strength at 0.2 per cent offset Rp0.2 for 0 o, 45o and 90o orientations for finer powder type 1 and coarsest powder type 3 [202].

Fig. 19. Effect of variation in layer thickness on the microstructure of laser sintered AlSi12 powder at laser power of 200 W, scan rates of 120 mm/s; and scan spacing of 0.1mm: (A) 1.0mm, (B) 0.5mm and (C) 0.25mm [28].

Fig. 47. Polished section of laser sintered parts shows the effect of graphite addition on the pore structures on a section cut parallel to the building direction; laser power is 215 W, scan rate is 75 mm/s, scan line spacing is 0.3 mm, and layer thickness is 0.1 mm (A) 0% C (B) 0.4% C (C) 0.8% C (D) 1.2% C [156].

Fig 18. Variation of the density of SLS processed AlSi12 powder with the applied energy density. Processing conditions: laser power (100–2 W), scan rate (80–200mms−1), scan spacing 0.1–0.3mm, and layer thickness 0.25–1.00mm [28].

Fig. 48. Characteristic microstructure of laser sintered iron-1.2 wt.% graphite powder mixture on a section cut parallel to the building direction shows heterogeneous carbon dissolution in the iron matrix resulting in the varying local hardness values (Table 2.4). Laser power is 215 W, scan rate is 75 mm/s, scan line spacing is 0.3 mm, and layer thickness is 0.1 mm [56].

Fig. 8. Typical microstructure of a LPS system with the phase diagram characteristics shown in Fig. 6 [60].

Table 3 Solubility effects on densification in LPS [60].

Table 4 Fractional amount of liquid phase generated in various SLS processed metal systems.

Fig. 34. Three-phase equilibrium for wetting and non-wetting systems [46].

Fig. 1. (A) A layered manufacturing (LM) paradigm (B) Generic fixturing [9].

Fig. 55. SEM images showing characteristic morphologies of worn surfaces of SLM-processed Ti parts at: (A) 900 J m-1, 100 mm s-1; (B) 450 J m-1, mm s-1; (C) 300 J m-1, 300 mm s-1; (D) 225 J m-1, 400 mm s-1 [176].

Table 1 Properties of pure and alloyed aluminium at its melting point [44]

Fig. 39. Effect of energy density on the fraction of primary phase in SLS processed AlSi12powder [28].

Fig. 9. Optical images of the polished sections of the laser sintered multi-component Cu-based metal powder with varying contents (wt%) of binder CuSn in the samples (a) 20 (b) 35 (c) 50 and (d) 65 [63].

Frequently asked questions

Q1. What have the authors contributed in "A review on selective laser sintering/melting (sls/slm) of aluminium alloy powders: processing, microstructure, and properties" ?

Due to the paucity of publications on SLS/SLM processing of aluminium alloy powders, the authors review the current state of research and progress from different perspectives of the SLS/SLM, powder metallurgy ( P/M ) sintering, and pulsed electric current sintering ( PECS ) of ferrous, non-ferrous alloys, and composite powders as well as laser welding of aluminium alloys in order to provide a basis for follow-onresearch that leads to the development of high productivity, SLS/SLM processing of aluminium alloy powders. The final part of this paper summarises findings from this review and outlines the trend for future research in the SLS/SLM processing of aluminium alloy powders. 

Q2. What is the main reason why the Nd:YAG laser is increasingly used in various applications?

With the development of high-output power, improvement in laser beam quality, and the possibility of glass fiber delivery, the Nd:YAG is increasingly used in various applications where the CO2 laser had previously held sway [44, 124]. 

Q3. What is the way to reduce liquation cracking in aluminium alloys?

Occurrence of liquation cracking can be reduced in SLS/SLM processed aluminium alloy parts by minimising the dissipated energy density on the powder bed/substrate. 

Q4. What is the way to achieve stable scan tracks?

With the development of high-power and high-quality lasers, stable scan tracks can be obtained, especially using CW Nd:YAG lasers. 

Q5. What is the reason why the 5000 series alloys are not susceptible to solidification cracking?

The 5000 series alloys are not susceptible to solidification cracking due to their high Mgcontents, whereas, heat treatable alloys have higher solidification cracking tendency as a result of greater amount of alloying which create a tendency to form low melting constituents and widen the critical temperature ranges [2]. 

Q6. What is the effect of the nitrogen atmosphere on the sinterability of aluminium alloys?

Since the processing conditions of conventional powder metallurgy sintering differ from that of the direct selective laser sintering, it will be good to explore whether changing from argon to nitrogen atmosphere enhances the sinterability of aluminium alloys other than AlSi12 powder in SLS/SLM. 

Q7. What is the nature of the microstructure obtained when laser power, scanning rates, scan spacing are?

The nature of the microstructure obtained when laser power, scanning rates, scan spacingand layer thickness are varied is dependent on the duration of the interaction between the powder and the laser beam. 

Q8. What is the reason for the increased solidification crack susceptibility in pulsed laser processed parts?

According to Cao et al. [168], rapid cooling rate may also be responsible for the increased solidification crack susceptibility in pulsed laser processed parts; though, pulsed-laser processing produces grain refinement and higher process control flexibility in comparison to CW laser processing. 

Q9. What is the effect of the disruption of the oxide film on the densification of aluminium?

the improved densification kinetics of aluminium alloy powders evident by the disruption of the oxide film covering the aluminium particles which resulted in the promotion of inter-particulate melting across the layers was found to be favoured by a high degree of thermal mismatch between the oxide film and the parent aluminium particles as well as a uniform oxide layer thickness. 

Q10. What is the likely mechanism by which lower oxide films are disrupted?

They then reasoned that Marangoni forces that stir the melt pool are the most likely mechanism by which lower oxide films are disrupted but not the sides, thereby creating the ‘walls’ of oxides. 

Q11. What is the reason for the increased dendrite arm spacing?

Increased secondary dendrite arm spacing as the energy density increased was attributed to the fact that at the highest energy density (100 J/mm3), there is the potential for greater melt superheat so it takes longer for the initiation of solidification therefore, a lower temperature gradient may result giving rise to a lower cooling rate, hence, coarser dendrite arm spacing (Fig. 38). 

Q12. What are the main reasons why a sintered sample may have inferior mechanical properties?

a sintered sample may attain full density; nevertheless, it may possess inferior mechanical properties as a result of microstructural defects, the presence of inclusions such as oxides (Fig. 52). 

Q13. What causes the occurrence of coarse structure at the bottom portion of the build?

The occurrence of coarse structure at the bottom portion of the build could also be attributed to the lower thermal conductivity of the powder bed at the first instance laser beam was impinged on it. 

Q14. What is the minimum layer thickness at which a pore-free structure is obtainable?

According to Agarwala et al. [50], the minimum possible layer thickness at which a pore-free structure is obtainable is determined by the maximum particle size of the powder deposited on the bed as well as the precision of the powder delivery mechanism employed in the sintering machine. 

Q15. What is the relationship between the scanning strategies and the properties of SLS/SLM processed parts?

To control the thermal gradient during powder heating and cooling and thereby fabricated unwarped and uncracked layers, various studies [42, 56, 98-102] have investigated the relationship between scanning strategies and the properties of SLS/SLM processed parts. 

Q16. How can a single laser beam be delivered to a number of laserprocessing work stations?

the light beam from a single laser source can be delivered to a number of laserprocessing work stations via fibre optic delivery [44]. 

Q17. What are the potential role of oxide films in aluminium alloys?

the potential role of oxide films has not been elucidated in SLS/SLM processed aluminium alloy parts due to abundance of some defects such as porosity, cracking, and loss of alloying elements.