Development of porous 316L stainless steel with novel structures by ...

Development of porous 316L stainless steel with novel structures by selective laser melting Z. Y. Wang*, Y. F. Shen and D. D. Gu The selective laser melting of a metallic/inorganic blended powder system consisting of 316L stainless steel powder and NH4HCO3 powder was performed. The SEM characterisation showed the formation of a honeycomb-like porous structure possessing unusually micrometre scaled pores (y2 to y5 mm), with the addition of 4?0 wt-%NH4HCO3 powder using a high laser power of 800 W. The EDX and XRD analyses testified that the obtained porous structure was stainless steel with high chemical purity. The developing mechanism of such a novel microcellular structure under various processing conditions was addressed. It shows that the cooperative action between bubbles escaping kinetics and metallic matrix solidification kinetics, accounts for the formation of the honeycomb porous configuration. The effects of component ratio and main processing parameters on the development of porous structures were also assessed. Keywords: Porous metals, Selective laser melting, Laser processing, Kinetics, Porosity

Introduction Porous metals offer opportunities in a wide range of structural and functional applications due to a number of interesting combinations of mechanical and physical properties, such as low specific weight, high gas permeability, and high thermal conductivity.1 There exists a great interest in their practical applications in terms of lightweight structures, filtration and separation systems, heat exchangers and cooling machines, etc.2 To date, a number of processing routes for the fabrication of porous metals have been reported, including powder metallurgy,3,4 foaming5,6 and casting methods.7–9 However, these conventional methods, especially the foaming approaches have inherent limitations such as contamination, presence of impurity phases, and limited control over pore features.10 More important, porous metallic components with complicated configurations cannot be easily fabricated using these traditional methods. Thus, novel fabrication methods for porous metals that can ensure complex part geometries, uniform pore structures and high levels of purity should be innovatively designed. Selective laser melting (SLM), as a typical solid freeform process, enables the quick production of complex shaped three-dimensional (3D) parts using a bed of loosely packed metal powders.11–13 Selective laser melting constructs parts by selective fusion and

College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, 29 Yudao Street, Nanjing 210016, China *Corresponding author, email [email protected]

ß 2011 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 22 April 2009; accepted 3 June 2009 DOI 10.1179/003258909X12450768326947

consolidation of powders in a layer by layer manner according to the 3D digital data of the part to be prepared, using a computer controlled scanning laser beam. Owing to its flexibility in materials and shapes, SLM is a promising strategy to produce complex shaped porous metallic components.14,15 Furthermore, the unique metallurgical process of SLM, e.g. the presence of significant Marangoni effect, the super high undercooling degree, may lead to the formation of some novel porous structures in SLM processed metals. Nevertheless, most of previous work using SLM has concentrated on reducing porosity and producing fully dense functional metallic components. Actually, few works have focused on the developing mechanisms of porous structures in SLM processed porous metals. In this work, SLM of a 316L stainless steel (SS) and NH4HCO3 blended powder system was performed. A novel honeycomb-like microcellular structure possessing a monolithic matrix was found in SLM processed porous metals. A reasonable metallurgical mechanism for its formation was elucidated. The influence of the content of the additive ingredient and the processing conditions on the development of porous structures was also addressed.

Experimental The starting powder system consisted of two components, i.e. the austenitic 316L SS powder with a nearly spherical morphology and a mean particle size of 45 mm (Fig. 1); and the additive NH4HCO3 powder with an analytical purity. A homogeneous powder blends of the SS–x wt-%NH4HCO3 system (x51?0, 3?0 and 4?0) were

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Development of porous 316L stainless steel

2 Schematic of SLM apparatus

Results and discussion Microstructural characterisation and formation mechanisms 1 Image (SEM) showing characteristic morphology of SS powder

prepared by mixing the ingredients in a Fritsch Pulverisette 6 planetary monomill, using a low rotation speed of 100 rev min21, a milling time of 30 min, and a ball to powder weight ratio of 3 : 2. To avoid the thermal decomposition of NH4HCO3 powder caused by an excessive temperature rise within the milling vessel, 5 min ball milling was followed by 5 min interval time. The used SLM system, as schematically shown in Fig. 2, consisted mainly of a continuous wave Gaussian CO2 laser (l510?6 mm) with a maximum output power of 2 kW, an automatic powder delivery system, and a process computer. When a sample was to be built, a steel substrate was placed on the platform and levelled. Afterwards, a uniform powder layer (0?20 mm in thickness) was spread on the substrate by the roller. Subsequently, a laser beam scanned the powder bed surface to form a layerwise profile according to the slice file converted from the CAD model of the specimen. The similar process was repeated until the sample was fully fabricated. The entire SLM process was performed in atmosphere at room temperature. A systematical investigation on direct metal laser sintering of 316L SS has been performed by Gu et al.16,17 From these previous results, the following suitable operating parameters were used: spot size 0?30 mm, laser power 200– 800 W, scan speed 0?01 m s21, and scan line spacing 0?15 mm. Rectangular specimens with dimensions of 5061066 mm were prepared. The porosity of the SLM processed porous specimens was calculated with Archimedes principle. One representative sample selected for metallographic examination was cut, ground, and polished according to standard procedures. Surface morphologies and microstructures of specimens were characterised by a Quanta 200 scanning electron microscope (SEM). Structural features like the mean value and the distribution of pore size were measured by quantitative image analysis software UTHSCSA ImageTool based on the SEM images. Chemical compositions were examined by an energy dispersive X-ray spectroscope (EDX). Phase identification was performed using a Bruker D8 Advance X-ray diffractometer (XRD) with Cu Ka radiation, operated at 40 kV and 40 mA.

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Figure 3a illustrates the typical surface morphology of the porous sample prepared at a laser power of 800 W with 4?0 wt-%NH4HCO3. It was clearly demonstrated that a honeycomb-like cellular structure with a homogeneous size distribution of unusually micrometre scaled pores (y3?5 mm in average) was formed. The pores size was typically in a range of y2 to y5 mm. In order to further study the porous features in Fig. 3a, SEM characterisation at a higher magnification was performed, as revealed in Fig. 3b. It was apparent that the honeycomb cells exhibited a near polygonal microcellular shape and the pore walls were successively interconnected (arrowheads, Fig. 3b). Moreover, composition and phase analyses of the honeycomb-like porous structure were implemented. Figure 3c gives the EDX results performed in zone A in Fig. 3b, showing that the honeycomb-like structure was mainly composed of elements from the starting SS powder (e.g. iron, chromium and nickel elements, etc.), while the nitrogen element originating from the NH4HCO3 powder could not be detected, which indicated a complete decomposition of the NH4HCO3 ingredient during SLM. As the additive NH4HCO3 powders possess a low decomposition temperature (y60uC), they decompose completely under laser induced ultrahigh temperature condition, undergoing the following chemical reaction18 NH4 HCO3 ~NH3 ðgÞzH2 O ðgÞzCO2 ðgÞ

(1)

Figure 3d depicts the typical XRD pattern of the honeycomb-like porous structure. The strong diffraction peaks of the SS austenite phase could be clearly observed, while no peaks of NH4HCO3 additive was detected. Thus, it can be preliminarily considered that the laser processed materials were mainly composed of the austenite phase of SS without residual additive NH4HCO3 or the formation of new phases. Figure 3e shows the internal pore structures on the polished crosssection. Both the shape and the size of the pores on the section were comparable to those on the surface (Fig. 3b), while only the thickness of pore walls on the section showed a slight decrease. Combined with the SEM, EDX and XRD results (Fig. 3), it can be reasonably confirmed that a novel honeycomb-like microcellular SS with high chemical purity is successfully prepared using SLM.

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a SEM image showing typical honeycomb-like microstructure on surface; b local magnification of a; c EDX analysis of honeycomb structure; d XRD analysis of honeycomb structure; e SEM image showing internal pore structures on polished cross-section 3 Microstructural characterisation of porous sample obtained at laser power of 800 W with 4?0 wt-%NH4HCO3

It is known that when interaction of the mobile laser beam with the irradiated powder occurs, the laser energy

is directly absorbed by particles through both bulk coupling and powder coupling mechanisms.19 During

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4 Schematic describing formation mechanisms of honeycomb-like microcellular structure during SLM

SLM of the 316L SS/NH4HCO3 metallic/inorganic powder system, the sufficient laser energy is deposited into the laser irradiated region, leading to a rapid elevation in the operating temperature above the liquidus temperature of the prealloyed 316L SS (y1398uC). Consequently, a resultant melting pool is rapidly formed and, meanwhile, the additive NH4HCO3 is expected to experience thermal decomposition process to generate polyatomic gaseous mixtures (see reaction (1)), which exist in the present steel melt in the form of suspended bubbles, as schematically described in Fig. 4. Owing to the usage of a Gaussian laser beam, a large temperature gradient between the centre and the edge of the melting pool will be formed, leading to surface tension gradients and the resultant Marangoni flow on free surfaces.20 Noteworthily, at a high oxygen concentration, the surface tension of steel liquid c may increase with increasing temperature T, i.e. dc/dT.0. In this case, it has been found that a deep and narrow flow pattern will be formed, since surface tension gradients result in shear stress and convective movement of the melt pool (Marangoni effect).21 The laser induced melting is followed by a rapid solidification, which is characterised by a high solidification rate, thus accelerating the advancing of solidification front. As SLM can be considered as ‘high power density short interaction time’ process,22 a combined consideration concerning suitable content of the gas generating additive and moderate laser processing parameters so as to yield a desired porous structure, is required. The amount of bubbles generation depends on the content of additive materials, while the pores evolution during laser induced melting and solidification is mainly determined by the processing parameters. During the rapid solidification process, the gaseous bubbles float to the melt surface swiftly due to a combined action of Stokes motion and Marangoni flow, and tend to accumulate near the peak site of the molten pool (Fig. 4). Owing to the well known piston effect occurring during laser irradiation, a significant backpressure on the surface of melt pool can be produced,19 which inhibits the bubbles escaping. The followed evolution of these bubbles, escaping from the melt pool or being trapped within the solidified structure, which is governed by processing conditions, is responsible for the resultant surface morphologies. When a high laser power of 800 W is used, bubbles with adequately high inner pressure may ‘break’ out of the melt pool completely. Meanwhile, the solidification of SS matrix completes, facilitating

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the realisation of a cooperative growth of the solid phase and the gas phase and, accordingly, the formation of honeycomb-like fine surface porous structure with a sufficiently high open porosity of 45% (Fig. 3a).

Influence of component ratio on pore features The obtained results (Fig. 3) and the proposed formation mechanisms (Fig. 4) reveal that suitable combinations of the additive contents and processing conditions are the key factors in controlling the pores development. In this section, the influence of component ratio on porous structures was investigated. Figure 5 illustrates the characteristic microstructures of SLM processed porous samples with various contents of additive materials. A close look at Figs. 3b and 5 reveals that a progressive transition from a compressed pores configuration to an expanded pores configuration occurs with increasing the content of the NH4HCO3 constitution. When 1?0 wt-% NH4HCO3 powders were used, the pores exhibited an irregular microcellular shape and the mean pores size was y2 mm. Nevertheless, pores were not well developed with discrete walls, and a relative low porosity of 35% was realised (Fig. 5a). Interestingly, at a higher content of 3?0 wt-%, a honeycomb-like microcellular structure was also present (Fig. 5b). Compared with the results shown in Fig. 3a, the value of the average pores size and porosity showed a slight decrease, being y3 mm and 42% respectively. It is known that the initiation for pore generation is that the inner pressure of the bubble exceeds the total pressure around it.14 As a higher amount of the gas forming additive (>3?0 wt-%) is used, the pressure in bubbles elevates, promoting the sufficient growth of bubbles and, accordingly, the formation of honeycomb-like configurations featured by expanded pores and a monolithic matrix (Figs. 3b and 5b).

Influence of main processing parameters on porous structures The effect of main controlling parameters (the laser power) on the porous structures is also taken into account (Figs. 3a and 6). At a relatively low laser power of 200 W, a large amount of close packed blisters with an average diameter of y3 mm were present, resulting in a near fully bubbly surface (Fig. 6a). With increasing laser power to 500 W, a small number of honeycomb pores together with many blisters were observed, as revealed in Fig. 6b. The pore formation mechanisms elucidated above can

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a 1?0 wt-%; b 3?0 wt-% 5 Images (SEM) showing typical surface morphologies of SLM processed porous samples with various contents of additive NH4HCO3 powders: laser power is ﬁxed at 800 W

be applied here. At a low laser power of 200 W, a relatively low temperature in the melting pool generates, limiting the internal pressure of bubbles and hence their floating rates. Under this condition, the quick advancing solidification front can trap these low rates floating bubbles within the solidified structure, forming a surface bubbly morphology (Fig. 6a). As the laser power increases, the temperature in the melting pool rises correspondingly, inducing a higher floating velocity of bubbles. On the other hand, for a given scan speed (0?01 m s21), the advancing rate of solidification front is almost constant under the same cooling conditions. Thus, with increasing laser power to 500 W, only partial bubbles may ‘break’ out of the melt pool, forming a certain number of honeycomb pores, while some bubbles remain trapped within the solid. Therefore, honeycomb pores and surface bubbling structure are

coexisted (Fig. 6b). In this situation, the obtainable open porosity of the prepared multilayer component was 21%. As the laser power further increases to 800 W, a desirable honeycomb-like porous structure possessing a high open porosity (45%) is yielded (Fig. 3a).

Conclusions Porous 316L SS materials with typically honeycomb-like microcellular configurations and unusually micrometre scaled cells (y2 to y5 mm) were successfully prepared using SLM at a favourably high laser power of 800 W, by adding 4?0 wt-% gas generating agent NH4HCO3 powder. The formation of such a honeycomb-like porous structure is attributed to the competition between bubbles escaping kinetics and matrix solidification kinetics, providing sufficient ability to ‘break’ out of

a 200 W, surface bubbly structure; b 500 W, blisters and honeycomb pores coexisted structure 6 Images (SEM) showing characteristic microstructures of samples at different laser powers: additive contents are ﬁxed at 4?0 wt-%

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the melting pool during solidification, and thus form the eventual honeycomb-like microcellular configuration. The contents of the NH4HCO3 additive and the applied laser powers exert a significant influence on pore characteristics. The decrease in the content of the NH4HCO3 additive leads to an unexpanded cells morphology and a lowered porosity. The increase in the laser power favours the formation of the open honeycomb pores.

Acknowledgement The authors would like to thank the financial support provided from the National Natural Science Foundation of China (grant no. 50775113).

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