Laser powder bed fusion (L-PBF)

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Laser powder bed fusion (L-PBF)

Most of the metallurgical alloy developments are driven by the technological needs of industries for a specific application. Various processes for manufacturing safety-critical parts have proliferated in recent years, one of them being metal additive manufacturing (AM), also referred to as 3D printing. Amongst the AM processes listed in ASTM F2792−12a [8], laser powder bed fusion (L-PBF) deals with selective laser melting of the metal powder, layer-by-layer, to achieve the desired complex parts, as shown in Figure 1-1a).
L-PBF involves powder bed fusion using a laser beam, which melts the powder in a specific location. That is why previously it was also known as ‘selective laser melting (SLM)’. Once the layer of powder is melted, the build piston on which the powder was lying, lowers down for the next powder layer to be deposited. This results in a deposition of fresh unfused powder on the build chamber, which is again fused by the laser. These steps are repeated consequently until the final 3-dimensional part is created (which is designed using the CAD software’s, as shown in Figure 1-1b)). This process of layer-wise deposition of powder and its simultaneous fusion using the laser is thus termed as laser powder bed fusion (L-PBF).
Figure 1-1 a) Principle of laser powder bed fusion (L-PBF), adapted from [9] b) Steps involved in L-PBF fabrication, adapted from [10].
L-PBF fabrication process mainly consists of the parameters shown in Figure 1-2a). Depending on the L-PBF machine, laser parameters are controlled by its laser power and the spot size (area of exposition). Similarly during the scanning, the speed with which the laser melts the layer (i.e. the laser scan speed) can also be controlled. Out of the ones listed in Figure 1-2a), laser power (P), laser speed (vlaser), hatch spacing between two successive scans (as shown in Figure 1-2b)) and the layer thickness are the most important process parameters for the L-PBF. Depending on these parameters the solidification can be affected. There are also parameters related to the powder feedstock used in the L-PBF. Usually, the size distribution, the morphology, and the flowability of the powder play a key role. In general two types of powder size distribution are used in L-PBF, 10-45μm, and 20-63μm [1]. Similarly, a good flowability of the powder along with a better aspect ratio of the powder particles is needed to achieve a dense final part.
Figure 1-2 a) Process parameters for L-PBF process and b) schematic of a scan strategy depicting the hatch spacing, adapted from [1]

Aluminum alloys in L-PBF

L-PBF has emerged rapidly into sectors such as aerospace, where a high strength-to-weight ratio and defect-free parts are key requirements. Therefore, for lightweight applications, the use of structural Aluminum (Al) alloys has the potential to be exploited using L-PBF.
Aluminum (Al) exists in FCC (face-centered cubic) crystal structure, having a lattice parameter of ~0.4nm [11]. Compared to other metals like steel, nickel-based alloys, Aluminum has a lower density. It also possesses higher corrosion resistance with good thermal and electrical conductivity. With such properties, Aluminum is frequently used in industries dedicated to automobiles, aerospace, naval-based industries, etc. However, pure Aluminum lacks mechanical strength. Several alloying elements are frequently added to the pure Al to form precipitates and phases which give strengthening to the material. Depending on the solute elements added to the Al, several alloys are classified into two major categories (casting and wrought), as shown in Figure 1-3. The typical numbering system consists of four digits. The first digit in the alloy numbering system denotes the alloy series (ranging from 1-8), the second digit refers to the purity or version number of the modifications to the original composition, the third and fourth digits are to identify specific alloys. Moreover, the classification into different series relies on the addition of specific solute elements (Si, Cu, Fe, Mg, Zn, Mn), and based on their heat treatability as shown in Figure 1-3.
Figure 1-3 Classification of Al-alloys based on the alloying elements, adapted from [12][1].
Of the different series listed in Figure 1-3, the 6XXX and 7XXX series have better mechanical properties compared to casting alloys (AlSi10Mg and AlSi12). During the casting process, to refine the microstructures, chemical modifiers are usually added. On the other hand, due to the high cooling rate (~106K/s) associated with the L-PBF process, refined microstructures can be directly obtained. Thus, with L-PBF processing of Al alloys, it is possible to achieve complex parts having fine microstructures, which can complement the high strength to weight ratio of the Al-alloys. The most easily manufactured aluminum alloys using L-PBF are the AlSi10Mg and AlSi7Mg alloys [1,2,13–15]. These alloys were originally developed for casting applications due to their near eutectic silicon composition. However, as already mentioned, their mechanical properties are inferior to the structural alloys from the 6XXX and 7XXX series [1,11,14,16]. One of the wrought alloys from the 6XXX series is the heat treatable 6061 Al-alloy composed mainly of Si and Mg as their main alloying elements. The 6061 grade can be used in various sectors such as the automotive or aeronautic industries using conventional manufacturing techniques, due to its high thermal conductivity [17], good corrosion resistance, and high yield strength after a T6-heat treatment [16]. However, welding of the 6061-grade still remains a challenge due to its cracking susceptibility during rapid solidification [18–20].
Similarly, under typical high cooling rate conditions of the L-PBF process, the 6061 Al-alloy is frequently reported to be crack-sensitive [2–4,21]. As mentioned previously, the refinement of microstructure under L-PBF conditions compared to casting is due to its solidification with high cooling rates occurring in a small region. From a macro scale of the L-PBF process as shown in Figure 1-4a), the molten region or the melt pools (where the solid gets melted) are formed on a micro-scale (see the red region of Figure 1-4b)). Owing to the dendritic morphology (Figure 1-4b)), the resultant as-built columnar microstructure of the structural Al-alloy (6XXX and 7XXX series), is frequently reported to be cracking, as shown in Figure 1-4c) [2–4,21].
Figure 1-4 a) Schematic of L-PBF process. b) Typical length scale in which solidification happens and different modes of solid-liquid modes. c) Typical as-built microstructure of an alloy from 6XXX fabricated by L-PBF, adapted from [4].

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Thesis aim and methodology

The cracking mechanism affecting structural aluminum alloys is well documented for casting and welding but not as much for additive manufacturing. According to the literature [22,23], hot cracks develop from a mechanism of either solidification cracking (occurring at the last stages of solidification) or liquation cracking (during remelting). While the hot cracking mechanism in welding literature of 6061 grade has been identified as liquation cracking [18,24,25], it is not obvious that this must be the case for laser powder bed fusion.
Moreover, Mauduit et al. [26] also reported the evaporation of Mg and Zn to cause a change in composition and thus increasing the cracking sensitivity. For example, hot cracking in laser powder bed fusion applied to other kinds of alloys such as nickel-based superalloys [27] is identified as solidification cracking. Thus, the mechanism of cracking in 6061 Al-alloy when fabricated by L-PBF is still unknown. A possible route to guide the alloy design strategy of Al-alloys dedicated to AM is to improve our understanding of the mechanism causing cracks.
Here, the objective of this thesis is to provide new insights into the hot cracking mechanism affecting the 6061 Al-alloy processed by L-PBF with experiments and modeling, as illustrated in Figure 1-5. One of the objectives is to resolve the ambiguity about the governing mechanism occurring in the 6061 Al-alloy. This allowed us to investigate the effect of (i) the processing conditions typically encountered in L-PBF, and, (ii) of alloying additions of major elements such as Si and Mg, on the cracking sensitivity. These results should enhance our knowledge about the cracking mechanism affecting the 6061 grade manufactured by L-PBF and help us to suggest guidelines regarding processing conditions and possible chemical composition modifications to improve its manufacturability.

Methodology:

In this thesis, we investigate by experiments and modeling the hot cracking mechanism of 6061 Al-alloy fabricated by L-PBF and its sensitivity to the processing and compositional changes. First, we optimize the processing parameters based on the stable melting parameters. Second, we characterize experimentally the cracking mechanism and the location of cracks relative to the melt pool geometry. The experimental observations are then rationalized with the help of modeling. Using the melt pool temperature field predicted by Rosenthal simulations [28], hot cracking sensitivity maps are generated using the hot cracking model of Rappaz, Drezet, and Gremaud [7]. The results allow the hot cracking sensitivity to be mapped as a function of the processing parameters. A qualitative comparison is made between experimental observations and predictions. Finally, the potential role of metallurgical parameters on the hot cracking sensitivity is discussed. The detailed microstructural study of cracking, the as-built microstructure along with the modeling approach, allowed us to identify:
The cracking mechanism operating under L-PBF conditions.
The cracking pattern and its sensitivity to L-PBF processing conditions. Role of solute elements present in the 6061 Al-alloy.
A cracking sensitive criterion for 6061 Al-alloy during L-PBF, based on the Rappaz, Drezet, and Gremaud [7] model.
The effect of chemical composition modification on the cracking sensitivity. Metallurgical parameters playing a major role in depicting cracking.

Table of contents :

1. Introduction 
1.1. Laser powder bed fusion (L-PBF)
1.2. Aluminum alloys in L-PBF
1.3. Thesis aim and methodology
1.4. Thesis Structure
2. Literature review 
2.1. Aluminum Alloys fabricated in L-PBF
2.2. Defects encountered in Aluminum alloys during L-PBF.
2.3. Cracking mechanism (Review)
2.4. Cracking sensitivity criteria
2.5. Cracking mitigation strategies in Al-alloys (L-PBF and welding literature)
3. Powder Characterization 
3.1. Chemical composition.
3.2. Powder size distribution
3.3. Powder morphology and flowability
4. Process parameter optimization and microstructural observations 
4.1. Process Optimization
4.2. Multi-scale characterization of the as-built microstructure (optimized parameters)
4.3. Summary of Chapter 4.
5. Developing cracking sensitivity criteria (based on RDG model) 
5.1. Rappaz, Drezet & Gremaud (RDG criterion)
5.2. Thermodynamic calculations.
5.3. Rationalizing cracking at HAGB’s
5.4. Estimating solidification conditions typical of L-PBF (thermal gradient and solidification velocity) 124 Introduction to Rosenthal analytical solution.
5.5. Predicting critical pressure drop
5.6. Summary
6. Effect of processing parameters on hot cracking sensitivity 
6.1. Introduction
6.2. Evaluating cracking sensitivity for L-PBF melting parameters.
6.3. Effect of preheating conditions.
6.4. Sensitivity studies for different parameters.
6.5. Summary
7. Effect of chemical composition modification on the hot cracking sensitivity 
7.1. Introduction
7.2. Effect of Solute content modification
7.3. Effect of Zirconium (Zr) addition
8. Conclusions and recommendation for future work 
8.1. Conclusions
8.2. Perspectives
Appendix
Laser tracks on 6061 Al-alloy (bulk substrates)
List of Figures
List of Tables
References

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