# Fluctuations of Tracks and Layers during Aluminium Laser Powder-Bed Fusion
## 1 Introduction  
Laser Powder Additive Manufacturing contains two main process variants. Processing blown powder is named Direct Energy Deposition, Direct Metal Deposition or Laser Metal Deposition, while building structures with pre-placed powder is named Selective Laser Melting Laser Metal Fusion or Laser Powder-Bed Fusion (LPBF). The LPBF process uses laser beams that locally melt the pre-placed powder of the powder bed. Layer by layer, complex shaped parts are possible to produce. The LPBF process is industrially used for applications where usually high-cost material and complex structures are necessary to be produced. The LPBF process enables the production of high-quality parts using several powder materials, including Ni [1,2], Fe- [3] and Ti-alloys [4]. In addition, certain aluminium alloys are more widely used in LPBF, showing advantages in part performances due to lightweight design possibilities [5]. For the typically used AlSi10 Mg alloy, studies showed that single and overlapping tracks can be produced using different scan speeds, showing a homogeneous distribution of alloying elements and no porosity, while, however, a higher layer thickness is not recommended to use due to the increased risk of defect formation [6]. Almost fully dense structures could be created with the alloys AlMg3, AlMgSiO5, AlSi12 and AlSi10 Mg [7]. Due to the higher thermal conductivity of aluminium, higher laser power and/or slower processing speeds are required compared to other metals like many steel grades [8]. Defects can occur that can influence the resulting part properties [9]. Therefore, a minimum of defects is usually required [10]. Typical defects during LPBF are, e.g., solidification cracks [11] or pores due to incomplete densification [12, 13]. Further possible origins of the pores and voids were identified to be the space in the bulk powder and trapped gases [14]. The part density is defined as the percentage of fully molten and connected material without interruptions of defects, while typically a density of 100% is desired. The processingparameters influence the occurrence of defects. In general, at constant process parameters, a larger hatch distance (distance between two consecutive tracks) and higher powder layers increase the risk of forming defects [14]. Oxidation effects are the main issue to consider during processing of aluminium alloys along with the low melting temperatures. The effects of oxidation can be seen already during laser beam absorption, while aluminium surfaces show in general a high reflectivity. In addition, the laser beam illuminates different surfaces, namely, the oxide layer on the melt pool and partly the aluminium melt. During solidification, the oxide layer forms, while the aluminium is still liquid, which results in the tendency to lower the wetting on the base structure and can even lead to balling effects [15]. Also, process spatter was found to show increased oxide layers when processing AlSi10 Mg powder, while no oxides were found in the bulk material [15]. The chemical composition of the aluminium alloy plays an important role regarding the process result. Element variations can lead to increased crack susceptibility, e.g., when changing from AlSi10 Mg to Al-Cu systems [16]. Defect occurrence can be reduced by proper process parameter selection [17]. When observing the laser process, the energy input by the laser defines the shape of the single tracks and must melt the powder and a part of the material underneath to ensure metallurgical bonding [18]. In case the laser beam energy is too low, voids can remain due to insufficient melting of the powder material [14]. However, [PERSON] et al. [19] found that machine parameters are often set very conservatively to avoid voids. This typically results in multiple remelting of the same volume, which decreases the process efficiency. [PERSON] et al.