# Powder Bed Selective Laser Processing of Alumina: Scanning Strategies Investigation
## 1 Introduction  
Aluminum oxide (alumina), characterized by its unique physical and mechanical properties, is one of the most important ceramic materials [1]. Its low density, high strength, high hardness, electrical insulation, corrosion resistance, biocompatibility, and heat resistance enable it to be used in many applications [2; 3; 4; 5]. Additionally, its high melting point and low thermal conductivity make it suitable for ultra-high temperature and thermal protection applications [6]. Several conventional manufacturing methods such as extrusion, injection molding, slip casting, and pressing can be used to manufacture alumina parts [7; 8; 9; 10]. These techniques need post-processing operations to reach the final product and its limited complexity. Additive manufacturing (AM) of ceramics is a developing technology that can offer several features such as flexibility and shape complexity for free. There are seven techniques for AM as defined by ISO/ASTM 52900 [11] as Binder jetting, Powder Bed Selective Laser Processing (PBSLP), and vat-photopolymerization. Some of these techniques can produce ceramic parts with acceptable quality. However, other methods are still under development. PBSLB, as described in Figure 1, is one of the AM techniques under development for ceramics. In PBSLP, a layer of powder is deposited on the printer bed and is then selectivelyscanned by a laser system according to the layer details. After finishing the first layer, another layer of powder is deposited and scanned. These steps are repeated until finishing all layers [12]. PBSLP of alumina can be either indirect or direct. In indirect PBSLP, alumina powder is mixed with another material acting as a binder, and then laser scanned [13; 14]. After scanning, post-processing operations are needed to remove the binder material and consolidate the alumina powders together. Removing the binder leads to dimension shrinkage, one of the main problems that indirect PBSLP faces [15]. Direct-PBSLP means that a pure alumina powder is fed into the printer bed and melted with a laser source. Due to the high melting point (>2000 K) of alumina and the rapid increase in temperature during PBSLP, thermal shocks occur, and consequently, high thermal stresses and cracks appear [16]. Direct-PBSLP holds excellent potential for the AM of alumina, where final products can be obtained without post-processing operations. However, direct-PBSLP faces many obstacles. Among these obstacles, as described previously, are the thermal shock, thermal stress, cracks, laser interaction with powder. Additionally, there are several process parameters (laser power, scanning speed, hatching distance, and scanning strategies) which need to be deeply studied and investigated to arrive at the proper values for these parameters. Many previous studies focused on LB-PBF of ceramic material to overcome the previously described problems. [PERSON] et al. [17] used a CO\({}_{2}\) laser to heat the powder layer also scanning with an Nd:Yag laser to reduce the developed cracks. Their results showed that the cracks reduced. [PERSON] et al. [18] developed a preheating system for yttria-stabilized zirconia (YSZ) powder. They used an Nd-YAG laser to preheat the powder bed before scanning with a fiber laser. YSZ parts with a relative density of 84% were manufactured with fewer cracks. Other studies [19; 20; 21; 22; 23; 24; 25; 26; 27; 28] investigated the interaction between the laser and alumina. Almost all available LB-PBF printers in the market are devoted to working with metal powder and equipped with a laser system with a tiny wavelength (about 1064 \(\upmu\)m). Alumina has low absorptivity for this laser, and researchers studied mixing some additives with alumina powder to increase its interaction with lasers. Additionally, studies [26; 29] have investigated the process parameters, especially laser power and scanning speed, to achieve acceptable relative density and mechanical properties.