## 4 PBF-LB/M Process
### Process Optimization
#### 4.2.2 Strategies to Improve Productivity
However, despite the productivity gains, this method can lead to defects such as a lack of fusion between layers, which can result in stress concentration points that may be problematic under mechanical loading [149]. Another branch of the current literature focuses on advancing multi-laser systems capable of simultaneously melting multiple regions within the powder bed, thereby optimizing build rates and overall system efficiency [152]. Multi-laser systems employ multiple laser beams concurrently to melt materials, resulting in a substantial increase in productivity. High-power configurations offer significant efficiency improvements, particularly those utilizing dual or quad lasers. These systems provide key advantages, including the ability to process a wide range of materials and manufacture large components more cost-effectively [156]. [PERSON] et al. demonstrated that using a machine with two lasers (whether both lasers are applied to a single sample or each laser works on separate models) can reduce build time by up to 50% compared to single-laser systems [149]. Commercial PBF-LB/M machines typically use a cartesian coordinate system with linear actuators positioned along three orthogonal axes. In this setup, parts are built vertically along one axis, the recoater moves along another axis, and the gas flow is managed along a third axis, sometimes parallel to the recoater movement. A notable drawback of this design is its need for sequential operation: the laser must pause during the movement of the axes, and the axes must pause during lasing [147]. To overcome this limitation, General Electric Company researchers have developed an innovative PBF-LB/M machine that uses a rotating powder bed and multiple scanning lasers, operating on a polar coordinate system instead of the traditional cartesian system. This polar arrangement is particularly well-suited for manufacturing ring-like parts with large diameters and small cross-sections, which are common in the aerospace industry. In this sector, AM plays a crucial role in enhancing performance and reducing costs through innovative design and part consolidation [157]. Figure 10a-d illustrate the configurations of multi-laser cartesian and polar coordinate systems, as well as the components and outputs of the rotary powder bed fusion machine. In rotary powder bed fusion machines, also referred to as spiral growth manufacturing [158], the system synchronizes the rotational movement of the powder bed with an ascending laser scanner and recoater to construct parts helically. A single-point powder feeder deposits metal powder near the inner radius of an annular build volume, while a recoater spreads the powder to the outer radius in a \"snowplow\" manner. Since the recoater and laser scanner operate at different angular locations, they function independently and simultaneously. This method has demonstrated build rates three times faster than traditional PBF-LB/M systems while reducing powder consumption by over fourfold [147; 157; 159]. Research by [PERSON] et al. on producing rings using 3D laser printing methods shows that the dimensional accuracy and the specific and nominal density of rings produced via polar printing are nearly identical to those achieved through cartesian methods. Additionally, the build rate efficiency of rings made with polar printing is approximately 5% higher than that of rings produced using the cartesian method [160]. Additional strategies to enhance PBF-LB/M productivity have been explored by [PERSON] et al., who investigated the use of shelled geometries combined with hot isostatic pressing (HIP) [161]. This approach focused on limiting laser melting to the outer shell or contour of the structure intentionally, leaving the interior powder unmelted. By reducing the volume of material that needs to be melted, this method significantly reduces PBF-LB/M production times.