Frequency domain measurements of melt pool recoil force using modal analysis

Recoil pressure is a critical factor affecting the melt pool dynamics during Laser Powder Bed Fusion (LPBF) processes. Recoil pressure depresses the melt pool. When the recoil pressure is low, thermal conduction and capillary forces may be inadequate to provide proper fusion between layers. However, excessive recoil pressure can produce a keyhole inside the melt pool, which is associated with gas porosity. Direct recoil pressure measurements are challenging because it is localized over an area proportionate to the laser spot size producing a force in the mN range. This paper reports a vibration-based approach to quantify the recoil force exerted on a part in a commercial LPBF machine. The measured recoil force is consistent with estimates from high speed synchrotron imaging of entrained particles, and the results show that the recoil force scales with applied laser power and is inversely related to the laser scan speed. These results facilitate further studies of melt pool dynamics and have the potential to aid process development for new materials.

Transition mechanism of melt depth during Laser Powder Bed Fusion using in-situ X-ray and thermal imaging

This research clarified the transition mechanism of melt depth in Ti powder bed during Laser Powder Bed Fusion process using in-situ X-ray and thermal imaging. A fiber laser beam of 150 W was irradiated on a powder bed at a scan speed of 15 mm/s for 3.5 s in a vacuum chamber. The obtained X-ray images showed a keyhole depth Ld increased immediately after laser irradiation, gradually decreased, and became constant. It also showed a keyhole width Lw increased immediately after laser irradiation and decreased afterward, after that, Lw increased again, and became constant. Furthermore, thermal images that measured the temperature on the powder bed showed the high temperature width Lh gradually increased and become constant. The model of the driving force which pushed the molten droplet was examined by analyzing the volume and scattering speed of the molten droplet. The model indicated the recoil pressure caused by the vaporization of powder metal was a driving force for the molten droplet scattering. The transition mechanism of keyhole depth was considered as follows. The increase of Ld at the beginning is due to the increase of the recoil pressure PT. This is because the decrease of Lw and large quantity of vaporization. Next, the decrease of Ld is due to the decrease in PT. This is because the increase of Lw and decrease of quantity of vaporization. At last, the transition to the constant Ld is caused by stabilization of Lw and Lh followed by stabilization of PT.

Frequency Domain Measurements of Melt Pool Recoil Force Using Modal Analysis

Recoil pressure is a critical factor affecting the melt pool dynamics during Laser Powder Bed Fusion (LPBF) processes. Recoil pressure depresses the melt pool, providing layer-to-layer fusion without introducing porosity. If the recoil pressure is too low, the process operates in a conduction mode where layers will not properly fuse, while excessive recoil pressure leads to a keyhole mode, which results in gas porosity. Direct recoil pressure measurements are challenging because it is localized over an area proportionate to the laser spot size producing a force in the mN range. This paper reports a vibration-based approach to quantify the recoil force exerted on a part in a commercial LPBF machine. The measured recoil force is consistent with estimates from high speed synchrotron imaging of entrained particles, and the results show that the recoil force scales with applied laser power and is inversely related to the laser scan speed. These results facilitate further studies of melt pool dynamics and have the potential to aid process development for new materials.

Acceleration of metal drops in a laser beam

Different processes require the detachment of metal drops from a solid material using a laser beam as the heat source, for instance laser drop generation or cyclam. These techniques imply that the drops enter the laser beam, which might affect their trajectory. Also, many laser processes such as laser welding or additive manufacturing generate spatters that can be accelerated by the laser beam during flight and create defects on the material. This fundamental study aims at investigating the effects of a continuous power laser beam on the acceleration of intentionally detached drops and unintentionally detached spatters. Two materials were studied: 316L steel and AlSi5 aluminium alloy. High-speed imaging was used to measure the position of the drops and calculate their acceleration to compare it to theoretical models. Accelerations up to 11.2 g could be measured. The contributions of the vapor pressure, the recoil pressure, and the radiation pressure were investigated. The recoil pressure was found to be the main driving effect but other phenomena counteract this acceleration and reduce it by an order of magnitude of one to two. In addition, two different vaporization regimes were observed, resulting respectively in a vapor plume and in a vapor halo around the drop.

Role of bronchodilation and pattern of breathing in increasing tidal expiratory flow with progressive induced hypercapnia in chronic obstructive pulmonary disease

Hypercapnia (HC) in vitro relaxes airway smooth muscle; in vivo, it increases respiratory effort, tidal expiratory flows (V̇exp), and, by decreasing inspiratory duration (Ti), increases elastic recoil pressure (Pel) via lung viscoelasticity; however, its effect on airway resistance is uncertain. We examined the contributions of bronchodilation, Ti, and expiratory effort to increasing V̇exp with progressive HC in 10 subjects with chronic obstructive pulmonary disease (COPD): mean forced expiratory volume in 1 s (FEV1) 53% predicted. Lung volumes (Vl), V̇exp, esophageal pressure (Pes), Ti, and end-tidal Pco2 ([Formula: see text]) were measured during six tidal breaths followed by an inspiratory capacity (IC), breathing air, and at three levels of HC. V̇exp and V̇ with submaximal forced vital capacities breathing air (V̇sFVC) were compared. Pulmonary resistance ( Rl) was measured from the Pes-V̇ relationship. V̇exp and Pes at end-expiratory lung volume (EELV) + 0.3 tidal volume [V̇(0.3Vt) and Pes(0.3Vt), respectively], Ti, and Rl correlated with [Formula: see text] ( P < 0.001 for all) and were independent of tiotropium. [Formula: see text], Ti, and Pes(0.3Vt) predicted the increasing V̇(0.3Vt)/V̇sFVC(0.3Vt) [multiple regression analysis (MRA): P = 0.001, 0.004, and 0.025, respectively]. At [Formula: see text] ≥ 50 Torr, V̇(0.3Vt)/V̇sFVC(0.3Vt) exceeded unity in 30 of 36 measurements and was predicted by [Formula: see text] and Pes(0.3Vt) (MRA: P = 0.02 and 0.025, respectively). Rl decreased at [Formula: see text] 45 Torr ( P < 0.05) and did not change with further HC. IC and Vl(0.3Vt) did not change with HC. We conclude that in COPD HC increases V̇exp due to bronchodilation, increased Pel secondary to decreasing Ti, and increased expiratory effort, all promoting lung emptying and a stable EELV. NEW & NOTEWORTHY The response of airways to intrapulmonary hypercapnia (HC) is uncertain. In chronic obstructive pulmonary disease (COPD), progressive HC increases tidal expiratory flows by inducing bronchodilation and via an increased rate of inspiration and lung viscoelasticity, a probable increase in lung elastic recoil pressure, both changes increasing expiratory flows, promoting lung emptying and a stable end-expiratory volume. Bronchodilation with HC occurred despite optimal standard bronchodilator therapy, suggesting that in COPD further bronchodilation is possible.

The Use of a Virtual Patient to Follow Pleural Pressure Changes Associated with Therapeutic Thoracentesis

Purpose Influence of therapeutic thoracentesis on the pleural pressure (Pp) has been discussed in many clinical studies, however reasons of Pp changes are not precisely established. The aim of the study was to use a previously elaborated virtual cardiopulmonary patient (VP) in analysis of impact of physiological factors on Pp during the procedure. Methods Simulations were performed on VP with default values of parameters for which VP simulated the respiratory system of the average 50-year-old healthy Polish woman according to spirometric examination. Alterations of Pp and the amplitude of Pp changes related to breathing (AP) were analyzed. Model parameters related to chosen factors were deviated from their default values to analyze the degree of their impact on Pp and AP. The analysis was based on and supported by our own clinical data. Results The Pp and AP alteration intensity appeared to be most sensitive to the compliances of the rib cage and mediastinum, and the nonlinearity of the dependence between the recoil pressure and the lung volume: the lower the compliances and the higher the nonlinearity were, the deeper the Pp fall during the procedure and the bigger the AP increase were observed. Conclusions Experiments in silico are very useful in analyzing sophisticated physiological and medical problems. They made it possible to show which factors are particularly responsible for changes in Pp during thoracentesis. In the future, they may be useful in establishing objective conditions under which thoracentesis needs to be stopped.

Evidence for age-dependent air-space enlargement contributing to loss of lung tissue elastic recoil pressure and increased shear modulus in older age

As a normal part of mature aging, lung tissue undergoes microstructural changes such as alveolar air-space enlargement and redistribution of collagen and elastin away from the alveolar duct. The older lung also experiences an associated decrease in elastic recoil pressure and an increase in specific tissue elastic moduli, but how this relates mechanistically to microstructural remodeling is not well-understood. In this study, we use a structure-based mechanics analysis to elucidate the contributions of age-related air-space enlargement and redistribution of elastin and collagen to loss of lung elastic recoil pressure and increase in tissue elastic moduli. Our results show that age-related geometric changes can result in reduction of elastic recoil pressure and increase in shear and bulk moduli, which is consistent with published experimental data. All elastic moduli were sensitive to the distribution of stiffness (representing elastic fiber density) in the alveolar wall, with homogenous stiffness near the duct and through the septae resulting in a more compliant tissue. The preferential distribution of elastic proteins around the alveolar duct in the healthy young adult lung therefore provides for a more elastic tissue. NEW & NOTEWORTHY We use a structure-based mechanics analysis to correlate air-space enlargement and redistribution of elastin and collagen to age-related changes in the mechanical behavior of lung parenchyma. Our study highlights that both the cause (redistribution of elastin and collagen) and the structural effect (alveolar air-space enlargement) contribute to decline in lung tissue elastic recoil with age; these results are consistent with published data and provide a new avenue for understanding the mechanics of the older lung.