Keyhole Wall Research Articles

Coupling effect of dynamic power and oscillation path on butt weld formation and melt flow behavior during aluminum alloys

The influence of the coupling effect between dynamic power and oscillation path on the formation of weld seams and the fluid behavior of the molten pool during laser welding of 5A06 aluminum alloy was investigated through numerical simulation and experimentation. Three power modes were compared in the experiments: equal power (EP), delayed dynamic power (D-DP), and non-delayed dynamic power (ND-DP). The experimental results show that both D-DP and ND-DP modes are favorable to improve the mechanical properties. Among them, the joints formed under the D-DP mode exhibit the best mechanical properties, with an average tensile strength reaching ≈ 99.1% of the base material, which is an increase of ≈ 35.3% and ≈ 5.6% compared to the EP and ND-DP modes, respectively. Numerical simulation results indicate that the D-DP mode effectively mitigates the average flow velocity of the molten pool, reduces the collision effects between liquid flows, suppresses the impact of fluid on the keyhole walls, and improves the stability of the keyhole and the welding process, thereby enhancing the mechanical properties of the joints.

Parametric study of the laser energy absorption in high-power laser beam welding

Laser energy absorption on the keyhole wall is decisive for the thermodynamic behavior and the resultant weld properties in the high-power laser beam welding process. However, its highly transient nature on a microsecond scale makes the quantitative analysis challenging. In this paper, the influence of the relevant welding parameters on laser energy absorption is studied statistically by utilizing multiphysical modeling, in which the three-dimensional transient keyhole dynamics and thermo-fluid flow are calculated. A dynamic mesh adaption technique and a localized level-set-based ray-tracing method are employed to improve the model accuracy further. The results show that the focus position has a remarkable effect on the time-averaged laser absorption, and in contrast, the laser energy distribution regime is only slightly influenced by the welding speed in the studied parameter range (1.5–3.0 m/min). The absorption ratio of the laser energy on the keyhole front wall decreases with increasing welding speed and increases with upward-moving focus positions. The comparison between the calculated results and the experimental measurements ensures the validity of the proposed model.

Porosity suppression mechanism analysis in narrow-gap oscillating laser-MIG hybrid welding of aluminum alloys based on keyhole stability and molten pool flow behavior

In this study, the keyhole dynamic behavior and molten pool flow were investigated to elucidate the effects of oscillating laser on porosity in narrow-gap oscillating laser-MIG hybrid welding of aluminum alloys. A weld with a porosity of 0.08% was produced at the laser oscillating frequency of 600 Hz and diameter of 1 mm, while 2.27% without oscillating laser. Compared to non-oscillating laser, keyhole opening size increased by about 2.3 times and keyhole depth fluctuation was suppressed. The forces acted on the interface between keyhole wall and molten pool became more balanced. The increased vapor recoil pressure overcame the combined closure effect of gravity and molten pool pressure. While applying oscillating laser, the vortex in molten pool was eliminated and the liquid metal flow was more stable. The combined impact force caused by droplets transfer and liquid metal convection due to constraining effect of narrow gap was buffered, which had less effect on molten pool and keyhole stability. The liquid metal tended to flow from the molten pool inside to surface, which favored the escape of bubbles from the molten pool.

Implementation of energy interaction between laser beam and keyhole wall based on algebraically reconstructing free surface: An experimentally validated numerical framework

The interaction of the laser beam with the material is critical for determining laser processing quality, with ray tracing methods mostly used in energy transfer research. These techniques generally rely on only the volume of fluid (VOF) method or level set (LS) function to track the molten pool free surface, resulting in either accuracy loss or difficulty in ensuring mass conservation of the interface. The few coupled algorithms are built on geometric notions that are difficult to understand while being limited to structural grids. In this paper, we have proposed a revised ray tracing framework, so-called ARCLS, to derive the exact intersections of the rays with the keyhole wall which is represented by a continuous LS surface algebraically rebuilt on top of the VOF interface. It is easy to code and convenient for unstructured meshes. This framework is tested through a V-groove case and a set of Ti6Al4V titanium alloy laser welding examples. The findings reveal that the reconstructed LS surface perfectly matches the VOF interface, and the predicted molten pool profile agrees with the actual data well. The keyhole morphology and molten flow patterns are also consistent with the physical truths. Spattering, porosity, necking, and swelling are observed in the developing keyhole, which are the possible reasons for the potential welding defects. Further investigation suggested that the ARCLS-based algorithm collected more laser rays at the keyhole bottom compared to the standard technique, achieving a deeper keyhole profile and a reflection pattern that was more optically logical. The work presents a reliable approach for estimating laser energy distribution, which can be effectively applied to the heat-fluid coupling simulation for laser manufacturing processes in order to forecast the dynamic behaviors of keyhole and molten pool.

Numerical simulation of keyhole-induced pores for TA15 in laser powder bed fusion (L-PBF)

In the complex laser powder bed fusion (L-PBF) process, the occurrence of pores is often triggered when localized temperatures of the materials exceed their boiling point, posing a significant impediment to the overall quality of manufactured parts. In this work, considering the thermophysical properties and latent heat of phase transformation of the material, the formation and evolution of keyholes and pores during the L-PBF single-track processing of TA15 titanium alloy was investigated using X-ray computed tomography (X-CT) and FLOW-3D finite volume analysis software coupled with multiphysics fields. The simulated molten pool characteristics generally agree with the experimental data, including internal pores The results showed that the evolution of keyholes is mainly attributed to surface tension and recoil pressure, the Marangoni flow and surface tension contributed to closed localized cooling regions on the keyhole walls to generate pores. Additionally, the Marangoni flow and buoyancy effects lead to pore motion within the molten pool. Furthermore, the laser energy density is inherently related to the total pore counts and the mean penetration depth of keyholes, respectively. Understanding the underlying physical mechanisms helps to optimize process parameters and reduce pores in L-PBF, ultimately enhancing the quality of the additively manufactured parts.

Towards implementation of alloy-specific thermo-fluid modelling for laser powder-bed fusion of Mg alloys

Multi-physics thermo-fluid modeling has been extensively used as an approach to understand melt pool dynamics and defect formation as well as optimizing the process-related parameters of laser powder-bed fusion (L-PBF). However, its capabilities for being implemented as a reliable tool for material design, where minor changes in material-related parameters must be accurately captured, is still in question. In the present research, first, a thermo-fluid computational fluid dynamics (CFD) model is developed and validated against experimental data. Considering the predicted material properties of the pure Mg and commercial ZK60 and WE43 Mg alloys, parametric studies are done attempting to elucidate how the difference in some of the material properties, i.e., saturated vapor pressure, viscosity, and solidification range, can influence the melt pool dynamics. It is found that a higher saturated vapor pressure, associated with the ZK60 alloy, leads to a deeper unstable keyhole, increasing the keyhole-induced porosity and evaporation mass loss. Higher viscosity and wider solidification range can increase the non-uniformity of temperature and velocity distribution on the keyhole walls, resulting in increased keyhole instability and formation of defects. Finally, the WE43 alloy showed the best behavior in terms of defect formation and evaporation mass loss, providing theoretical support to the extensive use of this alloy in L-PBF. In summary, this study suggests an approach to investigate the effect of materials-related parameters on L-PBF melting and solidification, which can be extremely helpful for future design of new alloys suitable for L-PBF.

The surface tension of boiling steel surfaces

Material properties of metals and metal alloys at high temperatures are often unknown, but necessary to understand physical mechanisms for prediction and improvement of high temperature processes, such as laser beam technologies. Surface tension is an elementary property that was measured in this study above the boiling temperature of steel using a laser-induced vapor channel in a steel substrate and the extraction of the vapor channel diameter from in-situ X-ray observations. The measurement principle is based on the pressure balance inside the keyhole, where the recoil pressure from keyhole wall vaporization works against the surface tension pressure from the surrounding melt pool. An increase in surface tension at increasing temperatures above the boiling point was measured against theoretical expectations. In order to create the keyhole shapes measured, the surface tension must increase to counterbalance the increasing recoil pressure.

Arc and keyhole behavior in narrow-gap oscillating laser-MIG hybrid welding of thick aluminum alloys

Narrow-gap oscillating laser-MIG hybrid welding was developed to weld thick aluminum alloys. Weld formation and porosity defect with various oscillating laser parameters were systematically analyzed. Furthermore, the corresponding influencing factors, i.e., arc and keyhole dynamic behaviors were discussed. The difference between both sidewall penetrations was small while wire feeding speed was high and oscillating laser diameter was lower than 3 mm. When laser oscillating frequency was higher than 300 Hz, porosity was less than 0.5 %. Arc burnt stably and the deflection angles were small while laser oscillating frequency was 300–450 Hz and diameter was 1–2 mm. The small fluctuation of arc resulted in small difference between both sidewall penetrations. With the action of oscillating laser, the charged particles increased and diffused to the upper part of the groove. Therefore, arc was attracted by the charged particles and struck at the center bottom of groove. While the arc burnt stably, the difference between both sidewall penetrations was small. The pore formation was significantly inhibited by oscillating laser, as the keyhole opening size increased and fluctuation degree decreased. With the action of oscillating laser, the ratio of keyhole opening width to depth increased, the probability of Rayleigh jet instability on the keyhole wall was reduced.

Keyhole Welding with Hybrid Plasma-Free Arc Source

Improving the keyhole welding process window is challenging work because the force and thermal state in the weld pool are hard to control. A hybrid plasma-free arc source was developed based on a plasma arc torch, sided tungsten was added close to the constraint orifice to form a free arc, and hybrid arcs formed after the free arc was fully absorbed into the constraint arc. In such a one-anode, twocathode hybrid arc system, the added free arc acts in an assistant role to adjust arc heat output without influencing the arc pressure peak, and the constraint arc acts as the guiding arc to control the arc pressure and slightly incline to the free arc side. Bead-on welding experiments were done to test keyhole welding process behavior with the hybrid arc source, including arc column, keyhole state, weld surface performance, and melting state in the weld pool. Results showed that (1) the hybrid arc can enlarge the stable keyhole welding process window in aspects of welding current and welding speed, and a more-obvious improving effect is obtained with the leading-sided tungsten; (2) the hybrid arc with the rear-sided tungsten has a smooth front weld surface; (3) the increased heat will enlarge the weld cross-section area with no obvious influence to the backside weld width; and (4) the leading keyhole wall is related to the outer tungsten position. The research gives a new method for improving keyhole welding stability.

Keyhole Welding with Hybrid Plasma-Free Arc Source

Improving the keyhole welding process window is challenging work because the force and thermal state in the weld pool are hard to control. A hybrid plasma-free arc source was developed based on a plasma arc torch, sided tungsten was added close to the constraint orifice to form a free arc, and hybrid arcs formed after the free arc was fully absorbed into the constraint arc. In such a one-anode, twocathode hybrid arc system, the added free arc acts in an assistant role to adjust arc heat output without influencing the arc pressure peak, and the constraint arc acts as the guiding arc to control the arc pressure and slightly incline to the free arc side. Bead-on welding experiments were done to test keyhole welding process behavior with the hybrid arc source, including arc column, keyhole state, weld surface performance, and melting state in the weld pool. Results showed that (1) the hybrid arc can enlarge the stable keyhole welding process window in aspects of welding current and welding speed, and a more-obvious improving effect is obtained with the leading-sided tungsten; (2) the hybrid arc with the rear-sided tungsten has a smooth front weld surface; (3) the increased heat will enlarge the weld cross-section area with no obvious influence to the backside weld width; and (4) the leading keyhole wall is related to the outer tungsten position. The research gives a new method for improving keyhole welding stability.

Frontal pyrometric snapshot measurements of the keyhole wall temperature in laser welding of pure aluminium

In deep penetration laser welding, the quality of the weld seam is strongly correlated with the process dynamics. Pronounced dynamics of the melt pool and the keyhole cause defects like pores and blow-outs which negatively affect the mechanical properties of the seam. The process dynamics are primarily governed by the fluctuations of the keyhole shape. Therefore, in order to avoid the occurrence of defects in the weld seam, it is necessary to gain control over the keyhole stability by understanding the underlying physical processes that contribute to its dynamics. Besides the intensity distribution of the laser beam on the keyhole walls, the keyhole wall temperature is a crucial influencing factor when it comes to the assessment of the local metal evaporation rate and the induced local recoil pressure acting on the keyhole wall. Using small high-melting tantalum probes as measuring channels, temperatures in the vicinity of the Knudsen layer at the keyhole front wall were measured at different depths below the sample surface by means of high-speed pyrometry. For this purpose, bead on plate welding was conducted at low process velocities in the Rosenthal regime using pure aluminium (EN AW-1050A) as substrate material. It is shown for different laser powers and process velocities that the melt temperature in the vicinity of the keyhole wall lies in the range of 3000 K ± 400 K, therefore intermittently exceeding the vaporisation temperature of the substrate material. Significant correlations between the keyhole wall temperature and the laser power as well as the measuring depth could not be identified.

Dynamic evolution of keyhole and weld pool throughout the thickness during keyhole plasma arc welding

Keyhole plasma arc welding (K-PAW) of medium-thick plates often faces instability in the initial keyhole penetration stage, so a thorough investigation of the keyhole and weld pool behavior in this stage is needed. In this paper, a special "sandwich visual sensing structure" was adopted to obtain images of weld pool and keyhole of the SUS304 stainless-steel plate during the plasma arc welding. This method allows direct observation of keyhole growth and weld pool flow throughout the thickness of the workpiece under different welding conditions. When the welding current is reduced, the plasma arc cannot melt through the metal, and the arc sweeps through the keyhole bottom like a “roller coaster”. The trends of keyhole dimensions along different directions during various welding processes were quantitatively investigated. As the weld proceeds to half of the plate thickness, the mutation zone of the rear keyhole wall becomes very common, and the rear keyhole wall's morphology frequently transforms between type I and type II. Weld pool flow characteristics during constant-speed welding were classified into four types. Last, the molten metal flow cycle and the molten metal cloud cluster have been observed by the new visual sensing method. This paper provides the industry with a more intuitive way to study the keyhole and the weld pool for K-PAW, as well as offers important guidance for the first normal penetration in the medium-thickness plate K-PAW process.

Investigation on the keyhole/molten pool dynamic behavior during adjustable ring-mode laser welding of medium-thick aluminum alloy

Adjustable ring-mode laser welding (ARM) provides a new solution for the pore defects of aluminum alloy joints. However, the inhibition mechanism of ARM for porosity defects in the aluminum alloy welds with medium-thickness is still lacking. In this work, the dynamic behavior of the keyhole/molten pool and the formation/transport process of bubbles during adjustable ring-mode laser welding of medium-thick aluminum alloy are investigated by experiments and simulations. The results display that the laser power ratio of central-ring beam of 6:4, the porosity is 4.19 ± 2.32 %, which is reduced by 52.2 % compared with the weld by single laser welding (SLW) of the same penetration depth. The addition of ring beam helps to enlarge the tilt angle of the keyhole rear wall, which increases the ability of the keyhole wall to resist the impact of the molten pool. Besides, it also lowers the radial pressure gradient and the local variation of the keyhole wall. These reasons reduce the formation of pores. Furthermore, the main eddy current direction of the molten pool changes less frequently, and the melt flow velocity impacting the keyhole wall only changes in the range of 0–0.5 m/s (SLW: 0.25–0.75 m/s). This diminishes the fluid dynamic pressure which is the primary driving force of the bubbles. The velocity of the bubble moving away from the bottom of the keyhole is decreased and it is more likely to remelt the keyhole and escape under the action of the laser beam at the next moment. This work provides engineering application and theoretical reference value for the suppression of pore defects in laser welding of aluminum alloy in medium-thick plates.

Vacuum laser beam welding of AZ31 magnesium alloy: Weld formability, microstructure and mechanical properties

Laser welding of magnesium alloy is still challenging because of the frequently occurred defects. In this work, a promising vacuum laser welding technique was used to join AZ31 magnesium alloy. The effect of ambient pressure on weld surface appearances and weld fusion cross sections was symmetrically investigated. It was found that the penetration depth increased and the weld width became narrower with the descending of ambient pressure from 101 kPa to 10 kPa. However, the penetration depth did not increase and even decreased when the ambient pressure further decreased to 1 kPa and 0.1 kPa. In addition, hump defects occurred at lower ambient pressure of 1 kPa and 0.1 kPa. The weld pool and keyhole behaviors were in-site observed using a high-speed camera at various ambient pressures. The results showed that unstable keyhole and turbulent flow were found in the molten pool at 101 kPa. The keyhole became more stable as the ambient pressure decreased from 101 kPa to 10 kPa. When the ambient pressure further decreased to 1 kPa and 0.1 kPa, the liquid metal thickness between keyhole wall and weld pool periphery became very narrow, which sometimes resulted in unstable absorption of the laser energy. Finally, a 10 mm-thick bottom-locking weld was made under vacuum. The average grain diameter of the weld in vacuum is 28.4 μm, which is much lower than 46 μm in atmosphere. The tensile results showed that the average ultimate tensile strength and the elongation of the joints welded in vacuum at 10 kPa was 8.8 % and 44.0 % higher than that of the joints welded in atmosphere respectively. The enhanced mechanical properties were attributed to the reduced porosity and refined microstrustures of the weld joint made under vacuum.

Challenges in dynamic heat source modeling in high-power laser beam welding

The amount of absorbed energy in the keyhole as well as its spatial and temporal distribution is essential to model the laser beam welding process. The recoil pressure, which develops because of the evaporation process induced by the absorbed laser energy at the keyhole wall, is a key determining factor for the macroscopic flow of the molten metal in the weld pool during high-power laser beam welding. Consequently, a realistic implementation of the effect of laser radiation on the weld metal is crucial to obtain reliable and accurate simulation results. In this paper, we discuss manyfold different improvements on the laser-material interaction, namely, the ray tracing method, in the numerical simulation of the laser beam welding process. The first improvement relates to locating the exact reflection points in the ray tracing method using a so-called cosine condition in the determination algorithm for the intersection of reflected rays and the keyhole surface. A second correction refers to the numerical treatment of the Gaussian distribution of the laser beam, whose beam width is defined by a decay of the laser intensity by a factor of 1/e2, thus ignoring around 14% of the total laser beam energy. In the third step, the changes in the laser radiation distribution in the vertical direction were adapted by using different approximations for the converging and the diverging regions of the laser beam, thus mimicking the beam caustic. Finally, a virtual mesh refinement was adopted in the ray tracing routine. The obtained numerical results were validated with experimental measurements.

Mechanism analysis on suppressing porosity in laser-MIG hybrid welding of aluminum alloy via external magnetic field

An external magnetic field with a high magnetic flux density (≥300 mT) has been proven to reduce the porosity of aluminum alloy single laser welded joints. However, the additional welding current exists in laser-arc hybrid welding (LAHW) and its physical mechanism of interaction between the external magnetic field and the molten pool is different from magnetic-assisted laser welding. It was explored whether the porosity suppression of the external magnetic field works in LAHW using experimental observation and numerical simulation. The results suggested that the presence of an arc would induce the interaction between the magnetic field with a lower magnetic flux density and welding current, giving rise to a strong backward Lorentz force up to 1.44 × 105 N/m3, which differed from that in the magnetic-field-assisted laser welding. Although the presence of an arc heat source extended the solidification time of the rear weld pool. The impact of the droplet on the rear keyhole wall deteriorated the keyhole stability and caused severe keyhole collapse. Additionally, some vortices also deformed the keyhole rear wall and prevented the escape of bubbles in LAHW, leading to a greater porosity rate of 13.7 %. However, the impact of the droplet was weakened and vortices were reduced when an external magnetic field of 24 mT was used, which significantly decreased the porosity rate of LAHW to 2.0 %. This study will provide a more underlying understanding of magnetic-field-assisted welding and widen the industrial application of LAHW.

Quantifying the mechanisms of keyhole pore evolutions and the role of metal-vapor condensation in laser powder bed fusion

Key to quality control for laser powder bed fusion (L-PBF) is the reduction of porosity in built parts. However, understanding the mechanisms of a complete cycle of keyhole pore evolutions, including the processes of the keyhole pore generation, collapse, and splitting, and the role of metal-vapor condensation, remains a great challenge. In this study, we employ a high-fidelity computational tool considering multiphase interactions and thermal-induced phase changes to reproduce the key observations and identify the critical physics underlying keyhole instability and the ensuing keyhole pore generation, collapse, and splitting. We demonstrate that the dynamic fluctuation of keyhole and keyhole pores is dictated by five interdependent factors: vapor condensation, liquid vortex, recoil pressure, surface tension, and keyhole morphology. The occurrence of protrusions inside the keyhole wall enhances the fluctuation of keyhole by re-directing the reflected laser rays and changing the transport of high-temperature liquid flows surrounding the keyhole. The locally generated liquid vortex joins with the overall melt pool dynamics to snap the lower portion of a keyhole to form a keyhole pore and further drive its motion, conditions for which are quantified from the numerical results. We further show that vapor condensation is the major mechanism that may cause two high-speed microjets of pores and result in pore collapse and splitting. Finally, we propose an optimization strategy based on a parametric study of the condensation rate to potentially eliminate keyhole pores during laser melting.

Dynamic vapor and keyhole behavior, and equiaxed dendrite formation in blue laser processing of copper

A coupled Cu vapor and keyhole numerical model including a simple virtual mesh refinement method was developed to investigated the Cu vapor and keyhole behavior in blue laser processing of a copper material. Electron backscatter diffraction (EBSD) was adopted to measure the crystal structure of the fusion zone. The Cu vapor mainly flowed upwards above the keyhole, while some downward flows were observed inside the keyhole. The recoil force and Marangoni stress caused the melt at the keyhole wall to flow downwards, and the Marangoni stress caused the melt at the pool edge to flow outwards. Owing to the multiple reflections and high absorption rate, the total blue laser energy efficiency can be as high as 83.1%. The EBSD, numerical and theoretical results indicated that the blue laser favored the fine equiaxed dendrite formation in the center of the fusion zone, which was attributed to the higher value of the cooling rate G × R, where G is the temperature gradient and R is the solidification rate, and the lower value of G/R at this location, unlike the previously studied infrared laser processing case.

Physical mechanism of material flow and temperature distribution in keyhole plasma arc welding at initial unstable stage

The initial unstable stage associated with keyhole plasma arc welding affects the stability and quality heavily, especially in the non-vertical welding position, which is difficult to control and restricts the process development and application. This work clarifies the temperature distribution and flow behavior of the process from the digging keyhole stage to the keyhole stable movement stage via an infrared thermography system and high-speed camera system. The experimental results showed that the molten metal on the rear side of the keyhole gradually accumulates and solidifies on the exit side and rear wall of the keyhole. When enough molten metal accumulates on the keyhole rear sidewall, it closes under the action of surface tension. The angle (86°–90°) of the keyhole rear sidewall formation can be used to determine whether the keyhole rear side is closed and reached the keyhole stable movement stage. Simultaneously, the influence of the shifting direction of gravity on the temperature distribution, flow behavior, and solidification morphology of the keyhole weld pool was investigated, which further revealed the initial unstable stage of the welding process. In the flat position, at this stage, the high-temperature area within the keyhole rear sidewall surface gradually accumulates close to the exit side of the weld pool. More molten metal flows to the keyhole exit side by gravity and shear forces, which causes a separation area on the keyhole rear side that is nearer to the exit side of the weld pool. This makes it harder to close the keyhole wall when it is in a flat position during the initial unstable stage. Furthermore, the temperature distribution and flow behavior of the keyhole rear side can be used as another factor to characterize the flow state of the weld pool in all positions and stages of welding.

Laser Welding Penetration Monitoring Based on Time-Frequency Characterization of Acoustic Emission and CNN-LSTM Hybrid Network

In-process penetration monitoring of the pulsed laser welding process remains a great challenge for achieving uniform and reproducible products due to the highly complex nature of the keyhole dynamics within the intense laser-metal interactions. The main purpose of this study is to investigate the feasibility of acoustic emission (AE) measurement for penetration monitoring based on acoustic wave characteristics and deep learning. Firstly, a series of laser welding experiments on aluminum alloys were conducted using high-speed photography and AE techniques. This allowed us to in-situ visualize the complete keyhole dynamics and elucidate the generation mechanism of acoustic waves originating from pressure fluctuations at the keyhole wall. Then, an adaptive time-frequency technique namely VMD (Variational Mode Decomposition) was proposed to characterize the acoustic energy distribution among the nine subsignals with low-frequency and high-frequency components under different welding penetrations. Lastly, a novel hybrid model combing CNN (Convolutional Neural Network) and LSTM (Long Short Term Memory) was designed to deeply mine the spatial and temporal acoustic features from the extracted frequency components. Extensive experiments demonstrate that our proposed approach yields a remarkable classification performance with a test accuracy of 99.8% and a standard deviation of 0.21, which obtains a high recognition rate. This work is a new paradigm in the digitization and intelligence of the laser welding process and contributes to an alternative way of developing an efficient end-to-end penetration monitoring system.