Low Surface Quality Research Articles

Micro-cracks generation and growth manipulation by all-laser processing for low kerf-loss and high surface quality SiC slicing

Low kerf-loss and high surface quality silicon carbide (SiC) wafer slicing is key to reducing cost, improving productivity, and extending industrial applications. In this paper, a novel all-laser processing approach is proposed by combining laser micro-cracks generation and growth manipulation. The first high fluence pulsed laser is applied to generate micro-cracks inside SiC, which increases its laser energy absorption. The second low fluence pulsed laser achieves the manipulation of micro-cracks growth and interconnection to separate SiC wafer. The optimal laser processing parameters are obtained to separate a 4H-SiC substrate at a thickness of 500 µm. The sliced surface is clean with average surface roughness (Sa) of 186 nm, standard deviation of 0.037, and kerf-loss of 915 nm. This laser slicing approach can be applied for high-hardness transparent material separation.

Investigation of the effects of heat treatment and hot isostatic pressing on micro-drilling of Ti6Al4V alloy fabricated by laser powder bed fusion

In this study, micro-drilling performance of laser powder bed fusion (LPBF) Ti6Al4V alloy with heat treatment (HT) and hot isostatic pressing (HIP) post-processing conditions was investigated and compared to wrought material. The tests were performed at two spindle rotational speeds (5000 and 10000 rev/min) and a feed per tooth (4 μm/tooth) under dry and minimum quantity lubrication (MQL) conditions using conventional drilling (CD) and peck drilling (PD) strategies. Micro-drilling performance was evaluated in terms of thrust force, surface roughness, subsurface microstructure and microhardness, burr formation, and tool wear. The test results showed that LPBF-HT and LPBF-HIP Ti6Al4V alloys exhibited different micro-drilling performances than the wrought material due to their different microstructural and mechanical properties. The LPBF-HIP Ti6Al4V characterized by higher strength and lamellar microstructure with higher β phase fraction (33.2%) led to higher thrust force, lower surface quality and higher burr height. On the other hand, the wrought Ti6Al4V with higher ductility, lower hardness, and lower β phase fraction (16.3%) showed lower thrust forces, surface roughness, and burr height. Notably, employing MQL conditions with the PD strategy provided good surface quality and reduced burr formation. A higher degree of tool wear was obtained in micro-drilling of LPBF-HIP Ti6Al4V. The major wear types were adhesion, BUE, and flank wear. Besides, chippings were observed in micro-drilling of LPBF-HT and LPBF-HIP Ti6Al4V.

On the notch sensitivity of as‐built Laser Beam Powder Bed–Fused AlSi10Mg specimens subjected to Very High Cycle Fatigue tests at ultrasonic frequency up to 109 cycles

AbstractThe notch effect significantly influences the fatigue response of components and is particularly relevant for parts produced with additive manufacturing (AM) processes, characterized by complex geometries and possible geometric discontinuities inducing local and critical peak stresses. Moreover, the low surface quality, as well as manufacturing defects and residual stresses, interacts with the local peak stress induced by geometric discontinuities, complicating the assessment of the notch effect for AM parts and requiring extensive experimental fatigue investigations. In the present paper, the notch sensitivity of as‐built AlSi10Mg specimens produced with the laser beam powder bed fusion in the Very High Cycle Fatigue regime is investigated. Ultrasonic fatigue tests up to 109 cycles have been carried out on rectangular bars and rectangular bars with a central through‐thickness hole. The notch effect has been found to significantly affect the fatigue response of the investigated AlSi10Mg specimens, with surface defects having a main role and pointing out the influence of the surface quality on the crack formation.

Machine learning models and machinability analysis for comparison of various cooling and lubricating mediums during milling of Hardox 400 steel

Martensitic steels are widely used in many areas such as automotive, mining, and agriculture mostly thanks to their thermal loading ability property. On the other hand, these special steels exhibit extreme tool wear tendency and low surface quality which can be associated with abrasive resistance. This situation makes this steel hard-to-cut and requires further investigation with several approaches. Sustainable machining environments are highly effective as modern strategies to improve the machinability index. Also, machine learning models have pivotal role on decreasing the total consumption in the way of lean manufacturing. In the light of above-mentioned information, this work focuses on the machining performances and optimization of dry, flood, and MQL conditions during the milling of Hardox 400 martensitic stainless steel. A novel approach was applied with using several cutting environments and machine learning models to enhance machinability of Hardox which is an industrially important material. Results were analyzed with different machine learning models using heat map and decision trees. Seemingly, cutting fluid assistance in the milling of Hardox steel is critical where flood and MQL provided a considerable effect on the tool wear for reducing it under some level. Also, this technology was found useful in determining the best conditions of machinability in terms of surface roughness, chip morphology, energy consumption, and cutting temperatures. Machine learning models provided hopeful results in analyzing the correlations between parameters used in the model. In machine learning, the heat map being close to 1 and the MSE and MAE values being close to 0 indicated that the model was suitable. This study is expected to observe the contributions of different types of cutting environments to the machinability criteria during milling of industrially important materials.

Multiple interaction electron beam powder bed fusion for controlling melt pool dynamics and improving surface quality

Low surface quality of fabricated parts and long processing times are two commonly stated reasons against a more wide-spread adoption of electron beam powder bed fusion (PBF-EB) as a commercially viable manufacturing technique. Aside from the large particle size distribution (45–150 µm) of processed powders, the surface roughness, limited structural resolution and process instabilities are attributed to unwanted dynamics of the melt pool, like spattering, balling, evaporation and melt pool displacement. As a result, a contouring step using slow but stable melting parameters is usually added to the process to improve surface quality, leading to increased processing times.In this study, we demonstrate how the surface quality can be improved by supplying the energy for melt pool formation over multiple, timely separated interactions. To investigate the effect of this multiple interaction processing (MIP), series of single line melt structures with increasing number of interactions were fabricated, characterized and corresponding thermal diffusion simulations performed. The experimental results show how MIP allows to lower surface roughness of melted structures through a reduction of spattering, balling and melt pool displacement during processing, while our simulation model identifies these benefits to be caused by a significant reduction in surface temperature and thus evaporation, with increasing number of interactions for melt pool formation. Our model further demonstrates, how a relatively simple, computationally inexpensive and thus scalable thermal diffusion model is capable of providing valuable information about thermal conditions during melt pool formation. Lastly, we introduce a few simple equations to calculate appropriate MIP parameters for any kind of scan strategy.

Improving the surface quality of additive manufactured polyamide parts using conventional treatment methods

Additive manufacturing processes have progressed in years past from not only being prototyping methods to also being end-product manufacturing methods. However, there are many challenges hindering higher industrial adoption rates of additive manufacturing such as low surface quality of the manufactured parts. Post-process surface treatment methods can play a key role in addressing this challenge. In this work, three mechanical methods were applied to selective laser-sintered polyamide 12 (PA12) parts with the aim of improving the surface quality. The effects of the methods regarding the surface integrity characteristics were evaluated. The results show improved surface roughness across all three methods: from initial Ra of 7.09 μm to 0.055 μm Ra, 0.28 μm Ra and 0.45 μm Ra by grinding, diamond turning and milling on the surfaces oriented upwards during fabrication respectively. Across the three methods, surface hardness was slightly reduced: with a maximum reduction of 2.5% by diamond turning.

An investigation into cut surface quality of Al6061/Al2O3/B4C composite using hydro-abrasive machining process

The modern era has realized the tremendous potential of metal matrix composites (MMCs) in slowly substituting the traditional materials used in automotive and aerospace industries. Traditional machining of second generation MMCs is quite challenging due to heat generation, tool wear, and low cut surface quality. This work presents an analysis on hydro-abrasive machining (HAM) of the Al6061/Al2O3/B4C composite by using an amalgamated method of gray incidence based response surfaces (GIRS) to produce a good surface quality. The influence of HAM variables [abrasive flow rate (FA), waterjet pressure (PWJ), jet traverse speed (SJT), and stand-off distance (DS)] on various cut surface characteristics [surface roughness (Ra), striation-free zone (SFZ), striation angle (ΘS), and kerf angle (ΘK)] is examined using L27 OA and response surface plots. A hybrid approach of the GIRS method is presented for necessary polynomial modeling and simultaneous optimization. The reduction of response values is primarily influenced by the waterjet pressure and jet traverse speed. An optimal cutting condition resulted in a significant reduction in surface roughness by 19.22% and striation angle by 10.82%. The improvements in striation-free zone and kerf angle were also notable with reductions of 7.89% and 4.74%, respectively. FESEM images of machined surfaces, profile graphs of surface roughness, and images observed using atomic force microscopy were also studied.

Influence of singular and dual MQL nozzles on sustainable milling of Al6061-T651 in different machining environments

Al6061-T651 alloy is widely used in the aerospace industry due to its superior properties and is a very difficult-to-cut material with high quality because of tool wear and thermal softening. This situation causes low surface quality, high cutting force, and limited tool life, as well as an increase in total consumed energy, total carbon dioxide emission, and total machining cost. Therefore, it is necessary to sustainably increase this aerospace alloy's machining performance. In the presented study, for the first time, milling performance and sustainability assessment of Al6061-T651 alloy under dry, minimum quantity lubrication (MQL), nano molybdenum disulfide (MoS2) reinforced nanofluid-assisted MQL (N-MQL) cutting conditions were investigated using the single nozzle and dual nozzles, two different MQL flow rates, and three different cutting speeds. In this context, the results of surface roughness, cutting force, feed force, cutting tool flank wear, total consumed energy, total carbon emission, and total machining cost were obtained. As the cutting speed increased, the cutting force, feed force, total consumed energy, total machining cost, and total carbon emission decreased by 12.9 %, 18 %, 7.8 %, 6 %, and 0.1 %, while the surface roughness increased by 34.9 %. In the MQL and MoS2 N-MQL cutting conditions at the same flow rate, using dual nozzles instead of a single nozzle improved machining response and sustainability indicators. In addition, increasing the flow rate improved machining performance and sustainability indicators. Using the double nozzle MoS2 N-MQL method reduced the surface roughness, cutting force, feed force, flank wear, total consumed energy, total carbon emission, and overall machining cost by 74.2 %, 63.9 %, 68.4 %, 66 %, 27.9 %, 0.4 %, and 6.39 %, respectively.

INVESTIGATING CUTTING FORCE AND CUTTING POWER WHEN TURNING AA6082-T4 ALLOY AT CUTTING DEPTHS SMALLER THAN TOOL NOSE RADIUS

Aluminum alloys are widely preferred engineering materials in the manufacturing industry due to their high formability, good mechanical strength, and low density. Machining problems in aluminum alloys include built-up-edge formation, chip rupturing, and low surface quality, particularly in the 6xxx series due to the high Si content in the machining area. The aim of this study was to investigate the influence of cutting depth smaller than the tool corner radius, and various cutting parameters on cutting force and cutting power in machining AA6082-T4 alloy. In this context, the Johnson-Cook material model was established for AA6082-T4 alloy, and machining behaviors in terms of cutting force, and cutting power were investigated by performing finite element method (FEM) analyses using a full factorial design and variance analyses with different machining parameters. In conclusion, the lowest cutting forces were achieved with a cutting depth of 0.3 mm and a feed of 0.1 mm/rev, and the lowest cutting power was obtained with a cutting speed of 300 m/min, a cutting depth of 0.3 mm, and a feed of 0.1 mm/rev. In addition, the most effective machining parameters have been determined as cutting depth with a ratio of 91.74% for cutting force and cutting speed with a ratio of 33.13% for cutting power based on the results of variance and regression analysis.

Evaluation on the performance of sustainable wire arc additive manufacturing using micro plasma arc welding

Metal additive manufacturing has gained popularity due to its significant advancement in minimising waste. The product design using additive manufacturing (AM) does not require mould and less material required, which is less waste. In this way, AM has the potential to transform the manufacturing industries towards greater sustainability. Besides, AM allow more complex designs and shape than conventional manufacturing. Wire arc additive manufacturing (WAAM) is one of the most popular metal AM that has received significant attention due to its usefulness for localising, repairing, and modifying instead of discarding and replacing entire components. However, conventional welding technology faced the issue of significant heat input, low surface quality, and low dimensional accuracy of WAAM components. Micro-plasma arc welding (MPAW) is a new alternative to the WAAM process. The speciality of MPAW is low current provided of 1A to 25A current range, leading to low heat input and energy-efficient manufacturing. The present study shows various metallic structures developed using an MPAW-based WAAM system, and the relative error between the CAD model and four different structures was determined. Overall, the study outcome shows the geometric uniformity of parts produced with an MPAW-based WAAM system and demonstrates smooth surface finish can be obtained.

Regulation of dimensional errors and surface quality of thin-walled components fabricated by blue laser directed energy deposition

The utilization of components fabricated by directed energy deposition (DED) faces certain limitations that hinder its widespread application. These critical challenges primarily include significant errors in dimension accuracy caused by the unreliable build-up of flowing powder and low surface quality arising from the interlayer cumulative effect and powder adhesion. The present study aims to comprehensively investigate the impact of process parameters and deposition height on the geometrical errors observed in the thin-walled structures fabricated by DED. By establishing a rational relationship between the dimensional error and the sample height, the mechanism underlying the influence of deposition height on printing stability and sustainability is elucidated. Furthermore, an innovative building strategy incorporating a variable speed is proposed to eliminate protrusions at the end of the components. The study also examines the surface roughness of inclined thin walls, revealing a correlation between roughness and both position and angle. The presence of adhering powder on the front side of the parts and the formation of a molten oxide layer on the backside result in a diminished impact of angle on roughness. Ultimately, the findings of this study are validated through the fabrication of a thin-walled axisymmetric curved structure, providing a guide for the printing of large special-shaped parts.

Investigation of dimensional and geometrical tolerances of laser powder directed energy deposition process

Laser Powder Directed Energy Deposition (LP-DED) is a promising Additive Manufacturing (AM) technology that offers the opportunity of repairing and revamping damaged or outdated parts and molds. Notwithstanding, it should be considered that the low surface quality of the produced parts makes post-processing operations necessary in order to meet the industrial requirements, and these operations strongly depend on the accuracy of the produced parts. In this paper, firstly an artifact for the LP-DED process, including classical features with and without curvatures, is proposed. Then, the dimensional accuracy and geometrical tolerances of the features are evaluated, limited to AISI 316L stainless steel.The findings of this work showed that the dimensional and geometrical tolerances of the LP-DED process are comparable with those of sand casting. On the other hand, tolerances are lower compared to Laser Beam Powder Bed Fusion (LB-PBF) technologies but are similar to those observed in Electron Beam Powder Bed Fusion (EB-PBF).

Analysis and development of elliptical tool path in trochoidal milling

Trochoidal milling has been developed to extend the cutting tool life as compared to conventional milling. It is widely used in the machining of slots and corners of molds and dies. There are several tool paths typically used in trochoidal milling such as true, circular, variable-feed, and variable-step trochoidal tool paths. However, these paths suffer from low material removal rates, low surface qualities, high CPU processing times, and/or the requirement of high-dynamic machine tools. Elliptical trochoidal tool path showed its capability to improve the material removal rate, but the expected low surface quality of the produced slot. In this paper, the elliptical tool path will be analyzed analytically and experimentally. The effect of elliptical tool path on material removal rate, cutting forces, walls waviness, and surface roughness have been investigated. A novel linear elliptical-based tool path has been proposed to enhance the elliptical tool path performance by improving both material removal rate and surface quality. A Performance index has been utilized to evaluate the performance of these processes. An analytical model for the local maximum chip thickness in elliptical tool path has been conducted to correlate this path with the cutting forces. The slot walls waviness has been modeled based on the cutting tool path geometry and validated experimentally with error less than 11%. The experimental results showed that the elliptical tool path improved the material removal rate by 11%, with a slight reduction in the maximum resultant cutting force by 3%. On the other hand, the walls waviness and surface roughness have been increased by 45% and 38% respectively as compared to the typical true trochoidal tool path. The introduction of linear paths at the elliptical path sides in the linear-elliptical tool path improved the performance index about 18 times by improving the material removal rate, waviness, and surface quality by 5%, 300%, and 74% respectively, without a significant effect on cutting forces. Therefore, the novel linear-elliptical paths are promising tool paths in increasing the performance of trochoidal milling due to the improved material removal rate, walls waviness, and surface quality, without a significant effect on cutting forces, which will improve the process machinability and cost-effectiveness.

EFFECTS OF VIBRATION CONDITIONS ON MACHINABILITY OF AISI 52100 BEARING STEEL

This study focused on determining the effects of the cooling condition in the conventional and ultrasonic-assisted turning of AISI 52100 steel. AISI 52100 steel has been widely used in the bearing, mold, and automotive industries because of its superior wear resistance and mechanical properties. However, its high wear resistance caused poor machinability in conventional machining methods. Low surface quality, rapid tool wear, high cutting temperature, and high cutting force are the main difficulties in the conventional machining of AISI 52100 steel. On the other hand, ultrasonic-assisted machining has become widely used in the past decades for the machining of materials that have low machinability in conventional methods. Conventional turning and ultrasonic-assisted turning were selected as machining methods. Ultrasonic-assisted turning experiments were conducted under 20 and 30 kHz vibration frequencies. Cubic Boron Nitride (CBN) cutting tools were used in machining experiments. In machining operations 50, 100, and 150 m/min cutting speeds were selected, and feed rate (0.1 mm/rev) and depth of cut (0.5 mm) were kept constant in experiments. Minimum quantity lubrication technique which used 1% Al2O3 nanoparticle additives cutting fluid and dry cooling methods were used. CBN insert was used as a cutting tool. The effects of cutting speed, cutting method, and cooling condition on surface quality, cutting force, cutting zone temperature, and cutting tool wear were studied experimentally. Analysis of Variance (ANOVA) was carried out to determine the significance of the effects of input variables on process outputs. As a result of this study, it is determined that combined UAT and nanoMQL have significant effects on the process outputs.

Experimental study on ultrasonic vibration assisted grinding of GCr15SiMn bearing steel

Many problems of GCr15SiMn high-carbon chromium bearing steel have been found in the ordinary grinding (OG) process. Aiming at the problems of high grinding force, low residual compressive stress and low surface quality, ultrasonic machining technology is combined with OG. The test of ultrasonic vibration assisted grinding (UVAG) GCr15SiMn high carbon chromium bearing steel is carried out. The residual stress, surface roughness, grinding force, and surface morphology are analyzed by changing machining parameters. The research showed that as the grinding depth and feed speed become larger, the grinding force increased, when the spindle speed rises, the grinding force becomes smaller. In the test range, with the ultrasonic amplitude from small to large, the grinding force from large to small. The ultrasonic grinding force is 27.8% lower than that of OG. The surface roughness increases with increasing grinding depth and feed speed and decreases with increasing spindle speed and ultrasonic amplitude, and at an amplitude of 6 μm, the surface roughness is reduced by 27% compared to OG. When the feed speed and grinding depth are lowered or the ultrasonic amplitude is increased, the surface residual compressive stress is increased. Compared to OG, it is increased by more than 13.5% under the same parameters. At the same time, the phenomenon of burrs and surface tears can be reduced, and the surface quality of the workpiece becomes better.

A comparative study of limit diagram of ultrasonic-assisted and conventional hot incremental forming of Ti-6AL-4V alloy

Electric hot incremental forming (EHIF) technique is a novel sheet metal forming method for low-formability materials like Mg and Ti-6Al-4V alloys. There are some defects in EHIF including low surface quality, unexpected sheet tear, and oxidation of the fabricated workpieces. These defects limit its widespread use. In this article, longitudinal ultrasonic vibrations of forming tools are used for Ti-6Al-4V sheets in the EHIF process. This technique led to a significant reduction of friction coefficient and improvement of workpieces’ surface quality. Also, a combination of mechanical vibrations and heat reduced the phase transformation temperature of α phase (HCP structure) to the more formable β phase (BBC structure) at 600 °C. It improved Ti-6Al-4V sheets’ formability. In this study, various tensile and compressive strains were created by manufacturing different geometries such as cone, frustum, torispherical, and five-lobe. These strains are used to compare the forming limit diagram of the ultrasonic-assisted and conventional hot incremental forming. The design of the experiment and analysis of variance were employed to identify the optimum condition for creating the largest strains. Also, effective parameters were optimized by response surface methodology. Then major and minor strains for both techniques were compared. Achieved strains in all types of geometries for ultrasonic-assisted technique were higher than those achieved by a conventional process.

The effects of scratching speed in ultrasonic vibration-assisted single diamond scratching process

Ultrasonic vibration-assisted machining has been widely used in manufacturing hard and brittle materials. Previous studies have shown that the vertical ultrasonic vibration in ultrasound-assisted single diamond scratching can affect the scratching forces, surface quality, and material removal mechanism by two mechanisms: changing the contact mode between the single diamond and the workpiece and increasing the speed of the single diamond relative to the workpiece. However, some studies point out that increasing the speed has little effect on the scratching force and material removal mechanism. Therefore, it is necessary to conduct an investigation that takes into account the effects of both scratching velocity and ultrasonic vibration on scratching force, surface quality, and material removal mechanisms. In this study, the single diamond conventional scratching (CS) and ultrasonic vibration-assisted scratching (UAS) experiments have been conducted at different scratching speeds to investigate the material removal mechanism of ultrasonic vibration-assisted machining. The effects of increasing the scratching speed on cutting forces, material removal mechanism, and machined surface quality are investigated. The results show that in both CS and UAS processes with low to medium scratching speeds (less than1000 mm/s), increasing the scratching speed leads to more brittle material removal and lower surface quality. At the same time, the scratching speed has almost no effect on the scratching speed force.

Effect of post-process treatments on mechanical properties and surface characteristics of 3D printed short glass fiber reinforced PLA/TPU using the FDM process

The extrusion-based fused deposition modeling (FDM) has become the most frequently used additive manufacturing method because of its shorten production time and cost. However, low surface quality remains a limitation to meet the regulatory standards set by traditional manufacturing, which requires post-processing to improve it. In this work, annealing and acetone vapor treatments are applied to short glass fiber reinforced PLA/TPU composites parts obtained by FDM, respectively. Their effects on the surface finishing, dimensional accuracy and mechanical properties of FDM parts using different post-processing methods are compared. The result shows that the annealing treatment dramatically improves the surface finish, but can reduce the dimensional accuracy and ductility of the FDM parts. Better ductility and dimensional accuracy but lower tensile strength are obtained by using acetone vapor treatment.

Ultrasonic assisted electrochemical discharge milling of complex glass microstructure with high-quality

IntroductionWith the rapid development of MEMS, the demand for non-conductive hard and brittle material microstructure is increasing. Electrochemical Discharge Machining (ECDM) is one of the most effective methods to machine non-conductive materials like glass and ceramics. However, in the milling process, the problem of discharge pits of different sizes and large thermal cracks caused by excessive discharge energy and heterogeneous energy distribution leads to low machining accuracy, poor stability and surface quality. ObjectivesIn order to solve the above dilemmas and improve the machining quality and stability, a newly developed micro Ultrasonic Assisted Electrochemical Discharge Milling (UAECDM) method is proposed in this paper. MethodsFirstly, a mathematical model is established to reveal the material erosion mechanism in the machining process. Secondly, the influence of ultrasonic vibration on gas film formation and discharge energy is analyzed by theory and simulation. Finally, the effects of ultrasonic amplitude and machining parameters on the quality of microchannels are experimentally studied and optimized by single factor optimization. The machining technology of UAECDM is creatively proposed. ResultsThe experimental results show that adding ultrasonic assistance and using the optimized machining parameters can refine the film thickness, optimize the energy distribution and stabilize the current signal. With assistance of ultrasonic vibration and optimized parameters, a smooth micro channel with width of 118.9 μm was fabricated, and its surface roughness was 0.23 μm, which was 80.99 % declined from 1.21 μm of ECDM. The successful fabrication of microchannel array, two-dimensional and three-dimensional complex glass micro-structures with surface roughness of about 0.2 μm also proves the excellent machining ability of UAECDM. ConclusionIt is proved that UAECDM can effectively improve machining accuracy, stability and surface quality. UAECDM is a desirable method to fabricate micro-channel on glass and will have a very broad industrial application prospect in the future.

Influence mechanism of laser delay on internal defect and surface quality in stitching region of 316L stainless steel fabricated by dual-laser selective laser melting

Multi-laser stitching additive manufacturing technology is an effective solution to break through the size limitation in single-laser additive manufacturing based on a powder-bed process, where excellent metallurgical bonding and high surface quality of the multi-laser stitching region (overlap) are the critical issues to ensure the final forming quality. The above metallurgical bonding and surface quality are not only affected by common process parameters but also sensitive to laser delay (including on and off delay) parameters because they influenced the starting and end regions of melt tracks. In this study, the influence of laser delay parameters on the forming quality of a single 316L stainless steel melt track was first investigated, thereby obtaining an optimization range of laser delay parameters. Then, the influence mechanism of laser delay on the internal pore defects and surface quality in the dual-laser stitching region was systematically studied. The results show that with the increase of laser on delay, there are four kinds of internal defects in the stitched region, namely keyhole, irregular pore, large-size continuous pore and non-overlap. In addition, due to the high energy density at the starting/end point of the melt tracks, powder denudation and solidified bulge were observed near the stitching line, which leads to the undulation problem with layer-by-layer accumulation on the surface in the stitching region. It is found that the surface undulation with a height of 202 μm and a width of 723 μm still exists at the optimized laser delay parameters (porosity of 0.96 %), which means new process strategies are also needed to be developed besides optimization of laser on/off delay parameters. In addition, a trade-off should be made between the low porosity and high surface quality due to the different influence trends of laser delay on them. This study provides a theoretical basis and technical support for reducing internal defects and improving surface quality in the stitching region fabricated by multi-laser selective laser melting (SLM).