Post-Processing Techniques to Enhance the Quality of Metallic Parts Produced by Additive Manufacturing

Additive manufacturing (AM) processes can produce three-dimensional (3D) near-net-shape parts based on computer-aided design (CAD) models. Compared to traditional manufacturing processes, AM processes can generate parts with intricate geometries, operational flexibility and reduced manufacturing time, thus saving time and money. On the other hand, AM processes face complex issues, including poor surface finish, unwanted microstructure phases, defects, wear tracks, reduced corrosion resistance and reduced fatigue life. These problems prevent AM parts from real-time operational applications. Post-processing techniques, including laser shock peening, laser polishing, conventional machining methods and thermal processes, are usually applied to resolve these issues. These processes have proved their capability to enhance the surface characteristics and physical and mechanical properties. In this study, various post-processing techniques and their implementations have been compiled. The effect of post-processing techniques on additively manufactured parts has been discussed. It was found that laser shock peening (LSP) can cause severe strain rate generation, especially in thinner components. LSP can control the surface regularities and local grain refinement, thus elevating the hardness value. Laser polishing (LP) can reduce surface roughness up to 95% and increase hardness, collectively, compared to the as-built parts. Conventional machining processes enhance surface quality; however, their influence on hardness has not been proved yet. Thermal post-processing techniques are applied to eliminate porosity up to 99.99%, increase corrosion resistance, and finally, the mechanical properties’ elevation. For future perspectives, to prescribe a particular post-processing technique for specific defects, standardization is necessary. This study provides a detailed overview of the post-processing techniques applied to enhance the mechanical and physical properties of AM-ed parts. A particular method can be chosen based on one’s requirements.

Numerical Simulation of the Influence of Different Surface Morphologies on Molten Pool Flow under Moving Heat Source

In order to study the influence of different initial topography on the molten pool flow under a moving heat source, the finite element analysis method was used to establish a two-dimensional transient model of laser polishing to simulate the evolution of the surface topography of the material during laser polishing. In the simulation process, a moving laser beam was used as the heat source, and the free surface of the actual material was profiled through a three-dimensional profiler. A very similar simulation model surface was constructed, coupled with the flow field and temperature field in the laser polishing process, and the capillary force was considered comprehensively. Combined with thermocapillary force. The results show that under the combined action of capillary force and thermocapillary force, the surface of the polished material has a peak-filling effect, which makes the surface of the material achieve a good polishing effect. The initial shape will affect the polishing effect, the greater the curvature, the faster the flow rate of the molten pool. In molten pools with large spatial curvatures, capillary forces dominate. Keywords: Laser polishing; molten pool; surface topography; numerical analysis; capillary force; thermocapillary force.

Laser Polishing of Additive Manufactured Aluminium Parts by Modulated Laser Power

In this study a new approach to laser polishing with periodic modulated laser power in the kilohertz regime is introduced. By varying the modulation frequency and modulation time, different periodic laser power curves with varying minimum, peak and average laser power can be created. The feasibility of the method is shown by polishing of vertical built AlSi10Mg L-PBF parts with an initial roughness of Ra = 12.22 µm. One polishing pass revealed a decreasing surface roughness with increasing energy density on the surface up to Ra = 0.145 µm. An increasing energy density results in a rising remelting depth between 50 and 255 µm and a rising relative porosity of 0.3% to 4.6%. Furthermore, the thermal process stability, analysed by the melt pool length in scanning direction, reveals a steadily increasing melt pool dimension due to component heating. Multiple laser polishing passes offers a further reduced surface roughness, especially at higher modulation frequencies and provides an improved orientation independent roughness homogeneity. The process stability regarding varying initial surface roughness revealed an almost constant relative roughness reduction rate with an achievable roughness variation after two polishing passes between Ra = 0.13–0.26 µm from an initial state of Ra = 8.0−19.2 µm.

A comparative study on surface topography and micro hardness of laser polished hardened AISI D2 tool steel

In this work, a laser polishing-hardening (LPH) method with integration and high efficiency for the treatment of AISI D2 tool steel was proposed, and the effects of laser hardening (LH), laser polishing (LP) and LPH treatments on the surface topography and microhardness were examined. The results show that LH method had a negligible effect on the surface roughness of the treated sample, while the surface roughness Ra of LP and LPH specimens was reduced by 74.6% and 80.9% respectively, indicating that the milled surface topography had been significantly improved, especially LPH was more effective in reducing the roughness. Besides, the polishing efficiency of LPH was 10 times that of LP approach. In terms of hardness improvement, the near-surface microhardness of LH and LPH samples increased by 1.5 times and 1.3 times respectively, and the effective hardened zone (EHZ) depth was 0.42 mm and 0.24 mm respectively, demonstrating that these two laser processing methods had a beneficial effect on the cross-section microhardness of D2 tool steel, while the increase of LP on the microhardness was insignificant. The comprehensive analysis of the surface morphology and microhardness of LPH specimen indicates that LPH was a feasible laser surface treatment method for D2 tool steel. On the premise of ensuring a high surface finish, the polishing efficiency can be remarkably improved, the subsurface microhardness and EHZ depth of processed specimen can be also significantly enhanced, which provided a feasible idea for the application of laser surface treatment technology in industrial mold production.