A vibration treatment of holes obtained by selective laser melting

. The article investigated the vibration treatment of blind holes obtained by selective laser melting. The object of the investigation was a printed stainless steel specimen with holes of various diameters. The experimental results showed that the most effective treatment occurs when the hole diameter exceeds the size of the abrasive body by 1-2 mm, but does not exceed it by 2 times. It was also noted that the rounding of the edge of the hole with a size not exceeding the nominal value of the abrasive occurs faster than that of a hole with a larger size. The time of rough processing of stainless steel products should not exceed 4 hours, while the most effective processing occurs in the first 2-3 hours. The results obtained allow us to give recommendations on the processing of printed parts with blind holes made by selective laser melting.


Introduction
One of the main tasks of modern mechanical engineering is to increase labor productivity and product quality.One of the ways to solve this problem is the use of advanced methods of metalwork processing of parts based on process automation.It is especially important to find a solution to the above problem for modern manufacturing technologies, in particular for selective laser melting.Currently, parts obtained using SLM technology, as a rule, have a sufficiently large surface roughness up to Ra = 15 μm, which often does not satisfy the design requirements imposed on them [1].
One of the effective ways to improve the quality of the surface layer is vibration treatment, which was considered in this study on the example of hole processing [2][3][4].The process of vibration treatment is as follows: a special drive creates vibrations that drive the chamber in which the free abrasive (rolling bodies) and the processed parts are filled in.These abrasive materials begin to affect the surface of the product, exposing it to friction and numerous micro-impacts, which contributes to its improvement of roughness.This method allows you to carry out a wide range of work, such as removing a defective layer, scale, rust, burrs, rounding sharp edges.With the use of this technology, the quality of finished products is improved by polishing their surfaces and bringing them to the required quality [5].
The method of vibration treatment has a number of advantages:  Uniform treatment: vibration movement allows to achieve uniform distribution of the abrasive, which ensures uniform treatment of all exposed surfaces;  High efficiency: the vibration treatment can be automated and process a large batch of parts in one cycle, which reduces production time;  Versatility: the vibration treatment is suitable for processing various materials, including metals, plastics, ceramics, etc.;  Cost-effectiveness: vibration treatment can significantly reduce the cost of surface treatment compared to traditional methods.In turn, the possibilities of selective laser melting are very large, and the resulting parts can have various complex shapes and hard-to-reach places.Such structural elements will not in all cases be able to be processed in a vibration treatment, therefore it is necessary to study the possibilities of this type of processing as much as possible and determine the limitations that arise in order to further predict the results already in relation to real products.
The aim of the work was to study the possibility of vibratory processing of holes of various diameters of a stainless steel specimen made by the SLM technology, as well as the process of rounding the edges of holes and monitoring their roughness.

Description of the experiment
The object of the investigation was a specimen made on a 3DLAM Mid selective laser melting unit with six blind holes with diameters of 8 mm, 10 mm, 11 mm, 12 mm, 14 mm and 20 mm (Figure 1).Before the experiment, it was cut into two halves by electroerosion cutting in order to at different time intervals of processing, it was possible to control the roughness of the holes and the rounding of their sharp edges.Before processing, the two halves of the specimen were connected using bolts and nuts.

Fig. 1. A manufactured specimen with holes of various diameters
The specimen is made of 316L spherical powder (stainless steel).The powder has a high frequency and medium fraction, which is ideal for the production of parts with thick walls and smooth surfaces [6].As can be seen in Figure 1, various types of defects are present on the specimen after printing: vapors, remelts, non-melts, stuck metal powders.Most of them can be avoided by selecting optimal melting modes, conducting morphological analysis of the powder, conducting its input control and correctly dividing the geometry of the part.But even with this, there will be undulation of the surface and high roughness, which will not always correspond to the required [7].In this specimen, we are only interested in hole profiles, the print quality of which is close to optimal, with the exception of sticking in the form of powders.
Due to the particularity of the technology, when printing sharp edges by selective laser melting, a small radius of rounding of the sharp edges appears.On this specimen, the radius of the face of the holes after printing is approximately equal to R0.1 mm.The fillets were measured using an AmScope ME 1400TC-INF microscope, and then processed in the KOMPAS-3D program by applying a ruler to an enlarged image of the edge and its further measurement.Then, using the ISHP-210 profilometer, the roughness of the holes after printing was determined.The measurement results are listed in Table 2. To carry out this investigation, an AVALON WR60 mini rotary vibrating machine was used, designed for grinding and polishing various parts.Using the recommendations obtained for flat specimens, a new plan was drawn up in which the specimen was processed in the same modes and after each half hour the roughness of its holes was monitored.At the same time, every hour of processing, the rounding of their edges was monitored.The structural scheme with the main elements of this installation is shown in Figure 2.These modes were obtained based on the processing of titanium alloy specimens, but most likely they are also relevant for stainless steel, except for the processing time, which will take less to achieve maximum results [8].

Results of the experiment
After the first 30 minutes, it was noticed that abrasive bodies get stuck in the cavities of the holes Ø10 and Ø20, due to which the efficiency of the process decreases Figure 3.As can be seen from the results, the improvement of the roughness of blind holes is not as effective as on flat samples.But still, the dynamics of the reduction of some holes is much higher compared to others.So, for example, Ø11 and Ø12 are handled better than the others.This is due to the fact that one abrasive body with a size of 10 × 10 mm can penetrate into holes of a close, but not nominal diameter without getting stuck, and at the same time interact intensively with the walls, improving their roughness.
Holes Ø14 and Ø20 are processed less efficiently, which is explained by a smaller contact surface during processing.Also, two abrasives can be placed in the hole Ø20, which leads to their jamming, and after the end of processing, it becomes necessary to extract these bodies, which is an additional and time-consuming operation.
The processing of the hole Ø10, the size of which coincides with the size of the abrasive bodies, had no effect for the first 3 hours and its roughness remained unchanged.After this time, due to mutual friction, the rolling bodies lost their nominal value and began to penetrate into this hole, as a result, a change in roughness began, which can be seen on the graph.But still, this process cannot be called effective, and it should be taken into account when selecting bodies.
As expected, the hole Ø8 was not processed and in this study took place only to control the rounding of the sharp edge.
The graph shows that the processing efficiency decreases over time, and after 4 hours it begins to show negative dynamics at all.As expected before the start of the experiment, to achieve the maximum possible result of the primary processing of a stainless steel product, it will take less time than for titanium, which was confirmed by the obtained results.After analyzing the rounding graph, it can be concluded that smaller diameter holes with low throughput or lack thereof are rounded noticeably faster than others.The sharp edge of the holes Ø8 and Ø10 reached rounding R0.21 mm and R0.2 mm, respectively, while the values of the others did not exceed R0.17 mm.As in the previous graph, the greatest efficiency of the rounding process is observed in the first hours of processing, which indicates the need to reduce the duration of the process.

Investigation findings
Based on the conducted experiment, the following conclusions can be drawn and certain recommendations can be given: 1.The product after printing should not have gross defects in the form of remelts, nonmelts, stuck powders, since some of them cannot be eliminated by vibration treatment.
In the presence of gross defects after printing, it is recommended to carry out sandblasting or waterjet treatment.2. For effective processing of blind holes, their diameter should exceed the size of the abrasive body by 1-2 mm, but not exceed it by 2 times, since in this case the bodies may get stuck in the treated cavity.3. The rounding of the edge of the hole with a size not exceeding the nominal value of the abrasive body occurs faster than that of the hole with a larger size.At the same time, the greater this difference, the less the intensity of the rounding of its face.4. The time of rough processing of products made from powder 316L should not exceed 4 hours, so after this time, there is a negative dynamics of roughness changes.The most effective treatment occurs in the first 2-3 hours.

Fig. 2 .
Fig. 2. Scheme of the WR60 mini vibrating machine: 1 -Control stand; 2 -Metering pump; 3 -Auxiliary liquid tank; 4 -Liquid drain; 5 -Frame; 6 -Working chamber; 7 -Water supply nozzle; 8 -Vibratory drive Processing was performed in the following modes: stage 1 -type of abrasive -cone, processing frequency -1650 rpm, processing time -2 hours, compound feed rate -100%; stage 2 -type of abrasive -cone, processing frequency -1950 rpm, processing time -4 hours, compound feed rate -20%.These modes were obtained based on the processing of titanium alloy specimens, but most likely they are also relevant for stainless steel, except for the processing time, which will take less to achieve maximum results[8].

E3SFig. 3 .
Fig. 3. Specimen after 30 minutes of processing Let's present the measurement results in the form of graphs to clearly show the change in the controlled parameters during processing (Figure 4, 5).

Fig. 4 .
Fig. 4. Graph of changes in the roughness of holes during processing

Fig. 5 .
Fig. 5.The graph of the rounding of holes' edges

Table 1 .
Chemical composition of the powder 316L

Table 2 .
The surface roughness of the holes before processing