2.1 Double-layer optimized printing of AlSi10Mg alloy SLM forming curved surface samples
In this paper, AlSi10Mg alloy powder with a particle size range of 23-25μm provided by Solutions group is used for SLM forming. The SEM and EDS pictures of the powder are shown in Fig 1, and the powder element composition table is shown in Table 1.
Table 1. AlSi10Mg alloy powder element composition table
Element
|
Weight %
|
Atomic %
|
Net Int
|
Mg
|
1.10
|
1.30
|
46.25
|
Al
|
88.78
|
89.22
|
3648.12
|
Si
|
10.12
|
9.48
|
155.64
|
Table 2. AlSi10Mg alloy SLM forming parameter table
parameters
|
Laser power
|
Scanning speed
|
thickness of the layer
|
Scanning interval
|
Spot size
|
Scanning way
|
The outer layer
|
250W
|
2200mm/s
|
75µm
|
100µm
|
80µm
|
67°
|
The inner layer
|
250W
|
1650mm/s
|
95µm
|
100µm
|
80µm
|
67°
|
The SLM®125HL selective laser melting forming equipment of German SLM Solutions 3D printing company is used to form AlSi10Mg alloy powder for SLM forming. The forming model refers to the aero engine blade structure for modeling [4-6]. The finished product is shown in Fig 2 (e, d). As shown, the forming parameters of the sample are set in layers inside and outside. As shown in Table 2, the outer scanning parameters can increase the surface hardness of the sample while ensuring a low surface roughness, and the internal scanning parameters can improve the compactness of the sample [2].
Fig. 2 (b, c) shows the on-site diagram of the finishing sample forming, and the physical diagram of the sample is shown in Fig. 3. Wire EDM is used to separate the sample from the substrate, and then the sample is supported by an ultrasonic cleaner Because the magnetic finishing technology selected in this article is high-precision finishing finishing, the original shape of the forming needs to be retained. In order to ensure the accuracy of the later test results, it is selected to retain the residual support sintering point of the sample.
2.2 Detection and analysis of the original morphology of the AlSi10Mg alloy SLM forming surface sample
The surface roughness and surface morphology of the AlSi10Mg alloy SLM curved sample were detected and analyzed by a metallurgical microscope and a white light interferometer. The detection results are shown in Fig 4. The surface of the unground sample is rough due to the particularity of its forming technology, Ra=2.12μm. In addition, due to uneven powder spreading, unbalanced laser density and laser power during forming, unmelted or overmelted AlSi10Mg powder will inevitably exist in the molten pool of the sample. After ultrasonic cleaning, the unmelted powder leaves holes on the surface of the sample, while the overmelted powder will form hemispherical protrusions, and the two combine to form a ravine and vertical microscopic surface morphology.
Vickers hardness tester is used to test the surface hardness of the AlSi10Mg alloy SLM curved surface sample. As shown in Fig 5, the hardness measurement pressure is F=4.9N. After pressing it for 15s, the test diamond appears as shown in Fig 5(c). The original surface hardness of the piece is 126.7HV0.5.
In this paper, a contact angle measuring instrument (OCA15EC) is used to test the deionized water contact angle of the AlSi10Mg alloy SLM surface sample as shown in Fig 6, and the inspection result is shown in Fig 6(c). Therefore, deionized water is divided by a stepped structure and crisscrossed surfaces and cannot exist in the form of large droplets. The contact angle of deionized water is 1.6°, which has strong hydrophilicity.
2.3 Design of permanent magnet finishing device for magnetic finishing of AlSi10Mg alloy SLM shaped surface samples
Magnetic finishing of curved parts needs to increase the finishing gap to ensure flexible finishing characteristics, so it is necessary to increase the magnetic field strength of the magnetic poles. At present, the method of increasing the magnetization of permanent magnetic poles is mainly to slot the finishing surface [7-8]. Before slotting, the overall size of the finishing magnetic pole needs to be calculated to ensure that the machining gap is 0~2.5 mm to provide more than 1T Magnetic field force.
Fig 7 shows a schematic diagram of the magnetic pole finishing of the magnetic finishing ball head. The material for the finishing magnetic pole selected in this study is a rare earth sintered permanent magnet material, and the parameter performance is shown in Table 3.
Table 3. Rare earth sintered permanent magnet materials
Remanence
Br(T)
|
Magnetic energy product (BH)max(kJ/m3 )
|
Coercivity
Hc( kA/m)
|
Density
ρ(g/m3)
|
Operating temperature
Tc(℃)
|
1.1~1.2
|
260~280
|
≥860
|
7.45
|
≤80
|
According to the principle of magnetic flux continuity and the Ampere’s loop theorem [9], (1-1) and (1-2) are derived, where Lt and rt represent the height and radius of the spherical magnetic pole shown in Fig 7, according to the rare earths shown in Table 2. The performance parameters of the sintered permanent magnet material are finally calculated as Lt=13 mm and rt=12.5mm.
According to the above dimensions, in order to explore the influence of different slot shapes on the magnetic field strength of the finishing magnetic poles, this paper designs a total of 6 types of slot simulation models with different shapes, as shown in Fig 8, and the angularity of the various magnetic pole slots is 30° The angle span is decreasing, and 180° is a non-grooving shape. The main parameter setting of the simulation is to use the magnetic field and the steady-state three-dimensional physical field without current for simulation calculation. The excitation source selects the magnetic field applied by the permanent magnet in the axial direction. The values of residual magnetic induction and coercivity are respectively Br=1T and Hc=860 k A/m.
The relationship curve in Fig 9 shows that the magnetic field strength is inversely proportional to the edge angle of the magnetic pole slot. The smaller the angle, the higher the magnetic field strength. When the angle reaches 30°, the magnetic field strength reaches 1.4T. But at the same time, it can be found from Fig 8 that the areas with higher magnetic field strength are mainly concentrated at the sharp points, which are not suitable for the design requirements of magnetic abrasive tools. However, according to the relationship between the magnetic field strength and the slotted edge angle, this experiment designed a 75° trapezoidal slotted magnetic pole with a higher magnetic field strength and a larger distribution area. Including 75°trapezoidal slotted magnetic poles on cylindrical surface and 75°trapezoidal slotted poles on spherical surface. The ratio of L:H is 2:1. The design dimensions and simulation results are shown in Fig 10 and Fig 11.
Through the simulation results shown in Fig 10 (a, b, c) and Fig 11 (a, b, c), it can be seen that the magnetic field strength of the slotted surface of the two types of 75° trapezoidal slotted magnetic poles can reach 1.2 T, The area with higher magnetic field strength is larger. Fig. 10(e) and Fig. 11(e) show the actual picture of the ground magnetic pole obtained by compacting the NdFeB rare earth material for high-temperature sintering and magnetizing for 1T longitudinally according to the design size. Fig. 10(f) and Fig. 11(f) are the effect diagrams of the magnetic poles adsorbing the magnetic abrasive. It can be seen that the magnetic abrasive is evenly distributed on the boss along the grooved corners.
In order to test the magnetic field strength of the actual magnetic poles, a surface magnetic tester is selected to detect the magnetic field strength of the magnetic pole surface. Since the magnetic finishing gap is maximum 2.5mm, the detection interval is 3mm. The test results are shown in Fig 13. It can be seen that the designed two types of magnetic poles can reach more than 1.1T within the range of 3mm, which can meet the requirements of flexible processing of magnetic finishing.
2.4 Finishing test and processing of magnetic abrasive AlSi10Mg alloy SLM forming surface sample
Table 4.Abrasive movement state classification table
In this paper, the spherical magnetic abrasive prepared by the atomization quick-setting method [3] is used. The abrasive phase is Al2O3 ceramic material, and the size of the abrasive phase is W7. Fig 14 is the scanning electron microscope image of the spherical Al2O3 magnetic abrasive. It can be seen that the iron matrix and the The abrasive grain phase has good wettability, and the abrasive grain phase can be firmly adhered to the iron matrix. At the same time, the abrasive sphericity is high, which is conducive to free flow during finishing, thereby ensuring flexible finishing [10-11].
Multiple papers verified [12-14] that there are three types of finishing states in magnetic finishing, namely, split plow, rolling shear and air running, as shown in Table 3. When the finishing gap is large, the air running phenomenon will appear, as shown in Table 4 (3). When the finishing gap cannot enable the abrasive to be processed flexibly during the finishing process, the tipping plough state shown in Table 4 (1) will appear. Only when the finishing gap is suitable can the rolling shear soft finishing state appear as shown in Table 4 (2).
The sample to be polished in this experiment should not only select a suitable finishing gap, but also consider the surface characteristics of the sample. Therefore, it is designed to use cylindrical 75° trapezoidal slotted magnetic poles with higher magnetic field strength to adsorb magnetic abrasives for the initial finishing of the workpiece, and set a higher finishing gap to avoid the interference of the surface features of the sample, so as to remove the bulk of the surface of the sample. The crusted part left by sintering completely AlSi10Mg alloy. Then, the spherical 75° slotted magnetic pole is used to absorb the magnetic abrasive, and the innate advantage that the magnetic pole does not interfere with the curved sample is used to carry out the perfect lamination and finishing of the sample. The schematic diagram of the finishing is shown in Fig. 15 and Fig. 16.
At the same time, this experiment uses a 3-level 4-factor orthogonal test table to carry out 27 sets of finishing experiments. Each group includes two parts: initial finishing and re-finishing, and empirically optimized finishing for important finishing parameters such as Spindle speed, Feed rate, gap and Abrasive consumption. Fig 17 is a diagram of the finishing site, and Table 4 is a table of finishing parameters.
Table 4.3 Distribution table of 3 level-4 factor orthogonal test
Level
|
Spindle speed
|
Feed rate
|
gap
|
Abrasive consumption
|
A(s)
|
B(f)
|
C(h)
|
D(g)
|
1
|
1800 r/min
|
12 mm/min
|
2.5 mm
|
10 g
|
2
|
1500 r/min
|
8 mm/min
|
2 mm
|
7 g
|
3
|
1200 r/min
|
5 mm/min
|
1.5 mm
|
3.5 g
|
Table 5. Summary Table of Magnetic Finishing Effects of AlSi10Mg Alloy SLM Formed Curved Surface Samples
Orthogonal interactive test table
|
Num
-ber
|
A
|
B
|
A×B
|
C
|
A×C
|
B×C
|
D
|
Ra
/μm
|
1
|
s1
|
f1
|
s1 f1
|
h1
|
s1 h1
|
f1 h1
|
g1
|
0.302
|
2
|
s1
|
f1
|
s1 f1
|
h2
|
s2 h2
|
f2 h2
|
g2
|
0.312
|
3
|
s1
|
f1
|
s1 f1
|
h3
|
s3 h3
|
f3 h3
|
g3
|
0.355
|
4
|
s1
|
f2
|
s2 f2
|
h1
|
s1 h1
|
f2 h3
|
g3
|
0.368
|
5
|
s1
|
f2
|
s2 f2
|
h2
|
s2 h2
|
f3 h1
|
g1
|
0.362
|
6
|
s1
|
f2
|
s2 f2
|
h3
|
s3 h3
|
f1 h2
|
g2
|
0.336
|
7
|
s1
|
f3
|
s3 f3
|
h1
|
s1 h1
|
f3 h2
|
g2
|
0.282
|
8
|
s1
|
f3
|
s3 f3
|
h2
|
s2 h2
|
f1 h3
|
g3
|
0.291
|
9
|
s1
|
f3
|
s3 f3
|
h3
|
s3 h3
|
f2 h1
|
g1
|
0.282
|
10
|
s2
|
f1
|
s2 f3
|
h1
|
s2 h3
|
f1 h1
|
g2
|
0.459
|
11
|
s2
|
f1
|
s2 f3
|
h2
|
s3 h1
|
f2 h2
|
g3
|
0.428
|
12
|
s2
|
f1
|
s2 f3
|
h3
|
s1 h2
|
f3 h3
|
g1
|
0.413
|
13
|
s2
|
f2
|
s3 f1
|
h1
|
s2 h3
|
f2 h3
|
g1
|
0.411
|
14
|
s2
|
f2
|
s3 f1
|
h2
|
s3 h1
|
f3 h1
|
g2
|
0.396
|
15
|
s2
|
f2
|
s3 f1
|
h3
|
s1 h2
|
f1 h2
|
g3
|
0.403
|
16
|
s2
|
f3
|
s1 f2
|
h1
|
s2 h3
|
f3 h2
|
g3
|
0.413
|
17
|
s2
|
f3
|
s1 f2
|
h2
|
s3 h1
|
f1 h3
|
g1
|
0.408
|
18
|
s2
|
f3
|
s1 f2
|
h3
|
s1 h2
|
f2 h1
|
g2
|
0.399
|
19
|
s3
|
f1
|
s3 f2
|
h1
|
s3 h2
|
f1 h1
|
g3
|
0.486
|
20
|
s3
|
f1
|
s3 f2
|
h2
|
s1 h3
|
f2 h2
|
g1
|
0.452
|
21
|
s3
|
f1
|
s3 f2
|
h3
|
s2 h1
|
f3 h3
|
g2
|
0.467
|
22
|
s3
|
f2
|
s1 f3
|
h1
|
s3 h2
|
f2 h3
|
g2
|
0.428
|
23
|
s3
|
f2
|
s1 f3
|
h2
|
s1 h3
|
f3 h1
|
g3
|
0.426
|
24
|
s3
|
f2
|
s1 f3
|
h3
|
s2 h1
|
f1 h2
|
g1
|
0.482
|
25
|
s3
|
f3
|
s2 f1
|
h1
|
s3 h2
|
f3 h2
|
g1
|
0.472
|
26
|
s3
|
f3
|
s2 f1
|
h2
|
s1 h3
|
f1 h3
|
g2
|
0.433
|
27
|
s3
|
f3
|
s2 f1
|
h3
|
s2 h1
|
f2 h1
|
g3
|
0.466
|
I
|
2.887
|
3.674
|
3.525 3.55
|
3.618
|
3.475 3.596
|
3.600 3.578
|
3.584
|
|
II
|
3.73
|
3.612
|
3.737 3.691
|
3.508
|
3.663 3.566
|
3.546 3.577
|
3.509
|
|
III
|
4.112
|
3.443
|
3.467 3.488
|
3.603
|
3.591 3.567
|
3.583 3.574
|
3.636
|
|
Ki
|
3
|
3
|
3
|
3
|
3
|
3
|
3
|
|
Ⅰ⁄k1
|
0.962
|
1.224
|
1.175 1.183
|
1.206
|
1.158 1.198
|
1.200 1.193
|
1.194
|
|
Ⅱ⁄k2
|
1.243
|
1.204
|
1.245 1.230
|
1.169
|
1.221 1.188
|
1.182 1.192
|
1.169
|
|
III⁄k3
|
1.370
|
1.147
|
1.155 1.162
|
1.201
|
1.197 1.189
|
1.194 1.191
|
1.212
|
|
极差
|
0.281
|
0.077
|
0.090 0.068
|
0.037
|
0.063 0.010
|
0.018 0.002
|
0.043
|
|
Table 5 is a summary table of the finishing effect, taking into account the interaction between various factors, the results of the three sets of interaction tests between (A×B), (A×C) and (B×C). By analyzing the orthogonal test table, the best combination of finishing parameters can be calculated, which are s1, f3, h2 and g2, and the corresponding finishing parameter values are 1800 r/min, 5 mm/min, 2 mm and 7g. The range distribution of each factor is A>B>D>C. It can be seen from Fig 18 that the influence ratio of the amount of abrasive is greater than the influence ratio of the finishing gap. For the magnetic finishing of the AlSi10Mg alloy SLM surface, the finishing state is affected. The primary factor is the amount of abrasive filling, too much abrasive filling will affect the flexibility characteristics of magnetic finishing.