CHAPTER 3 EXPERIMENTAL PROCEDURES
3.4 Optimization for RSW process
3.4.1 Initial optimization for RSW process
The high strength steel sheet was used in this study; its chemical composition is listed in Table 3-17. Plates 0.7 mm in thickness were cut into strips of size 30¯100 mm. The schematic diagram of high strength steel sheet specimen for resistant spot welding was shown in Fig.3-14. The resistance spot welder (FANUC α8/4000is type) had been utilized for the experiment is shown in Fig.3-15.
Table 3-17 Material used in RSW process (wt-%)
Material C Si Mn P S Fe
MJSC340D 0.062 0.48 0.95 0.013 0.004 Balance
Fig. 3-14 Schematic diagram of the specimens
Fig. 3-15 Resistance spot welder and prepared specimens
Control and noise factor of RSW process
By making reference to the existing parameter conditions in the production line, the range of experimental parameter value has been initially framed as below: welding current 6200 ~ 11000 A, welding time 8 ~ 26 cycles, electrode force 1.8 ~ 3.3 kN and the size of electrode tip φ3 ~ φ6 mm. The value of each welding process parameter at the different levels is listed in Table 3-18.
Surface condition of the welding area was selected as the noise factor in this study. The specimens at level one (N1), without any cleaning treatment, may have been tarnished with dirt and / or grease. The surface impurities were
removed and the surface cleaned with acetone at level two (N2). The initial conditions of production operation currently were welding current at 7800A, welding time at 8 cycles, electrode force at 1.8 kN and the size of electrode tip at φ4 mm.
Table 3-18 Control factors of RSW process
Factor Process parameter Level 1 Level 2 Level 3 Level 4
A The size of
electrode tip φ3 mm φ4 mm φ5 mm φ6 mm B Welding current 6200 A 7800 A 9400 A 11000 A
C Electrode force 1.8 kN 2.3 kN 2.8 kN 3.3 kN
D Welding time 8 cycles 14 cycles 20 cycles 26 cycles
Orthogonal array experiment
Four four-level control factors, in addition to one noise factor, were considered in this investigation. The interaction effect between the welding parameters was not considered. Therefore, there are 12 degrees of freedom owing to the 4 control factors. The degrees of freedom for the OA should be greater than or at least equal to those for the process parameters. L16 (45) OA that has 15 degrees of freedom was employed in this study. An experimental layout
with an inner array for control factors and an outer array for a two-level noise factor (N1 and N2) is shown in Table 3-19. Four repetitions (y1, y2, y3 and y4) for each trial are used with this experimental arrangement; y1 and y2 are N1 specimens (without cleaning); y3 and y4 are N2 specimens (cleaned with acetone). The Max. Load for tensile-shear test specimens are shown in Table 3-20.
Table 3-19 Experimental layout using an L16 orthogonal array
Control factor Noise factor
N1 specimens N2 specimens Trial no.
A B C D
y1 y2 y3 y4
1 1 1 1 1
2 1 2 2 2 3 1 3 3 3
. . . . .
15 4 3 2 4
16 4 4 1 3
Measure data
Table 3-20 Experiment data in RSW process
Control factors Max. Load Trial no.
A B C D Average
(kN) SNR (dB)
1 1 1 1 1 3.317 10.41
2 1 2 2 2 4.098 12.25
3 1 3 3 3 4.105 12.26
4 1 4 4 4 4.392 12.85
5 2 1 2 3 3.299 10.35
6 2 2 1 4 3.758 11.49
7 2 3 4 1 3.950 11.91
8 2 4 3 2 3.855 11.70
9 3 1 3 4 2.622 8.36
10 3 2 4 3 3.735 11.44
11 3 3 1 2 4.168 12.39
12 3 4 2 1 4.083 12.22
13 4 1 4 2 2.318 7.29
14 4 2 3 1 3.572 11.05
15 4 3 2 4 3.637 11.21
16 4 4 1 3 4.139 12.24
Total average of SNR for all trial ηˆ is 11.213 (dB)
Evaluation of initial optimal condition
The Max. Load of the specimens as discussed earlier belongs to the higher-is-better quality characteristic. The SNRs, which condense the multiple data points within a trial, depend on the three characteristics LB, NB and HB.
The equation for calculating the SNR for HB characteristic is
⎟⎟⎠
where n is the number of tests in a trial (number of repetitions regardless of noise levels). The value of n is 4 in this study. The SNRs corresponding to Max. Load value of each trial is shown in Table 3-21. The effect of each welding process parameter on the SNR at different levels can be separated out because the experimental design is orthogonal. The description of the SNR for each level of the welding process parameters is summarized in Table 3-21. Fig.3-16 shows the SNR graph obtained from Table 3-21. Basically, the larger is the SNR, the better the quality characteristic (tensile-shear strength) for the specimens. The initial optimal conditions of the RSW process parameter levels, A1B4C1D3, can be determined from Fig. 3-16.
Table 3-21 SNR response table for the Max. Load
Factor Process parameter Level 1 Level 2 Level 3 Level 4
A The size of
electrode tip 11.941 11.363 11.101 10.449
B Welding current 9.102 11.558 11.942 12.252
C Electrode force 11.634 11.507 10.842 10.871
D Welding time 11.399 10.905 11.571 10.979
Fig. 3-16 SNR graph for the Max. Load
Analysis of variance
When the contribution of a factor is small, as with factor D (welding time) in Table 3-22, the sum of squares for that factor is combined with the error. This process of disregarding the contribution of a selected factor and subsequently adjusting the contributions of the other factors is known as ‘Pooling’ [9]. The welding current and the size of electrode tip were the significant welding parameters in affecting the quality characteristic, with the welding current being the most significant, as indicated by Table 3-22.
Table 3-22 Results of ANOVA for the Max. Load
Factor Degree of
Mark * means the factors are treated as pooled error
Confirmation test and proper regulation
Refer to Table 3-21 and 3-22, estimated SNR ηopt is computed as
ηopt=11.213 + (11.941-11.213) + (12.252-11.213) =12.98 (dB)
With a CI of 95% for the tensile-shear strength, theF0.05;1;6 =5.99, and Vep=0.448, the sample size for the confirmation experiment r is 2, N =16,
DOFopt=9, and the effective sample size neff is 1.6. Thus, the CI is computed to be 1.738 (dB). The experimental results (Table 3-23) confirm that the initial optimizations of the RSW process parameters (Aφ3mmB11000AC1.8kND20cycles) were achieved.
Table 3-23 Confirmation experiment of RSW process
Max. Load
Although the conformity of reproducibility for the experimental results has been confirmed with an average Max. Load of specimens as high as up to 4.406 kN obtained; however, a phenomenon of spark taken place between the specimens and the electrode during the spot welding process that leads to a severely shortened life cycle of electrode and an collaterally affected joint
quality of weldment for its subsequent welding. With the ANOVA outcomes (Table 3-22) referenced, a proper regulation of welding current is necessary to cope with the foregoing defects. As learned from Fig. 3-16 (SNR graph), SNR thereof was slightly increased when welding current regulated from 7800A to 11000A, that is, the Max. Load of specimens was not heightened in big magnitude. Therefore, the optimal conditions of parameters obtained from the application of Taguchi Method remained unchanged except the welding current was regulated from 11000A to 7800A. Table 3-24 lists the results of experiment after adjusting the parameters (Aφ3mmB7800AC1.8kND20cycles).
Table 3-28 Results of the Taguchi method with proper regulation
Tensile-shear strength Trial no.
N1 specimens N2 specimens
Average
3.4.2 Real optimization for RSW process