Welded parts are common in mechanical engineering. As all manufactured parts, they also present residual stresses introduced by the corresponding manufacturing process. Residual stresses can be beneficial or not because they can increase or reduce the useful life of the mechanical components, particularly when they are subjected to a cyclic stress in which they can fail by fatigue. In this study, SAE 1045 steel samples were welded by metal inert gas process, varying the speed and welding current. The welded samples were thereafter milled, including the welded region. Residual stresses on material as received, welded, and welded and subsequently milled were evaluated through the microhardness method. A factorial statistical design was used, and the results were studied by analysis of variance. It can be concluded that, in general, welding introduces compressive residual stresses which are improved by posterior milling operation, and there is an optimal set of operating parameters for this condition.
Currently, it is necessary to understand and control the manufacturing processes and manufactured products to achieve higher efficiency and production quality as well as low-cost operations. The gas metal arc welding (GMAW) is a procedure considered as advantageous when compared to other welding processes due to production capacity, applicability, and the automation possibilities. Several parameters can influence a weld, which make adjustment procedure difficult [
Welding current is very important in the process and must be adequate to the wire speed feed [
Residual stress can be evaluated using indentation tests. This method compares the surface hardness of a material before and after manufacturing process. If the hardness is higher than the hardness of the material after processing, it indicates that a compressive residual stress is present on its surface. If lesser, then tensile residual stresses are present [
The experiment was done using a randomized factorial design. The influence variables were welding current, welding speed, the regions of the part, and two process conditions: parts only welded and parts welded and subsequently milled. The response variable was the microhardness, associated with residual stress. Welding was done in plates of 100 × 100 × 3.2 mm of SAE 1045 steel.
The plates were wet grinded and cut into two parts with a closed joint. An AWS ER-70S-6 electrode with a diameter of 0.8 mm was used. Protection gas used was constituted of 80% of argon and 20% of CO2. This gas mixture was used as recommended by Moyer [
After tests, three levels were chosen for welding current:
Three weld speeds were adopted:
Affected regions after welding.
A group of welded parts were milled to evaluate the residual stress in this condition to be compared with simple welded part. The parameters used in milling were depth of cut (
Measurement position (dimensions in mm).
An indentation test was done in the material as received to be compared with the parts welded and parts welded and milled. The results are shown in Table
Microhardness Vickers of the material as received by position of measurement.
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Replica 1 | 238 | 242 | 240 | 240 | 242 | 238 |
Replica 2 | 239 | 243 | 241 | 240 | 242 | 239 |
Replica 3 | 240 | 242 | 240 | 241 | 243 | 240 |
In Table
Microhardness of parts welded and parts welded and milled.
Parts welded | ||||||||||||||||||
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245 | 250 | 255 | 256 | 250 | 246 | 250 | 252 | 253 | 252 | 250 | 256 | 254 | 254 | 257 | 255 | 256 | 254 |
248 | 253 | 255 | 255 | 249 | 248 | 247 | 253 | 255 | 249 | 248 | 248 | 252 | 253 | 256 | 260 | 259 | 258 | |
247 | 253 | 256 | 256 | 250 | 249 | 250 | 252 | 254 | 255 | 250 | 248 | 251 | 253 | 258 | 258 | 256 | 253 | |
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248 | 251 | 254 | 254 | 249 | 247 | 249 | 249 | 254 | 253 | 251 | 250 | 259 | 261 | 260 | 257 | 252 | 250 |
251 | 253 | 257 | 257 | 248 | 252 | 250 | 249 | 252 | 254 | 254 | 253 | 253 | 260 | 255 | 254 | 250 | 252 | |
248 | 254 | 257 | 257 | 254 | 245 | 252 | 253 | 255 | 256 | 255 | 254 | 252 | 255 | 256 | 254 | 253 | 251 | |
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250 | 252 | 254 | 254 | 250 | 250 | 252 | 250 | 254 | 253 | 251 | 251 | 253 | 252 | 258 | 259 | 258 | 251 |
252 | 253 | 254 | 254 | 249 | 249 | 249 | 249 | 252 | 255 | 253 | 255 | 252 | 255 | 256 | 257 | 258 | 252 | |
249 | 250 | 252 | 252 | 250 | 248 | 253 | 255 | 257 | 254 | 255 | 250 | 255 | 258 | 260 | 259 | 258 | 255 | |
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Parts welded and milled | ||||||||||||||||||
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276 | 280 | 286 | 286 | 270 | 260 | 270 | 273 | 279 | 282 | 280 | 269 | 268 | 290 | 284 | 270 | 268 | 270 |
278 | 272 | 275 | 276 | 258 | 268 | 269 | 270 | 270 | 279 | 270 | 269 | 260 | 269 | 274 | 290 | 286 | 279 | |
261 | 269 | 270 | 280 | 271 | 266 | 262 | 270 | 275 | 277 | 269 | 263 | 267 | 271 | 280 | 274 | 270 | 273 | |
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255 | 264 | 268 | 271 | 277 | 275 | 275 | 276 | 280 | 282 | 281 | 271 | 273 | 279 | 273 | 275 | 268 | 270 |
254 | 257 | 264 | 270 | 269 | 269 | 272 | 272 | 275 | 280 | 278 | 270 | 276 | 280 | 274 | 285 | 280 | 280 | |
279 | 271 | 280 | 263 | 270 | 267 | 275 | 276 | 280 | 280 | 278 | 271 | 292 | 282 | 280 | 290 | 280 | 276 | |
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270 | 274 | 279 | 280 | 270 | 270 | 266 | 286 | 283 | 269 | 265 | 272 | 290 | 290 | 294 | 300 | 293 | 280 |
278 | 270 | 267 | 275 | 270 | 269 | 256 | 249 | 268 | 271 | 258 | 270 | 298 | 295 | 299 | 295 | 293 | 275 | |
270 | 271 | 273 | 275 | 274 | 270 | 246 | 270 | 278 | 265 | 260 | 270 | 290 | 284 | 295 | 299 | 293 | 280 |
An analysis of variance (ANOVA) with 95% of confidence was used. First, the results were analyzed for the parts only welded. Table
Analysis of variance of the welded parts.
SS | DF | MSS |
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Total | 1824.698 | 161 | — | — | — | — |
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12.308 | 2 | 6.15 | 1.503 | 0.227 | — |
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461.086 | 2 | 230.543 | 56.331 | 0.001 | Significant |
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538.623 | 5 | 107.725 | 26.321 | 0.001 | Significant |
VI | 30.246 | 4 | 7.561 | 1.847 | 0.125 | — |
VP | 46.876 | 10 | 4.687 | 1.145 | 0.336 | — |
IP | 106.765 | 10 | 10.67 | 2.608 | 0.007 | Significant |
VIP | 186.791 | 20 | 9.339 | 2.282 | 0.004 | Significant |
ERR | 442.000 | 108 | 4.092 | — | — | — |
From the ANOVA, it can be concluded that welding current and the measurement position are significant and influence the microhardness. The welding speed, however, proved to be not significant and does not influence the hardness of the parts.
The analysis of variance also showed an interaction between the welding current and the measurement position, and there was also interaction between the three variables. An orthogonal contrast test revealed that there is no difference between the results obtained with the welding current
The measurement position also showed to be different. The position
The analysis of variance to the welded and milled condition is presented in Table
Analysis of variance of the welded and milled parts.
SS | DF | MSS |
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Total | 15,222.94 | 161 | — | — | — | — |
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421.800 | 2 | 210.907 | 5.394 | 0.006 | Significant |
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3735.800 | 2 | 1867.910 | 47.774 | 0.001 | Significant |
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1479.900 | 5 | 295.981 | 7.570 | 0.001 | Significant |
VI | 3361.200 | 4 | 840.315 | 21.492 | 0.001 | Significant |
VP | 291.666 | 10 | 29.166 | 0.745 | 0.680 | — |
IP | 210.555 | 10 | 21.055 | 0.538 | 0.859 | — |
VIP | 1499.259 | 20 | 74.963 | 1.917 | 0.018 | Significant |
ERR | 4222.667 | 108 | 39.098 | — | — | — |
After milled, the three variables are influents. There is interaction between the welding current and welding speed, and there is interaction between the three variables. An orthogonal contrast test revealed that there is no difference between the results obtained with the welding speeds
Figure
Microhardness for welding current and measurement position: effect of welding current.
In Figure
Microhardness for welding current and measurement position: effect of welding speed.
Here, the distribution of microhardness in surface of the parts of the welding speed variable, verified in the points
As demonstrated by other authors, the hardness can be associated with residual stress. If it increases, the presence of compressive stress is noted. Other way, if it decreases, there is the presence of tensile stresses. In the case of welding of a SAE 1045 steel, it can be observed that compressive residual stresses are introduced. The intensities of these residual stresses are greater near the bead weld and decrease toward the edge of the part. Welding current increases still more the intensity of the residual stresses as detected in the welding current of 176 A. The effect of welding speed on residual stresses is lesser than the effect of welding current. The milling operation in a part after welding proved to increase the compressive residual stresses. In this case, the effect of the welding current of 176 A was very important. So, as a general conclusion, considering that a presence of a compressive residual stress is beneficial to fatigue strength and that the milling improves the surface quality of the product in relation to the welded surface, this work recommends the milling operation after welding of this kind of parts.
The authors declare that they have no conflicts of interest regarding the publication of this paper.
The authors want to thank FAPEMIG–Fundação de Amparo a Pesquisa do Estado de Minas Gerais for the financial support to this research.