Resistance spot welding (RSW) is widely used as a main joining technique in industry and the electrode caps are frequently replaced because of the degradation during service. In this study, the G type copper RSW electrode caps were coated with Fe and Ni based Fe3Al, FeAl, Ni3Al, NiAl alloys by Electro-Spark Deposition (ESD), providing resistance to hot deformation, oxidation and Zn evaporation from sheet metal. The ESD coated electrode caps were tested in-situ on a hot dip galvanized steel in order to assess the performance of RSW electrode caps. For this purpose, three different coating voltages were selected for each coated electrode, and 12 different cap coatings were produced in total. Fifty resistance spot welds were consecutively manufactured with the same parameters for each type of coating electrodes. Hardness measurements, macrostructural examination, Ultrasonic Testing (UT) and chisel tests were performed on welded samples produced. In addition, effects of different coatings on RSW electrode caps were investigated on microstructural development, hardness variations and deformation capacity of resistance spot welds. Results showed that chisel tests and cross section thickness values of the welded sample made with the caps that were ESD coated with the Ni3Al electrode produced better results than the other caps. The cross-sectional thickness of nuggets was lower in all 158 V coated caps. The performance of aluminide coatings on RSW electrode caps can be listed from the best to the worst in the order of Ni3Al, NiAl, Fe3Al, and FeAl.
La soldadura por resistencia por puntos se utiliza ampliamente como técnica principal de unión en la industria y las capuchas de los electrodos se sustituyen con frecuencia debido a su degradación durante el servicio. En este estudio, las capuchas de electrodo RSW de cobre de tipo G se recubrieron con aleaciones de Fe y Ni basadas en Fe3Al, FeAl, Ni3Al, NiAl mediante deposición por electro-chispa, proporcionando resistencia a la deformación en caliente, a la oxidación y a la evaporación del Zn de la chapa metálica. Las capuchas de electrodo recubiertas por estas fases intermetálicas se ensayaron in situ sobre un acero galvanizado en caliente con el fin de evaluar su rendimiento. Para ello, se seleccionaron tres voltajes diferentes para cada electrodo recubierto, y se produjeron 12 recubrimientos diferentes en total. Se fabricaron consecutivamente 50 soldaduras por resistencia por puntos con los mismos parámetros para cada tipo de electrodo y revestimiento intemetálico. Se realizaron mediciones de dureza, exámenes microestructurales, pruebas ultrasónicas y pruebas de cincelado en las muestras producidas. Además, se caracterizó la evolución de la microestructura de los diferentes recubrimientos, las variaciones de dureza y la capacidad de deformación de las soldaduras por resistencia por puntos. Los resultados mostraron que los ensayos de cincelado y los valores de espesor de la sección transversal de la muestra soldada realizada con las capuchas de Ni3Al produjeron mejores resultados que las otras capuchas. El espesor de la sección transversal de los electrodos fue menor en todos los casquillos recubiertos utilizando 158 V. El rendimiento de los recubrimientos de aluminuro puede enumerarse, del mejor al peor, en el siguiente orden: Ni3Al, NiAl, Fe3Al y FeAl.
Resistance Spot Welding (RSW) is one of the many joining techniques of mass production and used intensively in the assembly of sheet metals. A constant electrical resistance between spot weld electrode caps and sheet metal should be maintained for retaining the quality of spot welds. RSW electrode caps should have definitive material properties such as high thermal, creep and abrasion resistances, good electrical conductivity, and resistant to mechanical loads depending on their area of use (
Intermetallic alloys, such as aluminides, are the type of materials with mechanical and thermal properties between metallic and ceramic materials and have been intensively investigated in recent years owing to their interesting physical and corrosion properties i.e. having better wear and oxidation resistance compared to most metals used at high temperatures. The main disadvantages of intermetallic alloys are their undesirable properties such as low ductility and poor fracture toughness. Recent studies on Ti, Fe and Ni aluminides have focused on brittleness problems which may be eliminated by microstructural variations, grain structures and compositions by keeping the alloying and production processes under control (
Electro Spark Deposition (ESD) creates high frequency micro electric arcs between the electrically conductive (anode) electrode and the base (cathode) material, with a scarce metallurgical effect on the substrate. In the ESD process, the electrode rotation speed, frequency, and voltage/power are controlled. Vibration/axial rotation movement of the electrode is needed to break the electrical current from circuit to the material and the micro welds formed on the surface of the substrate, ensuring constant mass transfer at the micro-level (
In this study, RSW caps have been coated by the electro spark deposition technique with Ni and Fe based aluminide alloys in order to investigate their effect on wear and mechanical properties of resistance spot welds and caps.
In the experimental study, 13 pairs of copper electrode caps of G type were used, and 12 pairs of the electrode caps were coated with Fe3Al, FeAl, Ni3Al and NiAl by the ESD technique, subsequently heat treated and homogenized. Aluminide alloys were selected within the composition range in which the intermetallic structure can easily form and be produced by vacuum arc melting using high purity Ni, Fe and Al powders under the vacuum flushed with Argon gas. Alloy compositions were analysed with EDX (Energy Dispersive X Ray) detector attached to a LEO 1430 VP brand SEM (Scanning Electron Microscope). Coated and uncoated caps were used for RSW of a low strength galvanized 0.75 mm thick DX56D+Z quality steel sheet metal (EN10326 standard) suitable for cold forming. The chemical composition, physical and mechanical properties of the RSW electrode caps and chemical composition of the EN10346 quality sheet metal are given in
Welding Electrode | Copper caps | |||
---|---|---|---|---|
|
1% Cr, 0.2 % other, 98.8 % Cu | |||
|
Fe3Al | FeAl | Ni3Al | NiAl |
|
73% Fe, 27% Al | %53 Fe, %47 Al | %73 Ni, %27 Al | 51% Ni, 49% Al |
Sheet metal Composition | C (max.) | Mn (max.) | P (max.) | Si (max.) | Al (max.) | Ti (max.) | Nb (max.) |
---|---|---|---|---|---|---|---|
|
0.008 | 0.30 | 0.025 | 0.03 | 0.080 | 0.15 | 0.04 |
Twelve G type RSW electrode caps with a tip diameter of 6 mm were divided into 4 groups and manually coated with Fe3Al, FeAl, Ni3Al and NiAl alloy electrodes (rods) in a set of 3 (
The first row: 66 V, the second row: 112 V and third row: 158 V. Codes of coating alloys are given above the images.
DX56D+Z quality steel sheet metals were cut in 40x25 cm2 dimension, and 13+13 sheets were lap joined by RSW (
The 10th, 20th, 30th, 40th and 50th resistance spot sections of the welded plates were cut with SiC cutting discs as to reveal just the middle section, passed through 320, 600, 800, 1000G grinding stages and polished with a 1 µm alumina suspension. Specimens were then etched with 2% Nital and the microstructure was examined. Metallographic and microstructural analyses were made from the 30th samples taken from the assembly. Photographs were taken from spot welds with Nikon optical microscope and using LEO 1430 VP brand SEM. Microhardness measurements were made using Shimadzu HMV2 microhardness tester on the 1st, 10th, 20th, 30th, 40th and 50th spot welds, using 100 g load, by making 3 indents on the weld nugget of RSW and averages of results are calculated and presented. The welding electrode cap surface was visually and microstructurally examined after the coating and RSW processes. The 41st corner spot welds on each plate, as shown in
The microstructure of the 30th welded sample made by uncoated cap is shown in
Hardness (HV) | |||||
---|---|---|---|---|---|
Uncoated | |||||
0. | 10. | 20. | 30. | 40. | 50. |
102 | 133.6 | 149.0 | 149.6 | 156.4 | 160.1 |
The UT signals of the welded sample obtained by the uncoated cap are shown in
The post-weld chisel test of the welded sample made with the uncoated cap is given in
In the hardness tests performed on the 10th, 20th, 30th, 40th and 50th samples cut from the plates. The most noticeable variation in hardness occurred in the caps coated with the Fe3Al intermetallic alloy. As in
Electrode Alloy | Fe3Al | FeAl | Ni3Al | NiAl | ||||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
|
10. | 20. | 30. | 40. | 50. | 10. | 20. | 30. | 40. | 50. | 10. | 20. | 30. | 40. | 50. | 10. | 20. | 30. | 40. | 50. |
|
20 |
20 |
20 |
20 |
21 |
19 |
19 |
20 |
20 |
19 |
19 |
19 |
20 |
19 |
19 |
20 |
20 |
19 |
19 |
19 |
205.1 HV | 200.3 HV | 199.4 HV | 198.4 HV | |||||||||||||||||
112V | 21 |
21 |
21 |
21 |
20 |
20 |
20 |
19 |
19 |
20 |
19 |
20 |
19 |
20 |
19 |
19 |
20 |
20 |
19 |
19 |
212.3 HV | 199.3 HV | 200.2 HV | 200.5 HV | |||||||||||||||||
|
19 |
20 |
20 |
19 |
19 |
19 |
19 |
19 |
19 |
19 |
19 |
19 |
20 |
20 |
20 |
20 |
19 |
20 |
20 |
19 |
200.0 HV | 197.8 HV | 201.7 HV | 199.8 HV |
The interpretation of the ultrasonic test (UT) graphics obtained from the RSW specimens is given in
The horizontal line in the images shows the threshold of 1.05 mm thickness of the spot weld zone.
The cross-sectional images of the 30th RSWs are shown in
Coating voltages and codes are given in each image.
The microstructures of the coated caps taken at 100x magnification from the cross-section are shown in
1) 66 V, 2) 112 V, 3) 158 V, row B: the FeAl coated caps 1) 66 V, 2) 112 V, 3) 158 V, row C: the Ni3Al coated caps 1) 66 V, 2) 112 V, 3) 158 V, row D: the NiAl coated caps 1) 66 V, 2) 112 V, 3) 158 V. Scale bar is 53 µm.
1) 66 V, 2) 112 V, 3) 158 V, row B: the FeAl coated caps 1) 66 V, 2) 112 V, 3) 158 V, row C: the Ni3Al coated caps 1) 66 V, 2) 112 V, 3) 158 V, row D: the NiAl coated caps 1) 66 V, 2) 112 V, 3) 158 V.
The results from this study can be summarized as follows:
The Ni-based aluminide alloys produce better spot welds than the Fe based aluminide alloys coated caps based on the appearance of ruptured spot welds after the chisel test and the penetration values of Zn on spot weld caps.
The Zn diffuses into coating regardless of the type of coating; however, the Ni based coatings are more resistant to the diffusion of Zn towards the copper electrode matrix.
The coatings help increasing the nugget core hardness compared to the weldments with uncoated electrodes.
As the intensity of the coating voltage is increased, the coating thickness and the amount of deformation of spot welds also increase as a result of high interface electrical resistance, which causes an increase of the interface temperature and the accumulation of the heat in the nugget of spot welds.
Increasing the coating voltage levels showed that the post-weld coating microcracks increased, as this is related to the thickness of the coating and, hence, to the interface resistance between electrode and sheet metal.
The effects of type of aluminide alloy electrodes on the RSW quality can be listed as: Ni3Al, NiAl, Fe3Al, FeAl from the best to the worst.