Experimental Study on Metal Transfer and Welding Spatter Characteristics of Cellulose Electrode

LIU Haiyun , LI Hui LI Zhuoxin and SHI Yaowu

1 College of Material Science and Engineering, Beijing University of Technology, Beijing 100022, China

2 College of Material Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China

1 Introduction

Due to the fact that cellulose-type welding consumables are well suited for all-positional welding, they have played an extremely important role in long-distance pipeline projects, but welding spatter is also a serious problem that must be considered. The welding spatter often deteriorates the working environment and decreases the welding efficiency, particularly for manual welding. Many studies have been done to reduce the welding spatter and to make the welding process more efficient[1–4].

Usually, the spatter occurs when the welding arc becomes unstable, so maintaining a stable welding arc is very important. However, for the current shielded metal arc(SMA) welding, gas metal arc (GMA) welding and flux cored arc (FCA) welding, there are dozens of process variables that may influence the arc stability. These variables include welding voltage, welding current, welding consumable materials, arc length, electrode extension,shielding gas/slag, mode of transfer and the characteristics of the power source, etc[5]. From a fundamental point of view, the welding process consists of numerous dynamic metal transfer processes. Therefore, most of the GMA process characteristics, such as welding spatter, welding fumes, penetration and positional weldability, can be improved by controlling the metal transfer mode. One typical example is the pulsed GMA welding process which incorporates controlled metal transfer techniques[6–7].Welding spatter can also be reduced by the careful regulation of metal transfer behavior[8–10]. By recording the arc signals using a high speed data acquisition system,BAUNé, et al[11], found a good correlation between the arc stability and metal transfer behavior in both rutile-type electrodes and basic-type electrodes. Their results indicated that more spatters were produced during the welding process when the rate of molten droplets decreased.Therefore, using higher droplet transfer rate would result in less welding spatter.

Because the transfer behavior of weld metals is one of the most important factors affecting the welding spatter formation, we studied the welding spatter characteristics and the metal transfer of three commercial cellulose electrodes. Their correlation is particularly addressed by using a high speed camera and the spatter rater evaluation.The spatter formation during cellulose electrode welding was analyzed, which is expected to provide useful information for developing advanced welding materials and the design of improved welding power sources. The experimental procedures of the study are firstly described in section 2. Thereafter, the metal transfer mode of cellulose electrode, the welding spatter characteristics and their relationships are presented in section 3 by sequence.Finally several conclusions based on the experimental results are given.

2 Experimental Procedures

Three cellulose electrodes with a diameter of 4 mm were used in this study: Fleetweld 5P from lincon, Foxcel 6010 from B?hler, and Celwel-60 made from Ador of India. The power source used for welding is Kaierda zxg-300. The welding voltage is 30 V–35 V, the current is 120 A–130 A,in all the welding experiments.

The weld metal transfer behavior and spatter characteristics were investigated by using a high speed LBS-16 camera, with a speed of 1 000 frames per second,as shown in Fig. 1. To ensure that the whole transfer process can be captured by camera, the arc length was finally set at 4 mm, which approximates to the arc length encountered in practical welding process (2 mm–4 mm[12]).A non-chamfered specimen was used and the electrode was fed automatically in all the experiments. The recorded film was manually interpreted and analyzed carefully. The amount spatter from each different transfer mode was counted statistically as well as the frequency of welding spatter.

Fig. 1. Schematic of high speed cine-camera

The spatter loss coefficient was measured as follows:bead-on-flat welding was conducted on a specimen measuring 250 mm×50 mm×20 mm. The welding experiment was made on the interior surface of a rounded drum, with a diameter of 300 mm. After the welding experiment was completed, the spattered particles were carefully collected and weighed, as was the electrode. The spatter loss coefficient was obtained by calculating the ratio of the weight of the spatter to the loss of electrode.

3 Experimental Results and Discussion

3.1 Metal transfer of cellulose electrode

From the high speed photographic observation, it can be seen that the metal transfer of cellulose electrode welding consists of short-circuiting transfer, large droplet spraying,fine droplet spraying and explosive mode, as shown in Fig. 2.

Short-circuiting transfer happened when the large molten droplet contact the molten pool. Fig. 2(a) presents the typical short-circuiting transfer, which was observed in Foxcell electrode welding. This mode of metal transfer was rarely found for the cellulose electrode. In comparison,large droplet spraying was one of the main metal transfer modes, which was found repeatedly in the studied electrodes welding. Fig. 2(b) displays a series of photos illustrating the large droplet transfer process where the metal droplet was blown into the molten pool by the arc gas.On the other hand, fine droplet spring transfer was also formed in almost all the welding processes; it is another principal transfer mode of cellulose electrodes. As shown in Fig. 2(c), the dynamic gas and the arc disintegrated the molten metal of the electrode tip, resulting in the formation of small particles. These sprayed small particles were then partially transferred to the molten pool, with the remainder forming spatter. In the other words, welding spatter happened inevitably during the fine droplet transfer.

Explosive transfer was caused by the explosion at the electrode tip, mainly due to the internal metallurgical reaction or caused by gas expulsion and at the same time accompanied a large amount of welding spatter, as shown in Fig. 2(d).

Fig. 2. Metal transfer photography pictures of cellulose electrode by high speed camera

In summary, large droplet spraying and fine droplet spraying are the two main transfer modes for the cellulosic electrode welding.

3.2 Spatter characteristics

From the observation of the high speed film, three types of the welding spatter were formed during the cellulose electrode welding: spatter induced by large droplet deflection, explosive spatter, and fine droplet spraying spatter, as shown in Fig. 3. The first mode of spatter was formed when large droplets were deflected which resulted from the gas blowing and arc force, as shown in Fig. 3(a).The explosive spatter was caused by the explosion at the electrode tip where metallurgical/chemical reaction and internal gas expansion play an important role, as shown in Fig. 3(b). Moreover, the spatter formed in fine droplet transfer occurred during the whole process, as shown in Fig.3(c).

Fig. 3. Spatter photography pictures of cellulose electrode by high speed camera

3.3 Influence of the metal transfer mode on the spatter

It can be seen from the above-mentioned comparison that the metal transfer is closely related to the spatter formation.The large droplet spraying transfer mode does not easily lead to spatter, but the other transfer modes are prone to spatter. The worst one is the large droplet deflection, it may also influence the other process properties, such as arc stability, electrode usability, deposition efficiency etc, as discussed in LIU, et al[10]. The explosion-induced spatter occurred concurrently with the explosive transfer, and it is also one of the main forms of spatter. Spraying-induced spatter appeared inevitably during the fine droplet transfer process, which is a common feature of cellulose electrode welding.

On the basis of the high-speed photographic observation,the metal transfer of cellulose electrode and the resultant spatter can be described as follows, during the dominant process of fine droplet spray as well as the spatter, the large droplet builds up periodically and then either enters into the molten pool through short-circuiting transfer or droplet transfer. Some of them were transformed into explosive transfer, some of the droplets were deflected and then caused welding spatter.

The further investigation shows the correlation of the metal transfer mode and welding spatter, as displayed in Table 1 and Table 2. It can be seen that with increasing frequency of the appearance of explosive transfer, large droplet deflection and large droplets, the welding spatter tends to increase. Therefore, reducing the explosive transfer,limiting the large droplet deflection, and refining the droplet size lead to less welding spatter in cellulose electrode welding.

Table 1. Results of the spatter of the investigated electrodes

Table 2. Frequency of the metal transfer Hz

In practical cellulose-type electrode welding, operators often use a short arc. As a result, large droplet induced spatter is more likely to happen, but spatter induced by explosive transfer and large droplet deflection are also unavoidable. In fact, the large droplet spatter also can be observed for cellulose electrode welding, no matter whether short-arc or non-short-arc technique was used. Therefore, it is recommended for the practical operator to improve arc blowing force and to use finer droplet transfer, which will reduce the occurrence of large droplet deflection as well as the explosion transfer, particularly in vertical welding and overhead welding.

4 Conclusions

(1) The metal transfer behavior of cellulose electrode welding consists of short-circuiting transfer, large droplet spraying, fine droplet spraying and explosive transfer.Short-circuiting transfer appeared rarely while large droplet spraying and fine droplet spraying were the two main transfer forms in cellulose electrode welding.

(2) The deflection of large droplet, explosive spatter, and fine droplet spraying-induced spatter are the main forms of welding spatter of cellulose electrodes. Large droplet deflection was generated during the formation of large droplets, and the fine droplet spraying induced spatter was formed inevitably during the fine droplet transfer process.Minimizing the droplet size and reducing the explosive transfer lead to less welding spatter.

Acknowledgement

The authors would like to express their sincere gratitude to Mahlon Smith who looked closely at the final version for English style and grammar, correcting both and offering suggestions for improvement.

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