Investigation into the high-speed laser welding feasibility of tin-plated steels available for three-piece food packaging can manufacture

Abstract

This investigation was focused on the high-speed laser welding of 0.4-mm tin-plated steels used for joining together parts of three-piece food cans. The high-speed laser welding quality is generally restricted due to several welding discontinuities that occur with the change of traverse speed. A study on the production set-up by a food can manufacturer was first addressed, and reasons for introducing high-speed laser welding were further discussed. A rotary axis as a welding fixture was designed and made to achieve high surface speeds. Thereafter, an experimental investigation was conducted using a CO₂, neodymium-doped yttrium aluminium garnet laser and hybrid of plasma augmented laser welding applied to the typical food can material. Conventional welding defects found during high-speed laser welding were observed for all three laser welding techniques. However, humping gradients reduced with plasma augmented laser welding and penetration were evident up to welding speeds of 98 m/min. Furthermore, the high-speed laser welding defects were discussed, and possible solutions to eliminate the humps and further work into the application of the high-speed laser welding process for the Canning industry were mentioned.

Keywords

Lasers hybrid plasma augmented laser welding high-speed welding tin-plated steel food cans

Introduction

Background of food packaging can manufacturing

Increase in population in urban areas and fast-paced lifestyle requiring ease and convenience have slowly given a rise to a variety of new food products, which has increased the demand for food packaging containers around the world. In particular, the metallic food cans (FCs) are easy to recycle and successfully provide food storage. This surely affects the beverage, drinks and food packaging industry in the United Kingdom, as 3.8% rise around the world in the demand for food packaging product is expected year-on-year from 2008 to 2013.¹ Having said that the food and drinks manufacturing has only reduced by a small amount throughout the recent recession, and the recovery of production output for food and drink products returned rapidly to prior recession times in the United Kingdom² This goes to show that the future for food packaging products such as metallic beverage and FCs is certainly bright.

A rapid increase in the launch of new food products has led a UK-based FC manufacturer to produce volumes of 144 million units per annum. On account of this, it can be postulated that the demand for three-piece FCs is not only in the United Kingdom but also considerably large in Europe and around the world. In addition, the production speed at which the manufacturers are required to produce each unit is also significantly high since the packaging FC manufacturers are required to produce in masses rather than in batches. Moreover, the current technique of joining such three-piece packaging FCs is a mash seam welding (MSW), otherwise known as resistant seam welding (RSW) technique. This technique has several flaws within the process, which are namely high set-up times, lack of flexibility, technical limitations, wastage of material and lead-times. In particular, the MSW process comprises a lap joint configuration to form the weld seam. However, lap joints utilize excess material in comparison with butt joints, which would waste material every time a three-piece FC is manufactured. The numbers become considerable when adding the annual cost and the amount of the wasted material in dimension. In addition, MSW systems lack the versatility and the flexibility to weld different shapes and sizes of the FCs since the MSW machinery is only set to join one certain can diameter. Hence, upon a change in demand to join another can diameter would require lengthy machine set-ups and timely changeovers during MSW. On account of this, it is required that an alternate process such as high-speed laser welding (HSLW) is considered. This research was focused on a case study of a United Kingdom based FC manufacturer in relation to the constraints with the current FC manufacturing process and a feasibility study of implementing laser welding as an alternative as well as a superior joining technique.

Previous research in HSLW

An increase in the welding speed results in discontinuities within the weld bead. The welding defects are known as humping, undercut and ropy bead.^1–4 Humping is a continuous bulging effect of the weld face in the direction perpendicular to the traverse direction of the laser beam.^1–8 There are three stages in the process where humping is the final stage. As the weld speeds increase, irregular undercutting begins to occur, and this leads to the formation of a ropy bead. The second stage will consist of both of the features. However, undercut is formed consistently until the speed begins to increase. Formation of the hump occurs at the highest traverse speed. Humping was first identified by Bradstreet³ who followed an investigation into manual metal arc (MMA) welding defects. Gratzke et al.⁴ presented a theoretical model of the humping phenomenon during conventional HSLW. Main results found from the theoretical work of Gratzke et al. were that the quantity of width-to-length ration of the weld pool remains under critical value for humping to occur. In other words, by maximizing the length-to-width ratio could result to avoiding the hump within a weld pool.

An investigation by Mombo-Caristan et al.⁵ used an oblong beam at high-speed, bead-on-plate laser welding and reduced the humping effect on 1.3-mm galvanneal sheet steel. Braunsteiner et al.⁶ investigated the possibility of using a CO₂ laser for HSLW of various ST 14 type steels of 1.6 mm thickness and showed that a CO₂ laser would be a relevant source to use for this technique. However, no other type of discussion was held with regard to the quality and the associated weld. Cao and Jahazi⁷ used a 4-kW neodymium-doped yttrium aluminium garnet (Nd:YAG) laser to investigate the effect of the welding speed on Ti-6Al-4V alloy sheets of 1 and 2 mm thickness. The effects of welding speed on surface morphology and shape, welding defects, microstructure, hardness and tensile properties were investigated. Results were reported to consist of weld joints comprising only minor cracks, porosity and shape defects. Kabir et al.⁸ studied the weldability of 5.1-mm-thick Ti-6Al-4V sheets by employing continuous wave (CW), 4-kW Nd:YAG laser. Welding speeds were varied, and as a result, defects such as under-fill and porosity were observed, although welds without cracks were also obtained using a high-power Nd:YAG laser.

Blundell et al.⁹ conducted an investigation on the laser welding of thin sheet steels of 0.16–0.4 mm thickness using high welding speeds. Blundell et al. reported the occurrence of discontinuities such as humping and undercut resulting after reaching sufficiently high speeds. Experiments were conducted using a 2-kW Nd:YAG laser alone and then direct current (DC) plasma augmented laser welding (PALW) with speed ranging from 10 to 90 m/min. The HSLW discontinuities that were minimized with full penetration (of the plastics coated thin sheets of steels) was obtained when the hybrid welding process was employed.⁹ It was reported that it is possible to double the welding speeds in comparison with the laser-alone welds. The overall hybrid welding results show defect-free welds in comparison with the laser-alone welds. However, the work of Blundell et al. still showed the presence of undercutting and a raised bead centre. Page et al.¹⁰ later published a study with Blundell to demonstrate successful effects of PALW not only with high speeds but also with welding tailored blanks as used for the automotive manufacture. Moreover, Russ et al.¹¹ also investigated the weldability of metal sheets of various thicknesses using a CO₂ and a disc laser. The main results reported evidence of humping, particularly when using a welding speed up to 45 m/min. An investigation by Soderstrom and Mendez¹² reported the theory of humping from undertaking an experimental investigation. Soderstrom and Mendez reported that humping affects result when welding is conducted at high traverse speeds, particularly for thin sheet metals. This particularly focuses on the arc welding process, and it does not agree well with the work of other investigators.

Research rationale

The study herein first shows an in-depth analysis of the production set-up of one of the leading UK-based FC manufacturers. This demonstrates the potential cost reduction and savings if HSLW is to be implemented in place of a MSW process. Second, this study also demonstrates an experimental investigation of HSLW of tin-plated thin sheet steels used for FCs. Effects of the HSLW process were investigated by a microscopic analysis to observe whether the occurrence of the aforementioned welding discontinuities, namely, humping, undercut, ropey bead and drop outs after the HSLW process, took place. Series of experiments were conducted using a high powered CO₂ and Nd:YAG laser. Furthermore, a novel rotary fixture was made to obtain high traverse speeds. This was associated by a robotic motion system and also by applying an additional heat source of the plasma augmented torch. Owing to this, it was then not only possible to achieve high welding speeds but also to understand as a first step the effects of HSLW thereon the FC material.

Analysis of the FC production set-up

Production system for three-piece FC manufacture

From analysing the production set-up of a leading beverage and FC manufacturer in the United Kingdom, it was found that requirements are such that the production rate was 500 cans/min. The company uses 10 h per shift on a two shifts per day regime. This meant that 600,000 units of the three-piece FCs are produced per day. After calculating the quantity per week considering 48 weeks of operation per year, the production of the three-piece FCs was 144 million annually. The calculated values are further presented in Table 1.

Table 1.

A summary of a can manufacturing production rate.

Manufacturing/production rate	Quantity
Number of hours per day	10
Shifts per day	2
Production rate (units/min)	500
Units per day	600,000
Units per week	300,0000
Units per year (48 weeks/year)	144 million

The FCs come in two forms – either two-piece FCs or three-piece FCs, which differentiates them from the two-piece aerosol cans and the required process to join each can. Most of the cans are made from a two-piece body with the top part of the can as the second piece being made from the same material. The two-piece cans are made using press working and deep drawing process where the sheet metal is blanked into large discs and pressed into cup shape that is further deep drawn to form the body and base of the can.¹³ The drink is then filled, and the top is joined by a flange joint. However, manufacturing of a three-piece FC is somewhat involved and comprises several steps further described elsewhere.¹⁴

MSW and HSLW process steps

The FC comprises several steps for its manufacture from raw materials to a finished product as presented in Figure 1(a). First, a large coil of 0.4-mm-thick steel is cut into small sheets. Then, a layer of tin is applied to the surface of the sheet steel, which becomes the internal surface of the FC. The special tin coating provides a protection from any interaction of the metal to the content inside and possibility of corrosion. The tin-plated sheets are then cured in an oven at high temperature. The large sheets are then slit into smaller sheets to the exact dimensional size to form the body of each can. Each sheet is then rolled into a cylinder by rollers installed into the MSW machinery. The edges of the cylinder are welded by the MSW process. The material at the intersection of the welding point is free from the tin coating, which enables a sufficient joint to be produced. The welded surface (weld bead) inside the can is yet vulnerable to contamination and corrosion so it is then treated with the tin coating, continuously running over the seam line of the weld. The tin-plated surface is cured by blowing heated air on the outside of the can body. The cans are then passed through a flanger to allow the end of the can to be flanged (except the top and the bottom part). This is a specialized flanging machine which carries out this process. Blank ends are also prepared by the aid of press work to form the disc shape. This is the second piece of the can, which is attached to the flanged end of the can body by the aid of a flange joint technology. The cans are passed through a beader to give an added strength to the structure, and the beader forms a wrinkle structure on the can body, which adds rigidity and takes in account for the volume changes. Cans are then passed through a pressure tester for any impurities, pinholes or larger fractures, which may have occurred on the surface. Defective cans are automatically rejected. The finished cans are then palletized for dispatch to the filling plant. The third piece of the can is joined after the can has been through the filling process, which is also carried out at the filling plant.

Figure 1.

A schematic diagram of (a) the MSW process and (b) the HSLW process.

As seen from Figure 1(b) that the process steps needed for the HSLW are reduced in comparison to the MSW. This is because the tin coating process is not required to be completed twice prior to the MSW and after, for coating the weld bead in the case of the current MSW process (see Figure 1(a)). In comparison, the HSLW process could include the tin-plating step after the laser welding process has taken place. This would eliminate the lead-time to make the FC and increase the throughput within the production line.

Rationale for implementing HSLW

Overlapping joints have tight tolerances. The diameter of a three-piece FC is usually 78, 96 or 110 mm. FCs come in various lengths: 50, 90 and 110 mm. The MSWed bead of a three-piece FC is 1.5 mm, as shown in Figure 2. The MSW technique produces a lap joint by overlapping the two opposite ends of the FC. A cross section of a MSW lap joint is shown in the microscopic image in Figure 2 of a tin-plated three-piece FC. In comparison with a butt joint, the lap joint utilizes excess material, which means that there is a small amount of material waste with every single three-piece FC that is made. When the material waste is calculated annually, then the numbers become significantly large. The HSLW process on the other hand has the potential to weld with a butt joint configuration. This means that there will be a considerable saving in the use of material to form the three-piece FC body. The savings in the material usage, in turn, could result in extra production capacity or simply a reduction in lead-time. Consequently, this could result in considerable cost reduction by reduced labour, operational hours as well as reduced electricity costs and overhead.

Figure 2.

Optical image of the cross section of an MSWed joint, showing the thickness of the FC wall made from tin-plated sheet steel and the amount of overlap used.

In addition, the MSW machinery is not flexible enough to weld different shapes and sizes of the FCs. Each MSW machine is set to a limit for joining a certain diameter of the three-piece FC. For example, can diameters of 78, 96 and 110 mm for welding are generally set within the RSW machinery. When the demand changes for joining a different diameter, then the MSW system cannot easily accommodate the change since time-consuming set-ups are required. The smallest weldable can diameter is 38 mm due to the smallest sized rollers available within the MSWer, which prevents MSW of small-diameter cans such as two-piece aerosol cans and, therefore, are naturally forced to be joined using alternative techniques. In comparison, the HSLW system could accommodate the joining of a variety of can diameters since there are no restrictions on the use of rollers. Thus, it is feasible to join small to large can diameters of varying lengths including not only the two-piece aerosol cans but also larger cans used for storage of liquid food, animal food and other products such as containers for paint, oil and chemical storage.

The daily machine warm-up time of the MSW machines is around 15 min. However, it could take up to 8 h to set-up the machine if there is a requirement to change can diameter since a considerable amount of machine set-up is generally required. Consequently, five skilled operators are required to complete the task. In addition, the machine undergoes a maintenance procedure every 6 weeks. Most working parts are serviced and small size parts are replaced if wear and tear are present. This adds an additional maintenance cost. In comparison, a HSLW process requires a minimal daily maintenance to start up the laser system. Hence, the set-up times are in minutes rather than hours if appropriate jigs and fixtures are employed, and a sufficiently skilled, single operator is also capable of setting-up the new product/part on the laser system. Moreover, the new laser systems are now almost maintenance free and require very little attention on a regular basis. So the frequent maintenance is no longer required and rather than weeks, the maintenance procedure is conducted every 4–6 months for the HSLW process.

The MSW system contains 60-mm-diameter rollers rotating at 500 r/min, achieving the speed of 47.12 m/min at the welding interface. A laser system, in comparison, using a correct set-up is capable of achieving much superior speeds of 130 m/min with great accuracy and tight tolerances.^15,16 Furthermore, some welding impurities such as formation of nuggets on the weld seam tend to occur with the MSW process, which means that the FCs must be scraped eventually and sent for recycling. However, the feasibility of HSLW as yet is a question which needs to be answered? This will be the next step of the investigation, which will be presented with the aid of the experimental investigation as presented further in this article.

Cost reduction and savings upon implementing HSLW

Tight fit-up joints and good tolerances are achieved by the use of laser technology.^10,17,18 As mentioned before, the utilization of a butt joint configuration could minimize the amount of material required to join the FCs. If a butt joint configuration using HSLW is employed, then the amount of excess material saved can be calculated annually from the optical image (see Table 2). Considering a 96-mm-diameter FC as an example would result to material savings/can, as presented in Table 2, followed by the wastage of the total can production in a year for that particular can diameter, if the butt weld configuration is adopted using HSLW in place of the conventional lap welding method using MSW. Figure 3(a) illustrates a schematic representation of the flat sheet main body of the FC showing various dimensions. Figure 3(b) illustrates a schematic representation of the RSWed with dimensions and Figure 3(c) is an optical image of the RSWed main body of the tin-plated FC.

Table 2.

Analysis of production system.

Analysis of production system	Units
Can diameter	6 mm
Can height	110 mm
Can length	304.5 mm
Area	33495 mm²
Cost of manufacturing one FC (unit)	£0.12
Material wastage per FC (unit)	1.5 mm × 110 mm = 165 mm²
Material saved per FC (unit) (see Figure 3)	165 × 2 = 330 mm²
Material saved/year	330 mm²× 144,000,000 (units) = 47,520,000,000 mm²
Required material to form the first piece of a can (96 mm diameter and 110 mm length)	33,495 mm²
Material wastage per can (96 mm diameter and 110 mm length)	330 mm²
Material waste per year	47,520,000,000 mm²
Extra production subject to material saved for 96 mm diameter of FC	47,520,000,000 mm²/33495 mm² = 141,8719 units/year
Potential savings from material reduction for one type of three-piece FC diameter	1,418,719 units × 0.012 (£) = £17024.63
Lead-time savings per/year (for 96 mm type of FC)	1,418,719 units/500 (units/min) = 2837.43 min or (47.29 h)
Potential savings from having extra lead-time in production capacity (for 96-mm type of FC)	2837.43 min × 500 units = 1,418,715 units = 1,418,715 × 0.012 = £17024.58

FC: food can.

The above figures are a summary of the potential savings achievable from reducing the material usage and lead-time and are subject to change depending on the demand and the required production volume at any particular time.

Figure 3.

(a) A schematic representation of the flat sheet main body of the FC, (b) a schematic representation of the RSWed FC and (c) an image of the RSWed main body of the tin-plated FC.

This means that by saving material, it is possible to either increase production output or cut down the number of hours required to reach the current production volume, since it will be possible to manufacture more units in the same time period when implementing the HSLW technology. Therefore, either the production volume can be increased in the same amount of lead-time, or alternatively, the lead-time can be reduced to produce the equivalent number of units as required for the current demand. The reduction in material waste in turn would lead to commercial advantages to be gained by the aforementioned FC manufacturer, as can be seen in Table 2.

The savings presented in Table 2 appear to be considerably small; however, it should be noted that the potential savings are as a result of one type of can size, which in this case is a 96-mm-diameter can. When other can sizes are taken into account, then the savings and the accounted benefits will sum up to a large number. In addition, it is not simple to account for the amount of units which the HSLW process could produce at this stage in the research but it is worth a mention since a major advantage would result from the fact that a laser system could potentially weld at speeds twofold higher than the MSW technique, which is welding at a maximum speed of 47.12 m/min with 500 cans/min. This indicates that an increased productivity, considerable reduction in lead-times and an increased throughput as well as low operational costs, reduced labour and lower maintenance costs could be gained. On account of this, despite the high investment cost of a laser system, the implementation of HSLW could still aid gain and eventually become a profitable venture in a short term rather than a long term. This inherently could prove to be an investment for a fruitful future by not only increasing the output and production capacity, reduced lead-times, considerable material savings but also enabling FC manufactures to gain a competitive advantage and meet the rising demands.

Experimentation and analysis

Experimental material

The material under investigation was 0.4-mm-thick tin-plated steel consisted of electrolytic chromium/oxide coating as used for the FCs supplied by a UK-based food and aerosol can manufacturer. The tin-plated steel is coated on both sides with pure tin at coating weight ranging from 1 to 12 g/m². Flats sheets of size 110 mm (height) × 304 mm (length) were used to conduct bead-on-plate experiments (see Figure 4). The flat sheets of tin-plated test samples were fixed firmly onto the rotary welding fixture, as shown in Figure 4(b), which also limited distortion during HSLW.

Figure 4.

(a) Image of the Nd:YAG laser–assisted plasma torch and (b) a schematic diagram of the Nd:YAG laser–alone experimental set-up with the rotary axis fixture.

Sample preparation

The laser welded tin-plated steel was later cut through the cross section of the weld for microscopic analysis. Thereafter, the samples were set into 60% methylmethacrylate (polymer powder) mixture with liquid 40% tetrahydrofurfuryl methacrylate (liquid hardener) and were left over night to harden. The samples were then ground and finely polished in stages using 6, 3 and 1 µm polishing wheels for 5 min each. Care was taken during grinding and polishing to avoid excess heat transfer to the plastic moulds, which otherwise would affect the characteristics of the laser welded region. Thereafter, the samples were etched using 2%−5% Nital for 30 s so the cross section of the welds could be viewed under the optical microscope (Mitutoyo; LWD; Japan).

Nd:YAG laser welding technique

A 2-kW, CW, Nd:YAG laser fed via a 600-µm-diameter optical fibre to conduct trial on a bead-on-plate experiment. The laser emitted a wavelength of 1.064 µm and consisted of a TEM₀₁ beam mode with a beam quality factor – M² to be 1.3. Argon assist gas was employed at 20 L/min with a spot size of 1 mm with an applied laser power of 0.5–2 kW. In addition, traverse speed was obtained using a rotary axis fixture between 25 and 100 mm/min. The Nd:YAG laser was assisted by a KC 25, Kuka, six-axis industrial robot. The Kuka robot was programmed to traverse horizontally in the X direction at low speed in order to create spiral weld lines on the material surface (see Figure 4), as the rotary fixture was made to rotate at 300 r/min. The workpiece was held firm using small clamps, which avoided distortion to take place. The rotational traverse speeds are measured by a revolution gauge (taco meter) to initially set up the experiments. The use of the rotary fixture was employed since high speeds can be achieved. More importantly, the robotic motion system consisted of a low dynamic response, which meant that for the robot to traverse at a high welding speed required large area. This would allow the robot to ramp up to a maximum set speed from its initial position to weld the workpiece and maintain the speed over the sample and then reduce while it is on the path to the initial home position. However, this was not possible to program as it required large traverse area, which was not available within the experimental set-up. On account of this, a particularly designed rotary fixture was made and upon a correct set-up enabled to obtain high surface speed processing during the laser welding experiments.

PALW

The second set of trials used a Nd:YAG laser and a plasma torch as an additional heat source, as shown in Figures 4(b) and 5. The rotary welding fixture shown in Figure 5 was designed and made to achieve surface speeds of up to 150 mm/min. This was so because it would suit the nature of the experiments and would allow flexibility to be employed under both types of lasers used for this research. The PALW experiments were conducted using triangulated configuration where the electrical current of 40 V flowed through to complete a circuit. The PALW experiments were conducted in similar fashion to previous laser-alone experiments but in this case a DC plasma arc torch was attached to the Nd:YAG laser head at 45°. This was because the 45° allowed sufficient coupling of the laser beam and the plasma. In addition, previous experiments also employed similar settings.^9,10 Spot size of 1 mm was used with an argon assist gas of 20 L/min. The power was varied from 0.5 to 2 kW using CW mode, and the traverse speed was varied in the range of 5–100 m/min. Moreover, the Nd:YAG laser consisted of the same features as to the experiments conducted using laser-alone trials.

Figure 5.

A schematic diagram of the Nd:YAG laser and PALW set-up with a rotary welding fixture.

CO₂ laser welding

A 2-kW CW CO₂ laser was employed to conduct the first set of experiment. The CO₂ laser emitted a 10.6 µm wavelength and had a beam mode of TEM₀₀ with an M² beam quality factor of 1.2. Trials were conducted between 5 and 100 m/min. High traverse speed was achieved using rotational speed of the welding fixture with the laser power being varied from 0.5 to 2 kW. Argon was used as an assist gas at a flow rate of 20 L/min. The CO₂ laser was programmed to run in a straight line (X direction) at a low speed in the centre of the tin-plated sheet of steel as the rotary fixture was rotating at 300 r/min. This combination produced a spiral welded pattern. The laser beam was switched on and switched off at selected positions on the sample (see Figure 6).

Figure 6.

(a) An image of the CO₂ laser set-up and (b) a schematic representation of the CO₂ laser beam delivery system and the rotary axis during HSLW.

Tensile testing

The MSW, PALW and both the CO₂ and Nd:YAG laser welded samples of the tin-plated steel were tested for the ultimate tensile strength (UTS) of the weld seam. Five strips were cut to 10 mm × 50 mm rectangular shape for each of the welding processes and consisted of the weld seam being precisely in the centre of the sample with equal amount of material left on each side. An average was taken for each of the five samples and is further presented in this article. The samples were placed (one at a time) vertically in the Y-axis into the tensometer (LR50 K plus universal testing machine; Lloyds Instruments Ltd, USA) and pulled apart in a vertical orientation by the tensometer in ambient temperature. The load was consequently increased from 0 N until the sample was completely fractured to obtain a load versus extension graph (see Figure 12). The results were naturally generated by the operating software (NEXYGENPlus) of the tensometer and were tabulated in graphical format to show a comparison welded samples.

Welding speed configuration

Due to mechanical constraints, the laser systems employed herein were unable to obtain the desired welding speeds. It was therefore required that the workpiece being welded would traverse using rotational speed to obtain higher traverse speeds during the laser welding process. Therefore, a rotary fixture was designed to achieve surface speeds from 10 to 150 mm/min (see Table 3 and Figure 7(a) and (b)). This solved the problem of having insufficient dynamic response which the two motion systems had for both the CO₂ and the Nd:YAG laser. On account of this, the speed was first configured by equation (1)

V = π \times D \times N

(1)

Table 3.

Surface speeds configured at various distances on the rotary fixture according to equation (1) during all three laser welding techniques.

Selected distance from the centre of the workpiece ((r) mm)	Configured speed (m/min)
5	4.71
20	18.84
35	32.97
50	47.10
65	61.23
70	65.94
85	80.07
110	103.62
125	117.75
140	131.88
155	146.01

Figure 7.

(a) A schematic representation of the traverse speed configuration during HSLW and (b) an image of the rotary fixture designed for the HSLW experiments.

where V is the velocity, π is 3.141, D is the weld diameter and N is the number of rotations of rotary welding fixture. The applied welding speeds ranged from 5 to 120 m/min.

Results

Figure 8(a)–(e) illustrates the Nd:YAG laser welded images of the tin-plated thin sheet steels at varying speeds. As shown in Figure 8(a)–(e), humping phenomenon exists to some extent. In Figure 8(a), the optical image represents the cross section of the sample welded at 10 m/min. The depth of penetration at this position as shown in Figure 10 is about 50%. Then, as the speed increases, the depth of penetration reduces due to the lack of thermal energy induced into the material. Using the Nd:YAG laser–alone technique, beyond 70 m/min, humping was not found and only a small heat-affected zone (HAZ) was to be seen with penetration of about 5%. It is obvious that this affect would occur since the increase in the traverse speed would result to a decrease in the thermal energy induced into the material, which thereby reduces the laser–material interaction time and consequently reduces the depth of penetration. The results of the Nd:YAG laser–alone weld samples complied to the results of a few previous researchers that stated that humping resulted at lower speeds during welding of the thin sheet steels and with increasing speed the gradient of the humps reduced.

Figure 8.

Optical image of the Nd:YAG laser–alone welded sample in (a) at 10 m/min, (b) at 30 m/min, (c) at 50 m/min, (d) at 60 m/min and (e) at 70 m/min using a spot size of 1 mm, laser power of 2 kW and argon assist gas with a flow rate of 20 L/min.

The results in Figure 9 revealed that by applying the PALW technique, the humping effect was much reduced due to the extra heat source. The use of plasma torch enabled an increase in temperature at the laser–material interaction zone and allowed larger melt zone to occur in comparison to the laser–alone welded samples. The melt zones upon freezing resulted to a smaller gradient. Although penetration reduced with increase in the weld speed, there was still some effect of the PALW up to 98 m/min. This in comparison to the finding by Blundell et al. using the PALW welding process was similar.⁹ However, it was possible to obtain higher speed using the set-up of the study herein since a rotary axis welding fixture was employed. On the other hand, due to the lack of dynamic response of the motion system employed in the work of Blundell et al., the maximum weld speed achieved with only a few welding discontinuities was 90 m/min.

Figure 9.

Optical images of the PALW using the Nd:YAG laser in (a) at 10 m/min, (b) at 40 m/min, (c) at 77 m/min, (d) at 30 m/min, (e) at 65.94 m/min and (f) at 98.43 m/min using 2 kW laser power, 1 mm spot size and a voltage supply of 40 V through the plasma torch and argon assist gas with a flow rate of 20 L/min.

Figure 10 shows a speed versus penetration graph for the three welding processes used in this study. The curves in Figure 10 shows that the penetration of the PALW samples is much greater followed by the CO₂ and then the Nd:YAG laser welding technique. However, due to the laser beam quality factor M² of the CO₂ laser being a high-quality beam resulted to a much concentrated beam that exhibited large impact, which in turn during surface speed of 33 m/min and below produced considerable dropouts where the beam had created a hole into the tin-plated thin sheet steel. This effect was also seen with the other two welding techniques, however, to a much smaller extent at low speeds. As the traverse speeds increased, a reduction occurred not only in the penetration but also with the humping phenomenon.

Figure 10.

Curve shows the weld penetration with increasing speed when using the CO₂ and Nd:YAG laser alone and PALW.

Figure 11 shows the results of the CO₂ laser welded samples of the tin-plated thin sheet steels. In comparison to the results obtained by the Nd:YAG laser alone and to those of the PALW samples, it was found that the gradient of the humps observed was much steeper at lower speed and then became smaller. In any case, the humping gradient was yet much greater than with the previous two (Nd:YAG laser alone and PALW) techniques. At the same time, the penetration of the CO₂ laser beam began to deteriorate. This could be attributed to the absorption of the tin-plated steel being somewhat lower than that of the Nd:YAG laser and the PALW. Nevertheless, the absorption of different wavelengths by the tin-plated thin sheet steel is yet to be confirmed by an experimental study in future investigations. What is more, the causes for the humping effect could be wavelength dependent since the Nd:YAG laser produced smaller humps than the CO₂ laser. Having said this, further study is also required using more than just two wavelengths to conclude as to what extent the wavelength contributes to affect the humping phenomenon. It can be gathered that the characteristics of the laser beams used have first caused this effect to take place, whereby the CO₂ laser exhibiting a much higher beam quality would have induced higher thermal energy focused in a localized surface area. This caused an instant bulging of the melted material and created significantly large peaks in the form of humps while the Nd:YAG laser consisted of a top-hat beam. This distributed over a larger surface area and relaxed the hump to take place. Moreover, the PALW technique not only exhibited a broader beam but also applied an extra heat source, which trailed after the Nd:YAG laser, elongated the cooling rate and allowed the weld pool to solidify at a considerably slower rate than the CO₂ and the Nd:YAG laser–alone weld pool. This maximized the possibility of distributing the melt-pool evenly and produced a much relaxed humping gradient as well as penetrating much further into the material unlike the Nd:YAG and CO₂ laser–alone welding techniques.

Figure 11.

Optical image of the CO₂ laser welded samples (a) at 20 m/min, (b) at 42 m/min, (c) at 60.52 m/min, (d) at 80 m/min, (e) at 90 m/min and (f) at 105 m/min by applying 2 kW laser power, spot size of 1 mm and argon assist gas at a flow rate of 20 L/min.

Having observed the results obtained from the experiments in this study, it is an interest to first understand the laser–material interaction in order to know how and why the humping effect was caused so that it can be either avoided or eliminated in future investigations. From the optical images in Figures 8, 9 and 11, it can be seen that humping is considerably high during lower traverse speed welding and with increase in welding speed; the gradient of the humps began to slack. This change in the weld profile resulted from the actual melt-pool starting-off as a keyhole weld where the energy of the laser has time to absorb into the material surface and penetrate through the subsurface and the cross section while the speed increases, the keyhole of the weld becomes smaller as conduction welding begins to take effect with the laser beam not having enough time to absorb into the material. This allowed the humping effect to relax with increasing speed.

Furthermore, humping is also caused by a pinch effect on the longitudinal direction of the weld bead. The surface tension caused within the weld bead is due to the Marangoni convection where instability in pressure causes the regime of high and low curvature, which in turn could also lead to the humps.

During cooling of the weld, rapid solidification takes place since the high-power density of the laser was focused on a small surface area not allowing enough time for the surface and subsurface to absorb the thermal energy. On account of this, the molten material cooled much faster than the amount of time required for it to settle. This was also stated by Gratzke et al.⁴ during conventional welding as solidification in the range of 10–100 ms tends to occur. In any case, the solidification of the laser beam would be in the same range or faster due to its high monochromaticity, coherence and unidirectionality, which will cause similar effects to what Gratzke et al. justified. However, further work into accurate measurement of the cooling rate during HSLW of thin sheet steel is suggested herein, which could justify this effect further.

Discussion

Upon considering the laser–material interaction found in this investigation and suggestions from previous research with regard to eliminating the hump, it can be further suggested that in order for the humping effect to be minimal and almost eliminated, it is necessary that the weld bead length (in its cross section) was broadened. From this work, it was found that the technique, which produced the best result with smaller and relaxed hump gradients, was first the PALW and then the Nd:YAG laser alone. This was because the Nd:YAG laser had a lower beam quality in comparison to that of the CO₂ laser, which distributed a concentrated laser beam. In addition, the PALW produced the smoothest humps because of the additional heat source applied. This broadened the width and shortened the cross-sectional height of the laser welds. Having seen this, it can then be suggested that it is possible to minimize humping and even eliminate it by first applying a laser beam with slightly low quality (possibly a flat head, top-hat beam), which has a spread energy distribution with an additional heat source. In particular, the PALW technique acts as an elongated cooling tool for the weld, while the laser beam already heats-up the material and causes it to melt. At the same time, the plasma torch elongates the rapid cooling effect and aids in distributing the weld more equally so that the humping gradient is much smaller. This concept goes well with previous researcher’s statement of broadening the length-to-width ratio of the weld bead to eliminate the humping phenomenon.^3,4,9,10 By observing the optical images of all the samples within this study, it can be denoted that humping occurs with most samples to various extent. However, undercut was not apparent. Drop out of the weld pool was evident during the CO₂ laser–alone welding experiments, particularly, at low welding speeds, but was not consistent during the Nd:YAG laser–alone and PALW experiments (see Figure 11(a)). To investigate the occurrence of a ropey bead, a topographical analysis is required in future investigations, which would confirm this effect. However, from analysing the cross section of various samples as previously shown in Figures 8, 9 and 11, it can be postulated that ropey bead would be formed throughout the weld seam.

To compare the mechanical strength of the welds, a tensile strength test was conducted. The RSWed samples comprised an average UTS of 998.84 MPa before it failed. This was the highest as one can see in Figure 12 from the set of welded samples that were tested. The average UTS measured for the PALW samples was 694.22 MPa, for the Nd:YAG laser–alone sample was 565.20 MPa and for the CO₂ laser–alone sample was 492.42 MPa.

Figure 12.

Average UTS of MSW, PALW and both the Nd:YAG and CO₂ laser–alone welded samples of the FC.

For the MSWed sample, it was interesting to note that each sample fractured from either side of the weld bead rather than fracturing from the weld itself unlike the others. This proved that the MSW strength was much higher than that of the parent material itself, whereas for the other samples the fracture in turn occurred from the weld seam itself. Owing to the UTS value of the MSWed samples, it was indicative as to what mechanical strength the laser welded seam should comprise. However, all the laser welded samples were of much lower UTS than the RSW as seen from Figure 12. Having said that the PALW samples were of higher mechanical strength than that of the Nd:YAG and the CO₂ lasers. Following was the Nd:YAG and then the CO₂ laser welded samples. This result correlated with the level of defects found in each of the samples and corresponded to the penetration depth. Since the PALWs comprised the highest penetration and lowest level of welding defects than that of the Nd:YAG and the CO₂ laser, it was therefore clear that this sample comprised a slightly better mechanical strength followed by the Nd:YAG laser and finally the CO₂ laser, which comprised much higher hump gradients. Owing to this, it was obvious that the quality of the laser welded samples in comparison to the conventional MSWed samples comprises low mechanical strength within the laser welded seam. This was not sufficient enough to similarly match that of the RSW joint, since it had insufficient weld quality, as a result of the humping onset. Therefore, it is suggested that future studies could entail a similar method for comparing the weld strength of relevant samples to the conventional welding process already in place.

For storage of food products, it is extremely undesirable if the weld seam of baked beans can or a tomato puree, for example, would consist of weld defects. This is because the FCs store sufficient amount of liquid, which would be subject to leaking contamination, oxidation and exposure to corrosive metal. In addition, it is important that gases such as Ar and N₂ are used to preserve food inside the FCs, so that the laser weld is free from any defects to prevent the preservative gases to escape. This will otherwise contaminate the food product considerably. Therefore, welding defects, namely, brittle cracking in various forms, distortion, nuggets, porosity, lack of penetration and fusion, inclusions, humping and undercut, and the occurrence of a ropey bead will reject the weld, while the FC is passed through the pressure testing procedure. Thus, from a metallurgical aspect, the welded material must avoid the above defects and adhere to the appropriate mechanical strength and the quality standards set for canned food packaging products.

Furthermore, it is suggested that for future, experiments should be carried using butt weld configuration. This will enable one to understand how the humping behaviour may occur since humping has been observed during high-speed welding only on bead-on-plate samples. In addition, a dimensional analysis of the weld bead is also suggested, which will further enable one to know the dimension of the welding discontinuities and in turn could assist using a certain spot size of the laser as well as the intensity of the plasma torch to achieve the perfect height-to-length ratio in order to minimize and possibly eliminate humping. What is more, further work is also required on the suitability of the laser system by comparing with the current MSW machinery used for manufacturing the three-piece FCs. Further research is also suggested on the suitability of a correct motion system for the laser, which comprises a faster dynamic response. This will enable the welding process to achieve high speed since the work herein employed a rotary axis welding fixture, which will not compliment a butt weld configuration when welding in a linear motion. Investigating the suitability of the most appropriate motion system within a production set-up will require lengthy experimental studies once the laser welding process free from the aforementioned welding discontinuities is defined.

Moreover, a finite element model (FEM) using computational means could also be adopted to model the humping phenomenon at various speeds, which will allow the prediction of the onset of the humps with respect to temperature and time at any traverse speed and other laser parameters employed. This will enable one to also understand the perfect energy density and the associated laser parameters as well as the most ideal traverse speed to be used for the HSLW of three-piece FC bodies.

Conclusion

This article was focused first on a case study of a UK-based FC manufacturer and its production system and the current process employed – rationalizing the implementation of the new HSLW process. Second, an experimental investigation was also conducted using a Nd:YAG, CO₂ and a PALW process complimented by a specially designed rotary axis welding fixture to further investigate the technical feasibility of the aforementioned process. The combined study revealed the following conclusions:

Upon the high demand for manufacturing three-piece FCs, mass production of the FC has become a necessity in order to meet the high production requirements.

Employing the HSLW process could result in considerable savings on material utilization and lead-times. This in turn enables higher production capacity, production outputs, reduced labour, maintenance cost and overheads leading to cost savings and profit for the packaging can manufacturers not only in Europe but also around the world.

Experimental study showed that a regime of reduction in humping at increasing welding speeds resulted when all three laser techniques were applied. However, the CO₂ laser welded sample showed humping to be much steeper and narrow due to its high beam quality in comparison with the Nd:YAG laser–alone and the PALWed samples.

The Nd:YAG laser–alone samples showed humping effect to be much relaxed in comparison to the CO₂ laser–alone welded samples. The PALW technique showed a much relaxed hump, and welding speed of up to 98 m/min showed some level of penetration within the tin-plated thin sheet steel.

To avoid the humping effect, it is required that the melt-pool balances out evenly and a thicker weld bead is produced so that even spread of molten layer occurs. This is ideal with the aid of the PALW technique, which offers an extra heat source.

Footnotes

Acknowledgements

The author would like to thank Mr Trevor Johnson and Dr Houzheng Wu for providing technical guidance and to all those associated with this research from the Advanced Joining Research Centre, as previously known, at Coventry University (UK).

Declaration of conflicting interests

The author declares that there is no conflict of interest.

Funding

This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.

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