Application of solid-state joining to investigate the Bi-polymer (PEHD-PP) joints performance

doi:10.21203/rs.3.rs-4933590/v1

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Application of solid-state joining to investigate the Bi-polymer (PEHD-PP) joints performance

https://doi.org/10.21203/rs.3.rs-4933590/v1

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Over the past ten years, the use of polymer products has taken a huge step forward in all sectors. The multiple properties of polymer types are positive factors that increase their scope of use in all aspects of daily life. Obtaining an ideal product requires assembling the polymers as needed. Friction welding is considered the safest way to connect polymer parts without any additional means. Welding two parts of the same polymer depends on controlling the effective factors in this process, which are the rotation speed, the feed, the penetration force, and the residence time of the spot weld. In this article, pneumatic energy is used to weld HDPE to PP by friction stir (linear and point) by selecting the ideal welding agents that meet the mixture of these two types of polymers and respond to pneumatic energy. The response surface method (RSM) and analysis of variance (ANOVA) were used to analyze the thermal results. The tensile test also has a role in deducing the mechanical properties of the welded area resulting from the mixture of these two types of polymers.

Portable FSW/FSSW of Polymers

FSW by pneumatic source

Response surface method

Analysis of Variance

For centuries, natural polymers have been taken from plants and animals, such as wood, rubber, cotton, wool, leather, and silk, as well as proteins, enzymes, starches, and cellulose, which play an important role in the biological and physiological processes of plants and animals. The tools of modern scientific research have made it possible to determine the molecular structure of this group of materials and to develop numerous polymers synthesized from small organic molecules. Polymers represent an increasingly important class of materials [1]. They compete with traditional materials, such as metals and their alloys, advanced composites and ceramics. Some of the attractive properties of polymers are low density, combined with increasingly sophisticated thermomechanical properties, low production cost, very specific functional properties such as energy transduction and information recording, and compatibility with contemporary manufacturing processes brings forward polymer as demanding material of the future Polymer materials are currently manufactured by extrusion and injection. The production of larger, and more complex parts from polymers has led to an increased need for assembly, particularly of thermoplastic materials [2–6]. Precise joints for polymers can be produced by using adhesives, mechanical fastening, and welding. However, joints made by adhesives possess some disadvantages such as, requirement of surface treatment to increase adhesion, brittleness after drying, incompatible expansion coefficient and sometimes chemical resistance. Further, plastics can be bonded by mechanical fastening, but there is always a problem with waterproofing the joints and cleaning the surfaces during service condition. Furthermore, both adhesive and mechanical joining can be time consuming and involve higher manufacturing costs. Thus, joining polymers/plastics at atomic level can be the most promising process as it is carried out mostly by welding [9]. Likewise other material joining processes the welding of polymers can be broadly divided into two parts, fusion welding and solid state joining. Plastic welding in case of either solid state or fusion is carried out in three stages: (a) heating the welding surfaces to a viscous or molten state (b) bond formation by application of pressure and (c) maintaining pressure until hardening [10, 13]. Some of the very specific welding techniques which are used to join polymers/plastics are laser welding, ultrasonic welding and friction stir welding, hot gas, and hot plate welding. Although fusion welding is more economical, it induces high heat input during the joining which can induce softening inside the joints and nearby areas causing premature failures even below the strength of the base materials. However, solid state joining mostly joins the material by plasticizing the materials below melting temperature, thus avoiding all the defect generation related to solidification. Compared to fusion welding methods, solid state joining has many advantages. These do not require shielding gases, additional wiring, or personal protection measures and are environmentally friendly. Welding defects such as porosity, gas cavities, inclusions, etc. on welding are not observed [15]. Among all other solid state joining Friction stir welding (FSW), is one of the most effective processes as it can join materials in any position. [7]. This method was first applied to aluminum alloys [14–17], and in recent years it is being used to join polymers (LDPE, HDPE and PP) [18–23]. The results showed that it was possible to weld unfilled thermoplastic by the friction stir welding method, the parameters of pin diameter, rotation speed, and welding speed, have significant effects on the performance of the welded joint. Although there have been many studies on friction stir welding in this field, there is still a need to study its effects on surface quality, temperature change, mechanical and microstructure [26–27]. In addition, the relationship between welding parameters and mechanical and microstructural properties in friction stir welding is not yet properly understood. Meanwhile, the process of welding different polymers together is still in its infancy. Saeedy and Givi and al. [30], varied both the tool rotational and welding speeds ranging from 1000 to 1800 rpm and 12–20 mm/min, respectively, at a tool tilt angle of both 1º and 2 º. Among various combinations of parameter only one single set of parameters generated weld strength of 75% of that of the base material. These results pointed out the fact that the influence of process parameters on welding plays a vital role. Aydin and al. [31] performed single pass butt welding with pre-heating of the base material by using similar kind of process parameters and tool as suggested by the Arici and al. and Selale and al. [32]. However, they reported that pre-heating ensures adequate heating even at low rotation speeds resulting enhanced weld quality. Further they also concluded that Welds with surface morphology and global properties were also improved by pre-heating. Squeo and Quadrini and al. [33] reported enhancement of weld quality by using pre heated tool and base material. However, in previous researches most of the welds were carried out by big conventional welding machines, which are mostly bulky in size and lags portability. Hence, in this present study a novel investigation of polymers joined by a portable FSW set up has been carried out. The article investigated the ability to FSW ed two different types of polymers (HDPE and PP) operated by a pneumatic source, in linear and spot condition. RSM and ANOVA were used to improve process parameters, as well as tool positioning relative to the melt zone for appropriate heat input for each polymer. This study relates to generated heat, voltage and resistance.

The portable pneumatic system was installed to power the aluminum friction stir welding tool. The tool was installed in the rotary pilot. The Schneider Super Master 400-10-60 W compressor was used to develop and control the rotation speed and generate friction heat for joining the polymer plates. This experiment made it possible to use à speed margin of between 800 and 1400 rpm thanks to the specifications of the compressor used and a welding speed (advance) between 25 and 30mm/min by the machine table in the case of linear welding and a holding time margin of 15 and 35 sec for the case of spot welding. The FSW tool (in AA7075 aluminium) has a 14 mm diameter shoulder, a 3 mm diameter pin and a 3.75 mm length, while the opposite end of the tool has an 8 mm diameter neck which can be firmly fixed in the chuck (as shown in Figure 1). Total tool length is 65mm.

A holder for the pneumatic rotary actuator was used in this process. Important operations such as the rotation speed of the tool are selected using a stroboscope, while the crossing speed and penetration duration are set on the machine in the case of linear and spot welding, respectively.

resulting assembly after welding of the two polymers HDPE and PP by the friction stir welding process in linear mode and point mode.

Friction stir welding (FSW) and spot welding (FSSW) were studied to improve the welding temperature of copolymer (HDPE-PP). Weld zone temperatures were measured via an infrared thermometer for real-time monitoring. The optional maximum temperatures of the copolymer welding zone for each experiment for FSW and FSSW are shown in Table 4.

Table .1. Measured temperature for Bi-polymer welding zone during FSW and FSSW prices

		FSW			FSSW
#	Rotational speed (rpm)	Traversig velocity (mm/mi)	Temperature (°C)	Rotational speed (rpm)	Dwell time (sec)	Tempe rature (°C)
01	800	15	99	800	30	83
02	800	25	103	800	35	84
03	800	30	106	800	40	92
04	1000	15	102	1000	30	94
05	1000	25	107	1000	35	105
06	1000	30	110	1000	40	106
07	1250	15	105	1250	30	100
08	1250	25	109	1250	35	104
09	1250	30	111	1250	40	108

2.1. Response Surface Methodology (Rsm)

Response surface methodology (RSM) [9-10] is a simple and effective method in industrial processes, because it allows finding the optimal solution. This method uses statistical techniques and mathematics to design a solution that optimizes the outcome based on the factors that make up the phenomenon.

The expected response is Y, a is a constant, and ai, a_ii, and I_ij are the linear, quadratic, and intercept coefficients. Indeed, the I_ij is the interaction between the factors and the term e_i is the difference between the experimental value and that given by the polynomial. After performing nine experiments, a matrix can be developed as shown below:

The coefficients are found by the Eq. 3.

Since this work studies two modes of linear and spot friction welding, we apply Formula (3) to table (1) to extract parameter coefficients in order to formulate polynomials describing FSW and FSSW.

Table .2. estimated values of the coefficients for FSW weld.

Coef	a₀	a₁	a₂	I₁₂	a₁₁	a₂₂
value	106.01	2.90	3.42	-0.42	-1.16	0.37

The polynomial of FSW can be written as follow :

Table .3. estimated values of the coefficients for FSSW weld.

Coef.	a₀	a₁	a₂	I₁₂	a₁₁	a₂₂
value	87.89	9.61	10.98	-0.47	-7.57	-1.12

The polynomial of FSSW can be written as follow:

2.2. Influence of factors in FSW

The influence diagrams of the main factors in FSW were presented in the fig.5. The graphs show that increasing the rotation speed leads to an increase in the temperature generated between the aluminum tool and the dissimilar polymer welded parts, which means that pneumatic energy maintains the same welding pattern as mechanical energy.

The curves in fig.5 (a, b) show that within the specified range of tool rotation speed per minute and tool feed speed per minute, the temperature gradually increases until it reaches its optimum value 112 °C, to welding PEHD withe PP at 1250 rpm and 30 mm/min.

The curves in Figure 6 confirm that the individual effect of the two main factors is the same as their combined effect. The optimal welding temperatures for these two types of polymers are when the tool rotation speed is limited to between 1100 and 1250 rpm and the welding speed is between 28 and 30 mm/min.

2.3. Influence of factors in FSSW

In the case of spot welding, the rotational speed continues to remain the FSSW parameter, and the welding velocity is replaced by the dwelling time of the tool on the plates.

Rotational speed remains the most influential factor on temperature during spot welding, as the heat generated reaches 100 degrees Celsius at a rotational speed of 1100 rpm. The temperature begins to decrease if the rotational speed exceeds this value. Regarding the residence time of the tool, its relationship with temperature is a relative relationship. As the residence time increases, the temperature increases, taking into account the melting point.

In the case of spot welding, the effect of the interaction between the rotation speed of the tool and the penetration time of the tool into the material (welding time) shows that the temperature required for welding HDPE with the pp is ensured at a rotation speed equal to 1100 rpm and a residence time within the range of 25 to 30 seconds.

The portable pneumatically operated friction stir welding tool was used to join HDPE with PP. Figures 3 and 4 shows that the left side represents the upper and rear joints during pneumatic FSW, while the right side represents the pneumatic FSSW process; the FSW and FSSW processes have produced encouraging results. As crack opening on the polymer surface usually occurred during bending tests [18-19], making it less accurate therefore tensile tests were carried out on FSW and FSSW welded specimens using An Instron 3400 universal traction machine (Instron, USA) at room temperature according to ASTM D638, as shown in Figure 8.

Figure 10 shows the behavior of the maximum tensile load for both samples. Most polymers cannot be subjected to tensile testing. The curves revealed that the weld joint obtained after welding PEHD and PP with FSW is less elongated than the FSSW sample. The FSSW sample showed an increase in elongation up to 45% [24-25]. Tensile test [31] plots for BI-polymer joints by FSW showed a linear relationship up to 800N loads with 9% elongation was recorded (Figure 10a). Subsequently, the HDPE-PP FSW samples showed ductile behavior until failure. This new source (pneumatic) gave a combined efficiency of around 20%. However, the efficiency of the Friction Stir Welding seal is approximately 60% compared to Bi-polymer when manufactured on a suitable professional machine [28].

In this article

• the conventional Friction Stir Welding has been substituted by a portable pneumatic-powered Friction Stir Welding to enable the tool to rotate for Bi-polymer joining and repair. Besides being a portable facility, it is capable of performing on-site welding on an as-is-where-is basis on any route.

• To analyze the results, the response surface method (RSM) was used. The FSW and FSSW results indicate that excellent weld quality was achieved when the tool rotation speed was less than 1200 rpm. A welding speed of less than 30 mm/min (for FSW) and a Dwell time of 30 s were adequate to achieve high-quality welding.

• Superior welding quality was ensured by a temperature of 100°C in FSSW processes and 112°C in FSW processes. The results of the tensile test was very encouraging. Even though the flexural load of the manual pneumatic Friction Stir Welding is still lower than the conventional Friction Stir Welding for polymer sheets.

• the tool made of aluminum accomplishes friction stir welding of polymers successfully because of their lightness, melting temperature higher than that of polymer; price and surface condition of the weld joint resulting from the aluminum tool is very good.

• after encouraging results obtained by the use of the pneumatic source it can be called in the future PFSW (pneumatic friction stir welding)

Funding : The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.

Competing Interests The authors have no relevant financial or non-financial interests to disclose.

Author Contributions: Mohamed Serier: Conceptualization, Methodology. Ibrahim Albaijan: Conceptualization, Investigation. Abdalrhman Milad: Conceptualization.".

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Application of solid-state joining to investigate the Bi-polymer (PEHD-PP) joints performance

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1. Introduction

2. Materials and methods

3. Mechanical performance

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