Friction stir welding (FSW) was investigated as a solid-state processing technique for joining recycled high-density polyethylene (HDPE) plates manufactured from post-consumer bottle caps and previously used as construction formworks. The effects of rotational and traverse speeds on weld densification and mechanical performance were systematically evaluated to establish processing–structure–property relationships in recycled thermoplastic systems. Mechanical characterization included ultimate tensile strength (UTS), density, and Shore D hardness, and load–elongation response, while surface and cross-sectional morphological analyses were used to assess void formation, material consolidation, and weld integrity. Optimal conditions were identified at 1600 rpm with 12.5 mm/min and 1000 rpm with 25.0 mm/min, resulting in UTS values up to 4.7% higher than those of the unwelded material. Morphological analysis revealed that excessive heat input at elevated rotational speeds promoted void formation and reduced joint integrity, whereas moderate parameters enhanced material consolidation and produced defect-free welds. A strong correlation among UTS, density, and hardness confirmed their suitability as indicators of weld quality. These findings demonstrate that controlled FSW parameters can significantly improve the structural performance of recycled HDPE, supporting their use in sustainable construction-related and non-critical engineering applications within circular economy strategies.
JungHShinGKwakH, et al.Review of polymer technologies for improving the recycling and upcycling efficiency of plastic waste. Chemosphere2023; 320: 138089. https://doi.org/10.1016/j.chemosphere.2023.138089
2.
EslamiSFarahaniBVTavaresPJ, et al.Fatigue behaviour evaluation of dissimilar polymer joints: friction stir welded, single and double-rivets. Int J Fatig2018; 113: 351–358. https://doi.org/10.1016/j.ijfatigue.2018.04.024
3.
PereiraMARGalvãoICostaJD, et al.Joining of polyethylene using a non-conventional friction stir welding tool. Materials2022; 15(21): 7639. https://doi.org/10.3390/ma15217639
4.
KhalafHIAl-SaburRDemiralM, et al. The Effects of Pin Profile on HDPE Thermomechanical Phenomena during FSW. Polymers2022; 14(21): 4632. https://doi.org/10.3390/polym14214632
5.
PereiraMARAmaroAMReisPNB, et al. Effect of friction stir welding techniques and parameters on polymer joint efficiency—a critical review. Polymers2021; 13(13): 2056. https://doi.org/10.3390/polym13132056
6.
KhalafHI. Elucidating the effect of FSW variables on HDPE sheets using grey integrated with fuzzy. JOM2023; 75(7): 2684–2692. https://doi.org/10.1007/s11837-023-05839-x
7.
EslamiSMirandaFMourãoL, et al. Polyethylene friction stir welding parameter optimization and temperature characterization. Int J Adv Manuf Technol2018; 99(1): 127–136. https://doi.org/10.1007/s00170-018-2504-x
8.
RazaKShamirMQureshiMKA, et al.On the friction stir welding, tool design optimization, and strain rate-dependent mechanical properties of HDPE–ceramic composite joints. J Thermoplast Compos Mater2018; 31(3): 291–310. https://doi.org/10.1177/0892705717697779
9.
HapuwatteBHartwellATriebeMJ, et al.Recovery pathway assessment of recycled HDPE for circular economy: shorter-life vs longer-life products. Proced CIRP2024; 122: 366–371. https://doi.org/10.1016/j.procir.2024.02.011
10.
NugrohoAWArifinAImbaragaMR, et al.The effect of tool rotational speed and welding configuration on the mechanical properties of high-density polyethylene (HDPE) plate friction stir welded joint. Semesta Teknika2024; 27(1): 66–80. https://doi.org/10.18196/st.v27i1.22182
11.
KumarRSinghRAhujaIPS. Friction stir welding of thermoplastic composites: a review. J Thermoplast Compos Mater2019; 32(10): 1377–1406. https://doi.org/10.1177/0954406219848465
12.
AhmedHvan ToorenMJLJusticeJ, et al.Investigation and development of friction stir welding process for thermoplastics. J Thermoplast Compos Mater2019; 32(6): 812–833. https://doi.org/10.1177/0892705718785676
de SouzaJABenítez PonsFd’AlmeidaJRM. Tensile behavior of post-consumer recycled high-density polyethylene at different strain rates. Polym Test2013; 32(2): 338–342. https://doi.org/10.1016/j.polymertesting.2012.11.007
15.
GobettiARamorinoG. Application of short-term methods to estimate the environmental stress cracking resistance of recycled HDPE. J Polym Res2020; 27(11): 353. https://doi.org/10.1007/s10965-020-02332-w
BurgerJLloret-FritschiEAkermannM, et al.Circular formwork: recycling of 3D printed thermoplastic formwork for concrete. Technol Archit Des2023; 7(2): 204–215. https://doi.org/10.1080/24751448.2023.2245724
18.
KhanMAAhmadSAliM, et al.Optimization of friction stir welding parameters for enhanced mechanical properties of AISI 1018 carbon steel. Sci Rep2025; 15: 16668. https://doi.org/10.1038/s41598-025-16668-0
19.
MugadaKKAdepuK. Effect of knurling shoulder design with polygonal pins on material flow and mechanical properties during friction stir welding of Al–Mg–Si alloy. Trans Nonferrous Met Soc China2019; 29(11): 2281–2289. https://doi.org/10.1016/S1003-6326(19)65134-4
20.
LaskaASadeghiBSadeghianB, et al.Temperature evolution, material flow, and resulting mechanical properties as a function of tool geometry during friction stir welding of AA6082. J Mater Eng Perform2023; 32(23): 10655–10668. https://doi.org/10.1007/s11665-023-08671-1
21.
TejaPJJainR. Effect of external heat assistance on the weld quality of AA2024 aluminium alloy during friction stir welding. Manuf Technol Today2023; 22(1): 20–24. https://doi.org/10.58368/MTT.22.1.2023.20-24
22.
IftikharSHCherupurakalNKrishnapriyaR, et al.Friction stir spot welding of recycled scrap thermoplastics. Int J Lightweight Mater Manuf2024; 7(6): 838–848. https://doi.org/10.1016/j.ijlmm.2024.06.003
23.
GoswamiNKPalKBisoyiRK, et al.Monitoring of weld bead profile in friction stir lap welding for polycarbonate sheets using wavelet packets of power signal. Arabian J Sci Eng2025; 50(4): 2217–2249. https://doi.org/10.1007/s13369-024-09057-8
24.
ASTM International. Standard Test Method for Tensile Properties of Plastics (ASTM D638-22). ASTM International, 2022. https://doi.org/10.1520/D0638-22
25.
KumarSRoyBS. Novel study of joining of acrylonitrile butadiene styrene and polycarbonate plate by using friction stir welding with double-step shoulder. J Manuf Process2019; 45: 322–330. https://doi.org/10.1016/j.jmapro.2019.07.013
26.
NathRKJhaVMajiP, et al.A novel double-side welding approach for friction stir welding of polypropylene plate. Int J Adv Manuf Technol2021; 113: 691–703. https://doi.org/10.1007/s00170-021-06602-9
27.
LambiaseFGrossiVPaolettiA. Advanced mechanical characterization of friction stir welds made on polycarbonate. Int J Adv Manuf Technol2019; 104: 2089–2102. https://doi.org/10.1007/s00170-019-04006-4
28.
HajidehMRFarahaniMAlaviSAD, et al.Investigation on the effects of tool geometry on the microstructure and the mechanical properties of dissimilar friction stir welded polyethylene and polypropylene sheets. J Manuf Process2017; 26: 269–279. https://doi.org/10.1016/j.jmapro.2017.02.018
29.
VijendraBSharmaA. Induction heated tool assisted friction-stir welding (i-FSW): a novel hybrid process for joining of thermoplastics. J Manuf Process2015; 20: 234–244. https://doi.org/10.1016/j.jmapro.2015.07.005
30.
RudrapatiR. Effects of welding process conditions on friction stir welding of polymer composites: a review. Composites Part C: Open Access2022; 8: 100269. https://doi.org/10.1016/j.jcomc.2022.100269
31.
HajidehMRFarahaniMRamezaniNM. Reinforced dissimilar friction stir weld of polypropylene to acrylonitrile butadiene styrene with copper nanopowder. J Manuf Process2018; 32: 445–454. https://doi.org/10.1016/j.jmapro.2018.03.010
32.
AhmedSSahaP. Selection of optimal process parameters and assessment of its effect in micro-friction stir welding of AA6061-T6 sheets. Int J Adv Manuf Technol2020; 106: 3045–3061. https://doi.org/10.1007/s00170-019-04840-6
MishraDSahuSKMahtoRP, et al.Friction stir welding for joining of polymers. In: Strengthening and Joining by Plastic Deformation. Springer, 2018, pp. 123–162. https://doi.org/10.1007/978-981-13-0378-4_6
35.
Sheikh-AhmadJYDeveciSAlmaskariF, et al.Effect of process temperatures on material flow and weld quality in the friction stir welding of high-density polyethylene. J Mater Res Technol2022; 18: 1692–1703. https://doi.org/10.1016/j.jmrt.2022.03.089
36.
AhmadBKhanZASiddiqueeAN, et al. Thermomechanical modeling of material flow and weld defects during friction stir welding of HDPE. Polymers2023; 15(15): 3230. https://doi.org/10.3390/polym15153230
37.
WuMSCenteaTNuttSR. Compression molding of reused in-process waste: effects of material and process factors. Adv Manuf Polym Compos Sci2018; 4(1): 1–12. https://doi.org/10.1080/20550340.2017.1411873
38.
ClarkEBleszynskiMGordonM. High-pressure compacted recycled polymeric composite waste materials for marine applications. SN Appl Sci2022; 4: 36. https://doi.org/10.1007/s42452-021-04908-7
39.
WangXXiaoYShiL, et al.Revealing the mechanism of tool tilting on suppressing the formation of void defects in friction stir welding. J Mater Res Technol2023; 25: 38–54. https://doi.org/10.1016/j.jmrt.2023.05.184
40.
DuYMukherjeeTDebRoyT. Conditions for void formation in friction stir welding from machine learning. npj Comput Mater2019; 5: 68. https://doi.org/10.1038/s41524-019-0207-y
41.
SimõesFRodriguesDM. Material flow and thermo-mechanical conditions during friction stir welding of polymers: literature review, experimental results and empirical analysis. Mater Des2014; 59: 344–351. https://doi.org/10.1016/j.matdes.2013.12.038
42.
SadeghiBSadeghianBTaherizadehA, et al. Effect of porosity on the thermo-mechanical behavior of friction-stir-welded spark-plasma-sintered aluminum matrix composites with bimodal micro- and nano-sized reinforcing Al2O3 particles. Metals2022; 12(10): 1660. https://doi.org/10.3390/met12101660
43.
SongZXiaoKXiaoS, et al.Effect of porosity on the tensile strength and micromechanisms of laminated stitched C/C–SiC composites. Mater Des2024; 247: 113429. https://doi.org/10.1016/j.matdes.2023.113429
44.
Ramesh BabuSHudgikarSRKPoornachandra SekharY. Experimental investigation on friction stir welding of HDPE reinforced with SiC and Al and Taguchi-based optimization. In: VorugantiHKumarKKrishnaP (eds) Advances in applied mechanical engineering. Lecture Notes in Mechanical EngineeringSpringer, 2020, pp. 929–939. https://doi.org/10.1007/978-981-15-1201-8_99
KumarRSinghRAhujaIPS, et al.Weldability of thermoplastic materials for friction stir welding: a state-of-the-art review and future applications. Composites, Part B Eng2018; 137: 1–15. https://doi.org/10.1016/j.compositesb.2017.10.039
ZhouLZhangZLiY. Effect of process parameters on mechanical properties and microstructure of friction stir welded HDPE sheets. J Mater Res Technol2020; 9(3): 3091–3100. https://doi.org/10.1016/j.jmrt.2020.01.064
