In the early 15th Century the Yongle Emperor of Ming China decided to increase the power of China's naval presence in the South China Sea and the Western Ocean (Indian Ocean). Chinese Great Junks had plied the seas for centuries trading in the Indies, South Asia, Arabia and East Africa. Now the Yongle Emperor sent out great fleets under the command of his most trusted Eunuch, Zheng He. In OTL Zhen He made seven voyages. Six of these were under the Yongle Emperor. But this all ended when the Yongle Emperor died in 1424 after hunting Tatars raiders into the Gobi desert, failing to capture them, becoming depressed and suffering a stroke. His son, the Hongxi Emperor cancelled the policy as unnecessary and too expensive. He died the next year and his son, the Xuandi Emperor first renewed his grandfather's policy and financed the 7th Voyage and then changed his mind to agree with his father's view and stopped the voyages once and for all.
POD. But what if the Yongle Emperor had captured the Tatar raiders and never became depressed and didn't die from a stroke. He was 64 at the time. He could have reigned as emperor for another fourteen years if he lived as long as Kublai Khan. What if he had and continued Zheng He's voyages?
Most likely the 7th Voyage would have happened earlier than in OTL, probably 1424 to 1427. This leaves time for Zheng He to make two more voyages before his death in 1433. The 8th Voyage further explores the East Coast of Africa discovering Madagascar from 1428 to 1431, and in the 9th Voyage the exploration continues around the Cape of Good Hope and up the West Coast from 1431 to 1433.
It's on the 9th Voyage that Zheng He trades spices for gold and diamonds in South Africa and arrives back in China with enough treasure that covers the cost of all the 9 Voyages.
Zheng died at sea on the 9th Voyage. His squadron commanders Hong Bao and Zhou Man took the fleet back to China where Hong Bao died. The 10th Voyage, from 1434 to 1437, was commanded by Zhou Man with Ma Huan, who had been Hong Bao's chronicler, now second in command. Ma Huan takes a portion of the fleet up the coast of Western Africa while Zhou Man built a port in South Africa for his fleet to use for trading. Ma Huan discovers the Congo river.
After the Yongle Emperor died in 1438 his heir, the Xuande Emperor continues his grandfather's policy now permanently. He appoints Zhou Man Commander of the Western Ocean and Africa with order to establish a permanent Chinese presence at the port he'd built, which he named Qiwang. From then on the voyages were continuous and no longer numbered. Zhou Man commanded the fleet from Qiwang, sending subcommanders to voyage back and forth from Qiwang to China through the Western Ocean and South China Sea continuing trade with East Africa, Arabia, South Asia and the Indies.
Ma Huan based in Qiwang slowly worked his way up the coast of Western Africa in many voyages and reached the Guinea Coast in 1446 where his fleet encountered the Portuguese explorer Antionio Fernandes. The Portuguese had been exploring the west coast of Africa during the same time the Chinese had been doing their voyages. The Portuguese had established colonies in Madeira and the Azores. The Portuguese were finally reaching their goal, sailing past the Sahara to Western Africa so they could directly trade there for gold and slaves. But that was nothing when they found that they could trade with Ma Huan in Guinea for all the riches of the Orient.
But the Chinese were not about to give the advantage to the Portuguese. Ma Huan's fleet accompanied Fernandes' ships back to Portugal and sailed into Lagos Harbor in Portugal on March 3, 1447. The Ming Fleet had arrived in Europe. World History would never be the same.