Portugal's Prime Minister Antonio Costa touched down in Angola on Monday kicking off a two-day trip designed to improve ties between Luanda and its former colonial master.The diplomacy marks an effort to move beyond the bitter legacy of Lisbon's rule over Angola that ended in 1975 when Portugal withdrew without handing over power and Angola sank into civil war until 2002."Portugal and Angola have a long history, the tone of which should be defined by the future and not the past," said Costa as he set off for Luanda.Angola entered a new era last year when Jose Eduardo dos Santos, who ruled the country with an iron fist from 1979 to 2017, stepped down and was replaced by President Joao Lourenco."There's lots we can and should do together in terms of the economic ties between our countries which are already strong," Costa added shortly after arriving, becoming the first Portuguese head of government to visit for seven years. * Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and Facebook"Angola has a major challenge to diversify its economy and to increase domestic production, shrinking imports."Angola is a key trading partner with Portugal, and the third largest recipient of its investments.A key source of friction was removed in May when a Portuguese court decided that Angola's former vice president Manuel Vicente can be tried in Luanda, rather than in Portugal, on corruption charges.Lourenco had demanded that the trial take place in his country, "so that relations between Angola and Portugal can return to the level of the recent past".Investment plansCosta said that Angola's efforts to shift its economy away from heavy dependence on oil presented opportunities for Portuguese businesses."We've put in place a number of mechanisms so that economic players can invest," he said.The prime minister will attend a gathering of Portuguese businesses active in Angola as well as visiting the nation's new military history museum.He will meet Lourenco on Tuesday when the two will sign a slew of agreements that include deals on Angola's debts to Portuguese businesses - estimated to be in excess of $470 million. Portugal, battered by the global financial crisis, avoided bankruptcy with a bailout from the European Union, while Angola has become the tenth largest foreign investor in the country.The visit by Costa, which has been postponed several times, was a "very important step" toward normalising relations, said analyst Alex Vines of Britain's Chatham House think-tank.President Lourenco has pledged to fight corruption and rebuild the economy of the second-largest oil producer in sub-Saharan Africa, which has still not recovered from the plunge in oil prices in 2014.He has ousted relatives of his predecessor from leadership positions at state institutions and public companies in an effort to stamp his authority on the country and break with his predecessor's checkered past.